Tag Archives: Water

Water hardness, pH and bonsai

I live in London, a city sitting on a giant chalk deposit which formed in the Cretaceous period and stretches all the way to France (via the Eurotunnel)^ref Chalk is a form of limestone made up of the shells of marine organisms, and is comprised mostly of Calcium Carbonate (CaCO₃).^ref According to my water supplier (Thames Water) “When your drinking water seeps through this rock, it collects traces of minerals like magnesium, calcium and potassium. This is what makes it hard.”^ref

As you can see the water in my area is towards the harder end of hard. But there are plenty of places in Europe with hard water as well, as you can see in this map which comes from a study measuring groundwater in 7,577 sites across the region – most areas in fact are hard with exceptions in Scandinavia, Scotland and northwestern Spain (where igneous/volcanic bedrock dominates)^ref:

https://essd.copernicus.org/articles/13/1089/2021/#&gid=1&pid=1

What is also interesting from this research paper is the corresponding map of groundwater pH (see below). Groundwater pH determines your tap water pH if that’s where your drinking water comes from. Some areas source their drinking water from surface water as well, such as lakes and running watercourses – for example in Sweden it’s 50/50.^ref

pH is closely associated with water hardness, with higher levels of calcium carbonate leading to increased pH (in the world of agriculture a common practice to raise the pH of acidic soils is ‘liming’ – or adding calcium carbonate)^ref. Look at the areas in Southern Spain and France below which are pH 8 and above – their groundwater is also hard as shown in the map above.

The water in my taps is pH 7.75, so getting close to 8 which is relatively high. Not only that, but continued watering and drying of a bonsai medium with calcium-carbonate-rich water could increase the concentration of calcium carbonate in the pot and potentially make the pH even higher. But is this a bad thing?

To answer that question we need to take a detour into pH and what it actually means. At this point you can be thankful that usually I wait for a couple of days before posting, because otherwise you’d be deep in the weeds of ions, acids & bases and cursing my lack of editing skills! The (relatively) simple version is that pH is a measure of the concentration of hydronium (H₃O⁺) ions relative to hydroxide (OH^–) ions in water. In a neutral solution like pure water, they are at equilibrium and there is the same amount of each. The chart below shows the different ratios of hydronium to hydroxide ions at each pH. You will notice that in the red section there are more hydronium than hydroxide – this is acidic. In the blue section there are more hydroxide and less hydronium – this is alkaline (aka basic).

https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch17/ph.php#ph

pH is mainly a useful way of describing a chemical environment, as it helps to explain how other chemicals will react in that environment. For example, when a low pH (acidic) solution reacts with many metals, hydrogen gas and a metal salt are created.

pH is one of the fundamental attributes that affects living things – including plants. In living cells a difference in pH across the cell membrane is harnessed to drive some of the most fundamental processes for life itself – photosynthesis and respiration.^{ref1, ref2} Living things are generally very good at managing the pH inside their cells and have feedback processes to adjust it up or down according to their needs and the environment (called homeostasis). Studies have shown that pH within plant cells is maintained at a small range of 7.1–7.5.^ref

It’s when plant cells interface with the outside world, such as when taking in nutrients from the soil, that pH can make a difference to the efficiency (or not) of these reactions. Nutrients are taken up by plants as ions – ie. dissolved in water. This means that they need to be in solution for root hairs to take them up, and that solution can be acidic, alkaline or neutral.

Dissolved substances in the soil water (which change its pH) can also change the availability of nutrients – for example calcium ions will react with phosphorus ions to make calcium phosphate, so the phosphorus is unavailable for plants.^ref But plants adjust their uptake according to these changes, so when they detect pH levels which reduce nutrient availability, in many cases they adjust their uptake to compensate, and these forces work in opposite directions.^ref The overall effects of pH on the availability of nutrients to plants are a combination of the effects of pH on absorption by soils and the effects of pH on plant uptake.

Below is a chart showing the absorption of different nutrients by soil (in this case geothite, an iron rich soil). You can see that due to their different chemical makeup, each nutrient has a different absorption rate – the higher the absorption, the less available for plants.

https://media.springernature.com/lw685/springer-static/image/art%3A10.1007%2Fs11104-023-05960-5/MediaObjects/11104_2023_5960_Fig2_HTML.png

Negatively charged metals (‘anions’) have a more consistent soil absorption profile – and most are absorbed by the soil eventually when the pH is 6 or above. But uptake by plants is significantly increased as pH rises.

So far it seems like acidic soils might provide more nutrients – but also more toxins (eg. cadmium, lead & aluminium). But the release of organic matter, including nitrogen, sulphur and the activity of microbes which perform this breakdown, is increased at higher pH, and the uptake of metals is increased.^ref So it’s really a conundrum to work out the net effect of all these interactions! What do we actually know? Some findings include:^ref

Phosphate fertiliser is least effective near pH 7; it is necessary to apply more of it to achieve the same yield as at lower pH. It is most effective near pH 5
Boron uptake is consistent between pH 4.7 and pH 6.3, but a 2.5-fold decrease occurs at pH 7.4
Molybdenum uptake is eight time higher at pH 6.6 compared to pH <4.5^ref
Uptake of metal ions from solution by plants is increased by increasing pH – but their availability is decreased. This applies to toxins as well as nutrients. Magnesium and potassium are two important nutrients to which this applies.
Sulphate’s absorption by soil decreases markedly with increasing pH but plant uptake also decreases – the net effect has not been determined.

There is actually a fantastic diagram which shows the best soil pH range for each plant nutrient – you can see this all over the internet and it looks so useful! But unfortunately this diagram, which was created in the 1940s, is incorrect and has no real numbers behind it.^ref In reality “nutrients interact and different plants respond differently to a change in pH” as described above so there is no one-size-fits-all diagram.^ref

While I’m in mythbusting mode, there isn’t any such thing as ‘soil pH’ either! As noted in this excellent study from March 2023, pH can only be measured in a liquid. Unless you are over-watering, it’s likely your soil is not a liquid, therefore the soil itself does not have a pH. The pH that is being measured when ‘soil pH’ is measured is actually the pH when the soil is mixed with water – whilst this is indicative of the pH that might be present on individual soil particles, there is probably a range of pH instead across different particles. The pH of the water on a soil particle and the pH of the water on a root hair combine to create the true pH environment for a particular nutrient on a particular root. This is obviously not very easy to measure! See the end of this article for my bonsai media pH experiment.

The study mentioned above basically claims that most studies on pH and soils have failed to take into account the interplay between availability in the soil and plant uptake of a nutrient, which often work in opposite directions and so pH should not be taken to be the main factor in nutrient uptake except in specific circumstances. But looking at all of the above, it does seem like slightly acidic conditions should optimise all of the different reactions taking place – between 6 and 7 pH.

To bring it back to my bonsai, in my London garden with hard tap water of pH near 8, on the surface it would appear that this has the potential to cause a phosphorus deficiency in my plants, and perhaps affect their boron, molybdenum and metallic ion levels (we care about magnesium particularly which is used for photosynthesis – magnesium uptake increases at high pH but availability in the soil decreases).

But tap water is not the only thing affecting pH in the water in my bonsai soil. It’s also affected by the pH of my rainwater, which was 5.89 on the last measurement^ref, as well as the medium in my pots. I use composted bark, biochar and molar clay. Composted bark has organic components so is acidic, biochar is slightly alkaline and molar clay appears to be acidic – and this pH will become evident when particles of these components dissolve into the water. So the actual pH of the solution in my bonsai soil is anyone’s guess! All I can conclude from this is that a long summer without rain might cause my soil to increase in pH due to the removal of one acidic component – the rainwater.

The other thing to consider is that you can obviously adjust the availability of nutrients by adding them to your soil. So even if uptake is reduced by a particular pH, making more nutrients available could compensate for this. Hence the importance of regular fertilising for our bonsai, and using a range of different fertilisers which provide different nutrients.

Finally if you want to test the pH of your bonsai medium, a good approximation can be made by using a red cabbage and some distilled water (don’t use tap water, as this will affect the outcome if it’s not neutral to start with). Simply boil up a bit of red cabbage in (distilled) water, let it cool and while you are doing that put a representative piece of your bonsai medium into some water (also distilled). Allow them to soak for a while. Remove the cabbage from the cabbage water, strain the medium out of the bonsai medium water, and pour some of the cabbage water into the bonsai medium water. It should change colour according to the pH as follows (you can read more instructions here):

https://i0.wp.com/www.compoundchem.com/wp-content/uploads/2017/05/Making-a-Red-Cabbage-pH-Indicator.png?ssl=1

I performed this experiment on different bonsai mediums I had sitting around in my shed by soaking them in filtered water for 1 hour, then adding the cabbage indicator. The results were interesting! I was expecting the Kanuma to be acidic but it was actually neutral, as was my bonsai mix (which included some molar clay, bark, biochar, pumice and compost), and the pumice was surprisingly slightly alkaline. A rather small amount of biochar caused the indicator to go dark blue, which definitely tells me it needs to be used in moderation (although other mechanisms in biochar make nutrients available to plants, which you can read about in my biochar post).

What I conclude from all this is that my use of composted pine bark in my bonsai mix is probably a good thing as it will counteract the alkalinity from the tap water. This was a suggestion I learned from Harry Harrington’s website – although he recommends it for water retention, it would appear to balance a high pH medium or water as well. It also has the added benefit of being organic matter, which is a fertiliser in itself, creating more nutrient availability even if the calcium carbonate in my water locks some away. The need for applying fertiliser regularly is also apparent, as you just don’t know how nutrients are behaving in your particular bonsai soil and you need to give each tree every chance they have to access the nutrients they need. But overall other than causing annoying limescale marks on pots, my bonsai seem completely fine with hard water.

Transpiration

I’ve talked about transpiration in quite a few different posts on this site, but a recent thread on http://bonsainut.com caused me to think maybe I should have a post dedicated to it, so here goes…

Transpiration is the evaporation of water from the leaves of a tree. It’s actually a critical process for trees, because excess transpiration is one of the few ways in which a tree can die; so-called ‘hydraulic failure’ has been identified as the most prominent cause of tree death.^ref Hydraulic failure – the failure to access enough water to replace water lost mainly through evaporation – causes cell death, xylem failure and a fatal reduction in photosynthesis. So it’s really important for bonsai practitioners to understand this process.

The main driver of transpiration is not – as you might think – to cool the leaves (although this is one reason for it). In fact transpiration is a by-product, or ‘cost’, of photosynthesis, and it happens because of the way that leaves obtain carbon dioxide. You may already know that plants have small pores called ‘stomata’ which open up to let air – and CO₂ – inside the leaf. But you might not have known that gaseous CO₂ in air needs to be dissolved in water before it can be accessed by chloroplasts and used for photosynthesis (this is explained in Vogel (Chapter 5 – ‘Leaking Water’). This means that water needs to be available on the surfaces inside the leaf – which means that when stomata open up, this water is subject to evaporation.

Vogel says that “only if the relative humidity is 100 percent will water not be lost…[and]…if the leaf’s temperature is above that of the surrounding air, then water can be lost even at that humidity.” He also says that for every gram of CO₂ used by a leaf for photosynthesis, it’s estimated that 125 grams of water is lost.

Smith et al (Chapter 4.10 Movement of Water & Minerals) explain that this evaporation causes a constant flow of water known as the ‘transpiration stream’. As water evaporates from the leaf cells, pressure in those cells is reduced, and this negative pressure causes water from the xylem to move into the space, due to strong mutual attraction between water molecules. That in turn pulls more water behind it and so on. This hydraulic mechanism is responsible for pulling water all the way up the tree from the roots. Actually this process is fundamental to the health of the tree, maintaining cell turgor (stiffness), transporting nutrients, metabolites & growth substances synthesised in the roots throughout the tree, and providing a source of water for the phloem stream which flows in the opposite direction providing energy to the tree. When there is enough water available, all of this works perfectly – when there isn’t, problems arise.

The extent of evaporation from the leaves of a tree is determined by several different factors, which can be divided into environmental, tree-specific physical factors and tree-specific response factors.

The main environmental factor which drives transpiration is the ‘vapour pressure deficit’ (“VPD”) – this is the “difference between the amount of moisture in the air and how much moisture the air could potentially hold when it’s saturated.”^ref VPD is a function of both heat and humidity, and provides a measure of how powerful the evaporative force of the air is with any combination of these.^ref

Occasionally while writing articles for this blog, I end up in the world of cannabis cultivation. Maybe because they are very motivated to keep their crops vigorous, cannabis growers and their equipment suppliers sometimes have the best data and charts out there! This is just such an occasion, see below for an excellent chart from ‘Ceres Greenhouse Solutions’ showing the VPD for a given temperature and humidity (you can download a copy here). The VPD is low in the blue section and high in the red.

https://ceresgs.com/wp-content/uploads/2018/10/VPD-Bioengineering-Chart.pdf

What you will notice is that the relationship between humidity and temperature isn’t exactly linear. Also, VPD increases with higher temperature and lower humidity. Since a higher vapour pressure deficit means there is more ‘pull’ on the water in leaves, increasing temperature and decreasing humidity both increase transpiration – and they reinforce each other, so dry and hot is a high transpiration combination.

Another environmental factor is wind. One study found that wind actually improves water use efficiency, because whilst it does increase transpiration, it also increases CO₂ uptake, and the net effect is greater water use efficiency and not less.^ref But for the purpose of this article, wind does increase transpiration.

Coming onto tree-specific physical factors – these are all the attributes that relate to the size, shape, position and structure of the tree. In general the more foliage a tree has, the more it will transpire – so a large broadleaf tree will transpire significant amounts on a hot day – in one study they found a large canopy tree in the tropics (Eperua purpurea) transpired up to 1180 litres per day!^ref By comparison in the same study, smaller (presumably more shaded) trees transpired a lot less. Thomas (Chapter 2: Leaves the food producers) gives the following figures: “<100L/day in conifers, 20-400 L/day in eucalypts and temperate trees such as oaks, reaching perhaps 500 L/day in a well-watered palm and as high as 1200 litres per day in specimens of Eperua purpurea growing out of the top of the Amazonian rainforest canopy.”

The chart below shows the daily transpiration rate during the growing season for a sessile oak tree in Turkey which measured 18.5m x 34.5m – this maxed out at 160 kg/day (effectively 160L).

https://www.ewra.net/ew/pdf/EW_2017_59_34.pdf

As well as the volume of foliage, trees have different stomatal size and density (number of stomata in a given area) which are determined by genetics as well as environmental factors (such as intensity of light and VPD to which they are exposed when developing).^ref1 ^ref2 Low stomatal area (ie. density x size) will result in lower transpiration when compared to a tree with higher stomatal area. These researchers measured stomatal area for 737 plant species across 9 forests and at the lower end of the spectrum conifers such as Cunninghamia lanceolata (0.2%) and Picea koraiensis (0.4%) had 100 times less stomatal area than angiosperms such as Viburnum betulifolium (23.77%) or Quercus serrata (21.74%). You can download all their data here. Basically the more stomatal area which is open to the air, the more transpiration there will be.

Many trees have wax plugs in their stomata which reduce their efficiency, and transpiration at the same time. To copy a piece from my article on needle leaves, wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.^ref One study found that 81% of the species they looked at contained such plugs and that wax plugs are particularly numerous in conifers.^ref

Another factor is the level of transpiration via bark. This isn’t due to stomatal opening but simply due to partial permeability of bark to air – also genetically determined and due to the presence of ‘lenticels’ – small channels which allow passage of water and air for the metabolism of living cells in the bark. One study on Pinus halupensis found that “Bark transpiration was estimated to account for 64–78% of total water loss in drought-stressed trees, but only for 6–11% of the irrigated trees.”^ref This is because bark transpiration is passive and unmanaged, unlike leaf transpiration which can be somewhat controlled by the tree (see below).

Also relevant for individual trees is their position relative to other trees and the sun. Shade will reduce the temperature at the leaf surface and reduce transpiration, a mass of trees together along with undergrowth may increase humidity, also reducing transpiration. A tree standing alone or above others will be exposed to higher temperatures and lower humidity, thus increasing transpiration. Different areas on a single tree will be exposed to different combinations of these factors as well, so rates of transpiration will differ even from leaf to leaf on a given tree.

The final category of attributes which determine transpiration relates to the trees’ ‘behaviour’. That is, how they react to different environmental conditions. As we all know trees may be sessile but they are also incredibly dynamic and can adjust a wide range of parameters of their own biology. The main issue they need to address in this case is losing too much water, which could lead to death. As a result, they change their physiology to manage evaporation as well as water intake at the start of the transpiration stream.

To manage evaporation, trees adjust their stomata based on water availability, changing their ‘stomatal conductance’ to reduce transpiration if not enough water is available.

They do this in a couple of different ways – ‘passively’ and ‘actively’.^ref The passive mechanism is where lower water pressure within leaves causes guard cells around the stomata to lose their stiffness, which has the effect of reducing the stomatal aperture. The active mechanism relies on the tree producing abscisic acid (ABA) – this “triggers efflux of anions and potassium via guard cell plasma membrane ion channels, resulting in decrease of turgor pressure in guard cells and stomatal closure”.^ref

A study on Metasequoia glyptostroboides found that in most conditions of water availability the passive mechanism was in play, and it wasn’t until prolonged or severe water stress was experienced that the active ABA-mediated mechanism came into play.^ref The article explains that different gymnosperm species use different combinations of these passive and active processes to manage a lack of water availability by reducing transpiration. Angiosperms by contrast use a more sophisticated and more recently evolved version of the active process, mediated by ABA.^ref

Thomas says that stomata usually close when it is “too cold or dark for photosynthesis” or when the leaves are in danger of losing too much water and wilting”. The consequence of stomatal closing is an associated reduction in photosynthesis – so when a tree is drought stressed, it won’t be generating energy at the same rate as when it was healthy. A study measuring photosynthesis versus stomatal conductance for Pinus radiata (see in the chart below) found there was a roughly linear relationship, as the stomatal conductance increased, so did photosynthesis.

https://www.researchgate.net/publication/8690701_Variation_in_foliar_13C_values_within_the_crowns_of_Pinus_radiata_trees

There are several other ways that trees manage their transpiration – by adjusting their root conductance (ability to draw in water), changing their leaf expansion so that there are fewer/more leaves which are smaller/larger in area, pointing exposed leaves downwards during hot periods of the day, changing the root/shoot ratio to match water source to water use and by operating a daily cycle of metabolism which optimises transpiration (eg. increasing their root hydraulic conductance at night when there is lower evaporation, and ‘filling up’ to deal with higher transpiration during the day).^ref So they are very much active participants in responding to and controlled their transpiration rate.

But what does it all mean for bonsai? The first thing is, if your tree has plenty of water availability, transpiration should not become a problem, but you need to remember that up to 95% of water use is evaporation^ref so trees need a lot more water than you might expect. The best way to avoid issues associated with excess transpiration is to supply your trees with all the water they need. This is achieved by regular and sufficient watering, and by using a medium which has some water retention to avoid drought stress – but is also well-draining. A well-draining medium allows you to water more often without the risk of waterlogging roots or creating conditions for pathogens to take hold.

Also – a tree’s ability to handle water loss varies widely depending on the species – Thomas gives the examples of eucalypts and alder as species which cannot control transpiration effectively, and some oaks as species which can. So each tree in your collection will be different.

But let’s consider all the factors explained above that increase transpiration: high vapour pressure deficit (high temperature and/or low humidity), wind, lots of foliage, high stomatal area, clean (unwaxed) stomata, passive bark & leaf evaporation, a sunny/solitary/high position, and a lack of water availability to the roots which activate stomatal closure.

Some of these are adjustable for bonsai. If it’s going to be a hot, dry, windy day then your trees are going to transpire a lot more than normal and if their roots can’t keep up, you need to improve their environment; newly collected and recently root-pruned trees or trees in particularly small or shallow pots will be most affected. You can help them by providing shade (reducing the temperature), increasing humidity, and moving them out of the wind – and obviously by watering. For a temporary period, on a very hot day, it might even make sense to sit pots in water (do not do this for an extended period).

Transpiration can also be a problem in the winter as trees do continue to transpire, albeit at lower levels, even if they are deciduous. As such, they do need water to be available which means you need to keep an eye on moisture levels in pots. If they get dry, water them. If the medium is frozen, this will lock up water and can have a dehydrating effect so in this case you need to also water, ideally when it’s above freezing. Mulch is suggested to avoid hydraulic failure for trees in the ground^ref, a similar approach can be used for bonsai in pots, to reduce freezing and make more water available to roots. Even at night it is not the case that transpiration completely stops – typically it is 5% – 15% of daytime rates.^ref

Balancing the amount of foliage with the roots when repotting or pruning is another important way to help your trees manage their transpiration rates, so that there is enough root mass to meet transpiration demands. Root pruning in the heat of summer should be avoided unless a comparable foliage reduction takes place. If you’ve gone a bit far with the root pruning, use the approaches above – provide some shade, increase the humidity and maintain a watering regime. This is where the bagging method for collected trees comes from – it reduces transpiration by increasing humidity and can be used for trees struggling to recover from a severe root prune.

Anti-transpirant is a product that some bonsai aficionados use. This does what it says on the tin – it is a “film-forming complex of polyethylenes and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate”^ref The active substance is derived from conifer resins. In reducing transpiration these products also reduces photosynthesis, which is a consideration. I’m personally not a fan of disrupting a plant’s natural processes in this way, and successful use of the product depends on the individual tree and product selected (read more here).

Hopefully you can see from all of the above that transpiration is an extremely important concept to understand as a bonsai geek, but one which can be managed, as long as you are aware of the factors at play. Here’s to helping our trees avoid hydraulic failure!

What does frost do to bonsai trees?

Living in London means that even though spring has blossomed forth, there is still a chance of frost all the way through to the end of May. And many of us will have learned the hard way that frost and/or sub-zero temperatures can seriously damage our trees.

One of the key risks of frost and extreme cold is the problem of ice. Ice formation pulls water from plant cells, causing dehydration, which can be just as lethal as dehydration from underwatering.^ref Ice masses which form and thaw can deform and damage cell membranes, as well as buds and other plant organs, and ice in trees’ xylem vessels can cause embolisms (air bubbles) to form, cutting off the water flow above the bubble.

Another problem is that low temperatures make photosynthesis dangerous due to an excess of energy which ends up as damaging reactive oxygen species (“ROS”). This is why some trees convert green chloroplasts to bronze chromoplasts in cold weather – read more here, and others shed their leaves altogether to eliminate the problem.

Plants which are ‘hardy’ or ‘frost-hardy’ have mechanisms to resist the effect of ice – either by ‘avoidance’ such as supercooling (lowering a liquid to below freezing point without freezing), or ‘tolerance’ – using biochemical changes and physical adaptations allowing the tolerance of ice in their tissues.^ref Some pines have evolved specific mesophyll cells which allow the ice into the intracellular spaces without deforming the key structures within the needles – a physical adaptation. Oaks and ring porous trees regrow their conducting xylem every year, since embolisms during winter make last seasons’ xylem ineffective.

Crucially, since many cold-hardiness mechanisms rely on biochemical changes within the plant, they require time, so that the relevant proteins, enzymes and other metabolites can be synthesised in sufficient quantities to have the desired effect in plant cells. This process is called ‘cold acclimation’. Smith et al (2010) give the example of rye. 50% of non-acclimated rye plants will die at -6^oC, but after spending 2 days at 4^oC, they can go down to -21^oC before 50% of plants will die. The cold acclimation process is what’s known as an ‘epigenetic’ process – where environmental triggers such as shortening days and reducing temperatures turn on genes – in this case known as COR (cold-regulated genes).^ref These then produce the proteins which are used to create the cold-hardy biochemical changes.

There is an equal and opposite process known as cold deacclimation. Smith et al (2010) say that above a temperature of 10^oC cold hardiness is rapidly lost, which is why a spring frost at the end of May can be so damaging.

To complicate matters, shoot hardiness is not the same thing as root hardiness.^ref Studies have shown that roots are less hardy than shoots, even when exposed to identical temperature acclimation treatments.^ref For example, the stems of Pyracantha are hardy to -25.6^oC, whereas mature roots are hardy to -18.8^oC and young roots are hardy to -6.1^oC.^ref This is an important insight for bonsai because the scale of our trees and the fact they are suspended in pots means their young roots are particularly vulnerable. A symptom of winter damage to young roots is when a tree flushes later than normal, and has retarded growth during the season.^ref It may not kill the tree, but it will certainly give it a handicap for the next season.

So what does it all mean for bonsai?

For me the main risk boils down to root damage. Those lovely fine roots we work hard to encourage during the growing season are more vulnerable to frost than any other part of the tree, and unfortunately due to the size, shape and positioning of most bonsai pots, they are very exposed to cold temperatures. In one study it was found that container-grown trees were subjected to temperatures down to -15^oC when the night-time temperature reached -30^oC, whereas the soil only went down to -6^oC.^ref And since surfaces cool faster than air when it’s cold, resting up against a cold pot surface is the last place a young root wants to be! When the temperature gets really cold, mature roots can be damaged as well, which could be fatal to the tree.

Sure – leaves can be damaged by frost as well – mainly when deciduous trees have leafed out expecting above-zero temperatures and then a frost comes along. Most will deal with this and should regrow their leaves – there will be an energy penalty which will reduce the overall energy they have to devote to the growing season, and they won’t look great in the meantime, but it shouldn’t be fatal if the stems and roots are still healthy. But certainly to keep your trees looking and growing their best, you want to avoid deciduous trees leaves being exposed to frost if at all possible (see below for some strategies).

To be able to manage and prevent frost damage, we need to know when a frost will happen. This starts with monitoring the temperature during winter. Importantly, when you hear or see a temperature forecast for a location, it is a forecast for the air temperature. The temperature of the ground is often several degrees lower. The UK Met Office says “As a general rule of thumb, if the air temperature is forecast to fall between 0 °C and 4 °C on a night with little or no cloud and light winds, then you need to bear in mind there may be a frost outside in the morning. The closer it is to zero, the greater the chance of seeing frost. If the air temperature is forecast to be below zero, then the risk of seeing frost is much higher.”^ref This goes for surfaces like pots as well. Wind and cloud cover reduce the chance of frost at a given temperature.

The actual temperature which will damage or kill roots is species-specific, so there’s no hard and fast rule but knowing where a tree comes from should give you an indication. In general the species common in the boreal forests such as many Pinaceae and Betula pendula will cope best with freezing temperatures. Some ornamental tree ‘killing temperatures’ are provided in this list.

So what should you do to help your trees defend themselves against frost damage?

The first thing to do is to let them be exposed to the cold over time, before it freezes. Give them time to activate their COR genes and establish cold acclimation. This includes not putting them in a polytunnel or shed or wrapping them up until they’ve had some time in colder temperatures (but not freezing).

It’s also useful to have actual temperature data from the location where your pots are going to be over winter. This can be achieved using a digital thermometer/hygrometer which records the temperature and humidity at regular intervals – there are many wireless/bluetooth-enabled options now which are very reasonably priced (I just bought 3 for £35, one for my garden bonsai bench, one for my allotment bed and one for my allotment greenhouse). This will enable you to monitor what the temperature does relative to the forecast so you can better predict the forecast in your actual location – and you can see which locations might provide better winter protection. If you see the temperature approaching zero you can act to protect your trees.

Then, adopting a form of overwintering system could be beneficial. Somewhat counter-intuitively, this involves watering everything well, without overwatering.^ref The reason for this is to avoid dehydration. Then providing a physical barrier to the cold, effectively a ‘tree duvet’. This can move the frost surface away from the pot edges. You could use an insulating blanket of some kind, but do some research – one study found that ‘microfoam thermoblanket’ made a difference, but clear, black or white poly did not.^ref The covering would need to have insulating properties, and not get saturated with water such that it would then freeze anyway. You could also move your trees into a polytunnel, garden shed or greenhouse, with the walls and air inside acting as a form of insulation. If you use a digital thermometer you can monitor these to ensure they aren’t getting too cold – note that polytunnels have been found to get cold enough to damage roots.^ref

The absolute best is to provide some form of heat so that the temperature can’t go below a certain point – in practice this could be moving your pots closer to your house, using a (lightly) heated propagation bed, or putting them in a (lightly) heated greenhouse or outhouse. You don’t want them to think it’s spring so there shouldn’t be too much heat, just enough to keep the temperature above freezing. Be aware that in a warmer environment such as a shed your trees will deacclimate earlier as well, so make sure they don’t go back outside into a frost as they may have lost their cold acclimation.

Water Sources for Bonsai

While many of us might simply use the hosepipe to water our bonsai, there are actually a range of options for recycling or collecting water for this purpose. I’ve looked into some of these below to understand how suitable water from different sources is for watering your trees.

Dehumidifier water – good unless you have a dessicant humidifier and toxic air

Dehumidifiers remove water from the air in a few different ways. One common type is a compressor dehumidifer. This type pulls air through a filter and over cooled metal coils which cause water in the air to condense onto the coils and drip into a reservoir.^ref Since this is effectively distilling the water, it should be relatively pure. Where contamination could come into play is if the coils or the water reservoir are not kept clean, but for bonsai tree watering, this shouldn’t be an issue.^ref You should be cleaning the reservoir anyway to avoid Legionnaire’s disease (see below). The water from a dehumidifier won’t have any minerals or nutrients in it (unlike rainwater), so fertiliser would be needed.

A dessicant-type dehumidifier pulls water through a dessicant material – such as zeolite^ref (if you’re wondering where you’ve heard that name before, it’s used as a bonsai soil additive^ref). Water is absorbed in the dessicant then this material is heated and the water drips out into the reservoir. It’s not exactly the same as distilling because the water is in contact with the dessicant as it is condensed and could hold dissolved compounds. Researching the properties of zeolite will take you down an entirely new internet rabbit hole (including 1.24M research paper results on Google scholar). But this substance is known for extracting heavy metals and other contaminants from liquids^{ref1 , ref2}. So in theory the zeolite could hold other molecules which could be released into the water as it condenses. This might be an issue if you are using dehumidifier water from a location with particularly toxic or polluted air. If not, the risk to your bonsai should be fairly low. As above cleaning the water reservoir and dessicant regularly is important.

Tumble dryer water – good if condensing, less good if venting

Tumble dryers also work in slightly different ways but the main mechanism for extracting water is that warm air evaporates water from the clothes, then this air either passes over a condenser or is vented outside, in both cases water condenses from the air as it cools.

With a condensing dryer, the process is another form of distillation so should be relatively pure water and fine for use on bonsai.

If you are using a vented machine however there may be microplastics, particulates and lint from the drying clothes coming through the venting pipe. In 2021 tumble dryers were found to be a leading source of microfibre air pollution.^ref You might not think this would affect your tree very much, but it has been found that nanoplastics and microplastics can enter plants through their roots, carrying a range of toxic substances including pesticides, polybrominated diphenyl ethers (PBDEs), endocrine-disrupting chemicals (EDCs), polycyclic aromatic hydrocarbons (PAHs), phthalates, and bisphenol-A. PLA microplastics have also been shown to negatively impact arbuscular mycorrhizal fungi diversity and community structure.^ref If you want to avoid microplastics in your pots and tree roots you probably don’t want to use vented tumble dryer water.

Air conditioner water – good

Air conditioners work similarly to dehumidifiers and dryers – they pull air through a mechanism which cools it, and in doing so water is condensed from the air. So it’s fine to use for watering (although as above will contain no nutrients).

Boiler condensates – not good

I’m not sure how many people would be trying to use boiler condensates for watering but just in case you think of it, boilers which use fossil fuels produce condensates containing carbonic acid, sulphuric acid and nitric acid, all of which reduce the pH of the water coming from the system.^ref Although slightly acidic water (in the range 5.5-6.5) has been show to optimise plant growth^ref, boiler condensates can be as low as 3 which is toxic to plants.^ref

Rainwater – good – maybe the best

Rainwater is a different proposition to the previous water sources. Whilst it is distilled from air just like the others, it has to fall through the atmosphere to reach the ground. As it does this, rain absorbs compounds present in the air in particulates and gases. It also runs off roof surfaces, down drains and pipes and into storage tanks. So rainwater collects contaminants along the way. As atmospheric carbon dioxide is one of the molecules rain collects, it has an average pH value of about 5.6 (just at the lower end of preferred plant water pH which is 5.5-6.5)^ref

The chemical composition of rain varies geographically even before it hits the ground. For example in Samoa the rainwater composition is highly influenced by marine sources, which makes sense since it’s an island.^ref Locales near the ocean have rainwater with a similar (diluted) composition to sea water^ref but those inland vary depending on natural and manmade influences such as industry, topography and weather. Raindrops have also been found to contain airborne bacteria and fungi.^ref

Rainwater in the UK is monitored for ions of sodium, calcium, magnesium, potassium, phosphate, nitrate, ammonium, sulphate, sulphur dioxide and chloride, and for acidity/pH and conductivity.^ref The latest measurement at my nearest monitoring station showed the following contents from a 3mm sample – nutrients are there but nowhere near the same levels as a seaweed fertiliser.

The other factor to consider is where you store your rainwater. Outdoor water reservoirs are usually colonised by bacteria, fungi and other organisms, and have rotting plant matter, bird faeces, dead insects and other detritus. This may also be a good source of fertiliser, depending on how concentrated your detritus gets, or a source of toxic microbes and algae, particularly if there isn’t much flow and replenishment of the water.

But overall rainwater is a good choice for watering bonsai. It has a favourable pH for plants, and is a mild fertiliser containing a range of macro and micro nutrients. As it doesn’t have carbonates like groundwater, using rainwater can help you avoid limescale marks on leaves & pots if you live in a hard water area. Just try to avoid leaving it standing or stagnant for long periods particularly if the temperature is above 20^oC (see Legionnaire’s disease below).

Aquarium water – maybe depending on your tank

The subject of aquarium water almost warrants its own post. Anyone wanting to better understand the chemical and biological parameters inside a planted aquarium should read the brilliant book “Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise” by Diana Walstad.

The water in your aquarium is likely to be completely fine for bonsai if it’s fine for fish. They have high standards – and can’t handle large pH ranges, excessive levels of nitrites, ammonia or excessive nutrients like heavy metals. In fact this water can be an excellent source of nitrogen since fish in aquariums excrete ammonia, which is the main source of nitrogen for most fertilizers. Usually when doing a water change on an aquarium the levels of rotting organic matter have accumulated and ammonia levels are at their highest – one of the main reasons for doing water changes is to reduce them. Planted aquariums which use soil as a substrate and have fish (and fish food as an input) also contain other macro and micronutrients, to the extent that fertiliser isn’t necessary. So aquarium water could be a good addition to your bonsai, as a fertiliser.

On the other hand, if you have certain plants in the aquarium, some are known to be ‘allelopathic’ – that is they produce compounds to inhibit other organisms. For example the water lily Nuphar lutea kills duckweed and lettuce seedlings (Walstad, 2012). Aquatic algae and bacteria can also behave allelopathically, to the extent that Diana Walstad keeps her prized plants in their own substrate and even in separate tanks to stop them being killed by competitive organisms. So there is a risk with water from planted aquariums or aquariums with algae that there may be allelopathic compounds which damage your tree. It’s hard to make a recommendation since it’s impossible to know without testing whether toxic compounds are in your aquarium water. If you have no algae or plants, then the water is probably fine.

Greywater – not unless you treat it

Grey water is waste water from your bath, shower, washing machine, dishwasher and sink.^ref It can contain detergents, oil, dirt, organic matter like food, skin particles, microplastics, bacteria, fungi or anything that you wash off yourself, your dishes or your clothes. As such greywater isn’t suitable for watering your bonsai. Some larger properties like hotels recycle their greywater, but it’s treated first using UV light, chemicals or serious filtration systems.^ref So unless you have access to a high quality greywater filtration system, do not use this to water your trees.

Blackwater – no

The things you learn about when researching bonsai websites. We won’t go there – just – no.

Seawater – not unless you have a desalination plant

Unless you are growing kelp forests, do not water your bonsai with sea water. Excess salt is extremely detrimental to plants, and can kill them.^ref

Lakes, streams, rivers, boreholes, wells or ponds – it depends

Natural water sources such as these are not treated and can contain all sorts of things aside from water, but this really depends on what runs into them, what happens and lives on and in them. Some lakes & streams are very clean, others have industrial runoff, pesticides, sewage overflow^ref, algal blooms from excess fertiliser runoff and worse. What you want to avoid with a water source is dissolved contaminants which might harm your tree – usually these will derive from human activity going on upstream. So before using such a water source, it would be wise to investigate what might be entering it and perhaps to invest in some testing.

It’s thought that water obtained from aquifers underground (eg. via wells or bore holes) is of higher quality if it comes from a deeper confined aquifer than from a shallow one.^ref Shallow aquifers are more open to contamination from pollutants.

pH is a consideration when using a natural water source. According to the Kentucky Geological Survey: “Streams and lakes in wet climates such as Kentucky typically have pH values between 6.5 and 8.0. Soil water in contact with decaying organic material can have pH values as low as 4.0, and the pH of water that has reacted with iron sulfide minerals in coal or shale can be even lower. In the absence of coal or iron sulfide minerals, the pH of groundwater typically ranges from about 6.0 to 8.5, depending on the type of soil and rock contacted. Reactions between groundwater and sandstones result in pH values between about 6.5 and 7.5, whereas groundwater flowing through limestone strata can have values as high as 8.5.”^ref Since you want to keep water pH between 5.5 and 6.5 for plant watering, it would be a good idea to test the pH of any natural water source you are using.

A note on Legionnaire’s disease

Legionnaire’s disease is a potentially fatal form of pneumonia caused by inhaling mist or droplets of water containing Legionella bacteria^ref. This bacteria can grow in any non-sterile water^ref, feeds on algae, rust, scale or sludge, and thrives in temperatures between 20^oC and 45^oC.^ref So it could be present in some of the above sources, particularly when water is left standing. It’s worth being more careful when temperatures are above 20^oC as you really don’t want to get this disease, which may already be a risk to bonsai practitioners since another source of Legionnaire’s is compost & potting soil.^ref In warmer weather it may be safer to use tap water for bonsai.

So to wrap all of this up from a bonsai perspective – feel free to use rainwater or water which comes from the distillation process of condensing humidifiers, tumble dryers or air conditioners. Be careful though if this water has been standing for a long time in temperatures above 20^oC. Feel relatively free to use aquarium water (but keep an eye on your trees to monitor for allelopathic effects) or dessicant-type dehumidifiers. Test the water if you’re using an untreated natural water source like a river, lake or well. Avoid using water from vented tumble dryers. And definitely do not use sea water, boiler condensate, untreated grey water or black water. Happy watering!

How roots absorb water and nutrients

Unlike animals, plants do not have a digestive system, although the sustainable food trust makes a good argument that ‘soil is the collective stomach of all plants’^ref Trees synthesise all of the substances they need to live and grow from 17 nutrients. It’s important to understand that plants don’t ‘eat food’ in the sense of consuming sugars, fats or proteins like animals do. Aside from oxygen, carbon and hydrogen (which come from air and water), trees absorb nutrients through their roots.

Water and nutrients are transported around trees via the xylem, a network of narrow dead cells which act like a kind of pipe. Nutrients are dissolved in the water (‘solutes’) and travel with it in the form of ions (charged molecules). To get into the xylem in the first place, water is absorbed into the root tips.

In many species this is done through the root hairs. Root hairs are “long tubular extensions of root epidermal cells that greatly increase the root surface area and thereby assist in water and nutrient absorption.”^ref According to Thomas most live only for a few hours, days or weeks, and are constantly replaced by new ones as the root growing tip elongates. Some conifers do not have root hairs and rely on mycorrhiza instead to assist nutrient and water absorption.

In order to absorb water, the root tips need to be in physical contact with it, so having root hairs that reach into the soil provides contact with more water (and nutrients). Nutrients in the form of ions are ‘pumped’ into root hairs (or cells, if the species has no root hairs) using a process called active transport, which uses some of the energy from photosynthesis. Because the root cells have dissolved nutrients in them, water is then attracted into the space by osmosis.

From the roots tips, water and solutes make their way to the ‘stele’ – this is the central part of the root which contains the vascular system (xylem & phloem, shown in blue and red respectively in the left hand diagram below). Surrounding the stele is the endodermis – seen below in orangey-brown cells with red lines through them.

https://onlinelibrary.wiley.com/doi/10.1111/jipb.12534

The red lines represent cells known as ‘Casparian strips’. They are full of lignin and other hydrophobic molecules, which basically plug any gaps between the endodermis cells. This forces any water or solute to pass through the endodermis cells. After this they travel through the root parenchyma cells into the xylem.^ref

The existence of Casparian strips leads to a pretty important insight, which suggests that most molecules entering the xylem from the outside world are actively invited in, and have to be able to traverse a cell membrane. So the tree can theoretically control or at least limit what can enter. Vogel says “the sap that rises up a tree trunk has to be nearly free of dissolved material. So much water gets transpired that the accumulation of dissolved solids, coming out of solution as water evaporated in the leaves, would make big trouble as the growing season advanced.” So this implies there aren’t a lot of non-nutrients dissolved in xylem sap. But in fact, xylem has a microbiome (it’s part of the endosphere) and literally thousands of dissolved molecules in it (described more in xylem), so obviously the Casparian strips are not a 100% barrier.

It’s not all down to the root hairs or root tips though, symbiotic fungus known as mycorrhiza play an important role in enabling root function, read more about this in The Microbiome and Symbiotic Microbes.

Roots

The roots of your tree are *just* as important as the above-ground parts, with a lot of responsibilities which aren’t immediately obvious. I’ve summarised the main ones here but there is a lot more detail in separate posts with links provided below. So why are roots so important?

they absorb water from the soil to meet all the tree’s needs (both for photosynthesis and transpiration)
they absorb all the nutrients that the tree needs from the soil (using a different process to water, hence a separate point)
they transport nutrients & water up to the above-ground parts of the plant, and photosynthates (the products of photosynthesis) down to the root tips
they produce exudates (secretions) which sense and control the rhizosphere (the environment in which the roots are growing)
they produce plant growth regulators for signalling and enabling growth within the plant
they store food for later use
they provide structural strength and stability for the tree by attaching it into the soil

Points 1 and 2 are fundamental to the health and growth of the tree – the roots are the mechanism for the tree to obtain all of the water and nutrients it requires (despite the mythic popularity of foliar feeding, this is only a way of augmenting nutrient absorption and not a primary mechanism). The mechanics of how they do this is described in more detail in how roots absorb water and nutrients – in summary it’s the fine roots and their root hairs which do the majority of the absorption since they have the closest and most expansive contact with the soil. There needs to be enough root surface area to supply the stems, shoots and leaves with the water and nutrients they require.

Point 3 reflects the fact that roots are part of a tree’s vascular system, that is to say, they transport the fluids necessary for growth around the tree. Above the ground the vascular system is present in stems, shoots and leaves, and below the ground it is present in the roots. Water and nutrients are transported up from the roots through the xylem and photosynthates (the products of photosynthesis) are transported down from the leaves and other storage organs in the tree via the phloem, to provide the energy and nutrients for the roots to grow and function. Do roots grow all year round? Find out here: when do roots grow?.

Points 4 & 5 show that roots are very much an active participant in tree growth and not simply a set of supply pipes. They produce both cytokinin and auxin (read more in the post about plant growth regulators), they also produce a huge variety of substances known as exudates which both sense and control the rhizosphere (the environment in which the roots exist). Researchers believe that roots use exudates to “regulate the soil microbial community in their immediate vicinity, withstand herbivory, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species”^ref. Read more about exudates and how they are produced in root exudates.

Point 6 reflects the fact that roots are used to store food, in the same way that the trunk and branches do this aboveground (throughout the ‘woody skeleton’ (Ennos)). I was going to tell you that a lignotuber is an example of this and show you a lovely picture of my eucalyptus, but then I read “contrary to common assumptions…the lignotuber in young eucalypt trees did not appear to be a specialized starch storage organ. Rather, the lignotuber resembled an extension of the stem because its starch concentrations and temporal fluctuations mirrored that of the stem.”^ref How roots store food and how much of a contribution to the plant’s overall storage capacity they make is debated. More on that in Root Food Storage (or, can I root prune before bud break?)

Finally as per point 7, the roots are responsible for physically holding the tree steady and stable against wind and gravity. They do this in many ingenious ways by adopting different root architectures – combining vertical taproots, lateral roots & sinker roots, creating ‘buttress’ roots, sending roots far from the trunk when needed and managing new root development in ways which stabilise the tree. More about this in root structure and architecture.

What all of this means from a bonsai perspective is that you need to pay just as much attention to the health and care of the roots of your tree as you do to the above-ground parts. Never mind developing a strong nebari for aesthetic purposes, you need to ensure that even though the roots of your bonsai trees are squashed into teeny-tiny pots, they are still able to perform the vital functions outlined above. Neglecting the roots will negatively affect the overall health of your tree.

Practically speaking, this is why you should aim to develop a well-ramified fine root ball, to provide the tree with lots of root surface area for nutrient & water uptake – taking into consideration the amount of biomass above-ground as this will determine how much root mass is needed.

The growing medium plays a huge role as well – this is your tree’s rhizosphere. It should provide the water, nutrients and micro-organisms the tree needs as well as (some) oxygen for root cell respiration, and ideally should not be disrupted so much so that exudates and microbes (fungi or bacteria) are lost. The risk associated with bare-rooting a tree (or excessive repotting) is that it destroys the rhizosphere all at once, leaving the tree vulnerable to pathogens and forcing it to regenerate exudates it has already created (which can use up to 40% of its stored carbon).

Your tree’s roots need to have a regular supply of nutrients, so they require fertiliser of some kind. Even if good compost is added during potting, the small size of bonsai pots will mean the nutrients won’t stay in there for very long. Trees will need added fertiliser – either home-made (for example, regular doses of diluted compost leachate), or purchased. And obviously – watering is critical. Given the role of symbiotic partners (such as fungi & bacteria), you can also add these to the soil – if your tree senses their presence and wants them to stick around – it will probably produce exudates to achieve this.

Will water drops on my trees burn the leaves?

This is one you sometimes hear in gardening circles – that you might burn the leaves of your plants if you water them in the heat of the day – because the drops of water act as a magnifying glass, focusing the light onto the plant and theoretically burning it. I have never observed water-droplet-shaped burns on a leaf which one would expect from this kind of behaviour. But what does the science say?

This article^ref references another one behind a paywall which says that although droplets can increase light 20x at their focal points, in most of the species tested a layer of leaf trichomes hold droplets above the leaf surface, and beyond the focal point.

In another study^ref “sunlit water drops on horizontal leaves without waxy hairs cannot cause sunburn regardless of solar elevation and drop shape.” – this is because “the focal region of water drops falls far below the leaf at higher solar elevations and can fall on to the leaf only at lower solar elevations, when the intensity of light from the setting sun is generally too small to cause sunburn.”

Unfortunately both papers conflict somewhat as the first says trichomes specifically hold water droplets too far away from a leaf to enable sunburn, and the second paper says only horizontal ‘hairy leaves’ can get sunburn (they found this could happen for floating fern but this plant has quite specific trichomes/hairs). In reality most bonsai trees do not have horizontal leaves, instead leaves are at a multitude of angles, and the water droplets if stuck to a leaf would probably be angled away from the midday sun. But do send me a picture if you ever see a bonsai tree burned by a water droplet!

Stomata

Stomata are “microscopic pores which mediate the uptake of CO₂ and loss of water from terrestrial plant leaves”^ref The pores exist in the cuticle of the leaf (refer back to Leaf Structure to learn about the cuticle). You can see a scanning electron microscope image of stomata below:

The stomata are the dark holes in the pictures, and each one is controlled by two guard cells. The guard cells bend or straighten to enlarge or close the hole, this controls the amount of air which can enter, and the amount of water vapour which can get out. The world of stomata is illustrated in beautiful detail on the Plant Stomata blog, and what you notice is just how symmetrical and perfect looking stomata are, even though they are only 20-70μm in size. In fact the creation of guard cells is choreographed by a gene known as MUTE – it triggers one round of cell division, then acts to stop any further division, resulting in one stomata with two guard cells^ref.

Stomata are the interface between the inside of the leaf and the outside world. They are “typically fully open under conditions favouring photosynthesis, but close when water supply is limited.”^ref They operate a control system which responds to several factors including CO₂ and water levels – lower CO₂ levels within the leaf space will open the stomata as will higher water levels and/or humidity. In most plants stomata close at night, since CO₂ is not being used by photosynthesis, but some also operate on a circadian rhythm – opening before dawn or closing for a period at midday (Vogel).

Stomata control the most fundamental life-giving processes of plants, and as such are an ancient structure, found on plant fossils from 400 million years ago^ref, basically from when plants first grew on land. As a result, stomata patterns can be used for paleontology, and for genus (and sometimes species) identification.

Stomata are distributed on bottom of leaves, and sometimes on the top as well. The guard cells have different shapes, including crescent, rectangular, dome and triangular^ref, and in conifers they are often sunk into the leaf, surrounded by structures and/or contain wax plugs. They are arranged in different patterns as part of the overall epidermal structure, so appear in rows in certain species, and in between the pavement cells in different patterns in others.

This photographer (http://www.foto-vision.at/) produces amazing microscope images of leaf and stem cross-sections. Below is a pinus mugo needle – look closely at the cuticle and you can see dark spaces where the stomata are, surrounded by the guard cells stained in bright orange.

Guard cells work by inflating with water – since they are pinned at each end, and stiff (in conifers the guard cells often have lignin in them) – when water enters the cells they bend outwards. To inflate, they transport positively charged potassium ions inward – this attracts negatively charged ions (like chloride) and water then is attracted as well to dissipate the concentration of ions back to baseline levels. Pressures generated by guard cells are surprisingly high – from 2-40 atmospheres,or 16-320x the normal blood pressure generated by humans (Vogel).

So aside from providing enlightenment, how does knowing about stomata aid your bonsai practice? Well to start with, more stomata provide more photosynthesising capability and hence more growth potential (assuming water availability). The number of stomata created on a leaf is not just genetic, but is impacted by the environment -“in a number of species both light intensity and CO₂ concentrations have been shown to influence the frequency at which stomata develop on leaves.”^ref So putting your trees out in the sunlight will increase the number of stomata – this is determined by the mature leaves being in the sunlight – they use ‘long-distance signalling’ to developing leaves to produce more stomata^ref. Researchers hypothesise this signalling is probably mediated through plant hormones, but it’s not currently known exactly how.

One bonsai practice which relates to stomata is the use of anti-transpirants. This is sometimes used after collecting a yamadori. It’s promoted to ‘protect leaves’ from various environmental challenges (heat, dryness, wind) and to ‘reduce excessive transpiration’. The product is “a film-forming complex of polyethylenes and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate”^ref – so basically you are spraying plastic onto the leaves and blocking the stomata.

My guess on this product is that most people are not spraying the bottoms of the leaves which is where the majority of stomata are located. This will indeed reduce transpiration (from the top of the leaf) but not prevent photosynthesis or gas exchange, because really most of the stomata are unaffected. I don’t really like the idea of spraying plastic on my trees though, and don’t think it should be necessary – if a plant is transpiring ‘excessively’ it needs more water, or it needs to be removed from the environment causing the transpiration (out of the wind or direct sun). Creating more humidity should have a similar effect (for example by covering with a plastic bag).

One situation where it may be justified might be when collecting yamadori, when more root has been removed than foliage, and the roots simply can’t keep up with the transpiration rate. Reducing the transpiration for a period of time would allow the roots to grow whilst keeping the foliage (otherwise in bonsai you would have to remove the foliage to match the root capability). But again, a plastic bag might work just as well, without the need for spray.

Watering bonsai trees

They say that a lack of watering is the number one reason that newbies kill their bonsai trees. It is quite a surprise when you first learn about the hobby to find out that you need to water your trees *every day* and sometimes multiple times a day! It suddenly feels like more of a serious commitment than you might have been expecting. Taking a more zenlike attitude and instead learning to enjoy the time with your trees when they are being watered is just one of the delightful things you discover as you become more obsessed with bonsai.

As you’ve read elsewhere on this site, water is essential for bonsai trees. Water is essential for plants in general, including trees. It’s a key ingredient in the process of photosynthesis, along with CO₂ and sunlight, it’s a component of plant cells’ protoplasm, it’s essential for the structural support of leaves and stems (water creates ‘turgor’ ie. the water pressure which helps plant cells keep their shape), and it transports nutrients and photosynthates in the xylem and phloem sap. Water is estimated to comprise over 50% of the weight of woody plants.^ref

Surprisingly, the majority of water taken up by a tree (90% or more) is actually lost through transpiration (which means evaporation from the leaves)^ref. This is partly a by-product of having open stomata on leaves to enable the entry of CO₂, but also performs a useful function for the tree, pulling water and nutrients up from the roots by hydrostatic pressure – as the evaporating water causes a pressure differential in the xylem which pulls more water up.

What this all means is that trees need a LOT of water. They also store water for times when water is low – in this study^ref they found that Cryptomeria japonica can store 91.4 ml of water per kg of mass, distributed among leaves, sapwood and elastic tissue. For the first 2 hours of transpiration when photosynthesis started in the morning, they found that the water transpired was supplied exclusively from the tree’s leaves – it wasn’t until later in the day when stored water was low that the tree started to take up water from its roots.

OK so bonsai trees are small, they will need less than a full-sized tree of the same species, but sufficient water is necessary not just for photosynthesis but to maintain turgor in the cells, to allow the stomata to open and close, to resupply the water lost through transpiration, to bring nutrients up to its cells and sugars away from leaves, to build new cells and to avoid embolisms.

Trees in nature will spread their roots out to access water sources deep in the ground, but your bonsai doesn’t have that option. Trees in pots – such as bonsai – depend on their humans for water.

Furthermore, the water requirement of your tree (and thus how much watering is needed) will depend on several factors. In general, a tree will need more water if:

It has a lot of foliage, since the level of foliage determines the level of photosynthesis *and* the level of transpiration, both of which require more water (but the latter being the largest driver)
It gets a lot of sun, since sun exposure drives increased photosynthesis and transpiration (assuming foliage is present)
The weather is hot, dry or windy – all of these increase transpiration
Its growing medium is very open, free-draining or lacking moisture retaining components (such as bark). A more open, draining medium will lose water more quickly.
Its pot is very shallow, as this means the water quickly drains out.
It’s going through a growth spurt – making fruit, flowers or seed, or pushing sap up to push out embolisms
It’s in a low-CO₂ environment – conversely if you have your bonsai tree indoors where there are lots of people, it may benefit from the increased CO₂ by reducing its water requirements^ref

When and how should bonsai trees be watered? The unscientific answer is – whenever their owner is most likely to be available and remember to do it! Convenience is important, since missing a watering could damage the trees.

But from a scientific point of view…the latest time when watering is needed is when the tree is approaching the point of running out of water. Obviously you don’t want it to actually run out for the reasons explained above. Bonsai lore is actually well-founded in this case – look at the growing medium and check how dry it is, this gives you a good indication of whether the tree needs watering.

Trees don’t need a lot of water at night, because many/most of them close their stomata which reduces transpiration – except when they are getting ready for sunrise – this article says they open their stomata up during the night in order to get water up into the leaves to be able to photosynthesis immediately that the sun comes up: they “can calculate the time of sunrise in advance”^ref This is a one-time occurrence prior to sunrise though, and a lot less than the continuous transpiration that happens during the day. According to another article trees actually do the majority of their growing overnight (that is, creating new cells), due to the increased water availability and humidity during this time (due to the lack of transpiration)^ref These points have two implications for bonsai enthusiasts – 1. if you want your tree to grow, make sure it has enough water at night but 2. it’s not going to be at its highest water usage rate overnight, so this is likely not the time when it requires watering.

At night, there is also a water gain from dew, depending on location. This article shows how much net water loss happens overnight in different geographies^ref – “in parts of the tropics and at high latitudes” dew is actually greater than nocturnal evaporation. But on average there is 8% net water loss on land overnight.

The point at which a bonsai tree is going to start running out of water will depend on all the criteria above – foliage mass, pot size, growing medium, dryness, heat and its stage of growth. In most cases this will happen at some point during the day, after the tree has been transpiring. Depending on these factors, it may require a top-up again during the day. So maybe mid-morning to noon is a good time, with a possible follow-up water later in the day if it’s excessively hot or dry.

Most bonsai enthusiasts dream of the perfect automatic watering system. Unfortunately this is quite hard to find, for a number of reasons.

Firstly, the amount of water needed for each tree varies based on pot size/growing medium/transpiration rate. The only way to achieve this is to have individually controlled watering devices for each tree. Secondly, you ideally want to avoid wasting water by watering outside the pot or when it’s not needed – this again requires individual control for each tree, plus a spray pattern which covers just the pot area and nothing else.

The final issue is that there is risk associated with relying on an automated system. This summer when I went on holidays I set up timed sprinklers and grouped my trees together for a twice daily watering. This worked great – until one of the hose connectors popped off the tap. I had quite a few losses but on reflection probably could have avoided these by setting up two independent systems. An enthusiast from Twickenham Bonsai Club which I attend has used mini soaker hose and a garden irrigation system for his holidays which he says has worked well – but it doesn’t look good enough for continual use due to soaker hose being coiled on top of every pot.

The compromise most bonsai nuts end up with is hand-watering the majority of the time and a sprinkler or similar system while they are away.

Can you use water sources other than the tap? Find out in this post.

The water system of a tree

One of the first topics you come across when starting to study trees is the question of how they manage to lift water all the way to the leaves at the top of the canopy.

Different organs play their part in this system, starting with the roots where water is absorbed into the xylem. Xylem is a network of interconnected cells, which die quickly after birth, so that the cell contents is eliminated leaving a large space for water to enter. New xylem is constantly being created in the roots, trunk, branches and leaves, and this is all connected so that water can pass from one to the other.

But what causes it to rise up towards the leaves? The phenomenon is well described in pretty much any tree biology book you care to pick up (see references page). The answer (as is beautifully described in Ennos’s book ‘Trees’) is that it is pulled from above.

The force which pulls up the water actually starts at the leaves. Cells in leaves need gases to photosynthesise and respire (carbon dioxide and oxygen), and the waxy epidermis (outer layer) is impermeable to gas. So, leaves have small holes called stomata which are pores in the epidermis allowing gas to enter the leaf interior. These holes also allow water vapour to escape from the leaf, and as this water vapour evaporates from the leaf it pulls up the water underneath it by hydrostatic force. Water is strongly attracted to its own molecules (a force known as cohesion), and when they move upwards by evaporation it creates tension pulling more water up. This is known as the ‘cohesion-tension’ theory (Smith et al) and the process is known as transpiration. This is why trees need far more water than their size would suggest – the majority is evaporated from the leaves during transpiration.

As most bonsai enthusiasts know, when you cut a branch, water does not spurt out. So it’s obviously not being pumped from the roots. But you can make water spurt out, if you put a cut branch in a pressure vessel and apply pressure which is equal to the tension that the water was under. Experimentally this has shows stretching forces of over 20 atmospheres (294 p.s.i) (Ennos), evidence which has supported the cohesion-tension theory. There are those who disagree with this as the exclusive mechanism for water movement against gravity – one paper argues that there is an “interplay of several forces including cohesion, tension, capillarity, cell osmotic pressure gradients, xylem-phloem re-circulation, and hydrogel-bound gradients of the chemical activity of water”.^ref

Whatever the nuances of the forces involved, the transpiration flow is essential for other processes within the tree – it helps maintain cell turgor (stiffness), maintains solute levels in cells which are needed for metabolism, draws nutrients, plant growth regulators and metabolites up through the tree from the roots via the xylem sap, cools leaves via evaporative cooling, and supplies water to the top of the phloem for the transportation of photosynthates (Smith et al).