Tag Archives: Leaves

Bonsai Tree Growth Stages

Most bonsai trees progress through stages of development, each with a different objective. In general the progression is thicken trunk -> achieve branch & root structure -> achieve branch, foliage & root ramification -> reduce leaf size -> evolve as branches grow/fall off. The faster we can move through the first few development stages, the faster we will have beautiful, well-proportioned bonsai – harnessing the tree’s natural growth is a way to speed this up. We also want to avoid doing things which slow down a tree’s growth during these phases, as this will mean it takes longer to get the tree we want. Read about how trees grow before starting at #1 below. Also consider what do old trees look like?

1. Trunk

Some bonsai enthusiasts collect mature trees for bonsai specifically so they can start with a thick trunk, following a collection process which minimises damage to the tree. The alternative is growing your tree’s trunk. Once a tree has its roots and foliage reduced in size in a bonsai pot, it won’t generate the energy needed to make significant sapwood additions and its girth will only increase by small increments every year. So you really need to be happy with the trunk size first before you stick it in a tiny pot. But – how big should a bonsai tree’s trunk be?

2A. Branch Structure & Overall Shape

Arranging the branches is what gives you the canopy and overall foliage shape that you’re after and the first step in this process is growing (or developing) the branches you want in the positions they are needed. Growing a branch starts with a new bud, which, unless it’s a flower bud, becomes an extending shoot and eventually a new branch. So firstly you need to work out where new buds will grow on your tree and then deal with the extending shoots as needed to get the required internode length.

You may need to remove some buds and shoots if they don’t help achieve the shape you are looking for – this should be done as soon as possible to avoid wasting the tree’s finite energy reserves. You have a trade-off to make here because leaving more foliage on the tree will provide more energy overall which contributes to its health and ability to recover from interference. However, growing areas of the tree which won’t be part of the future design is a waste of energy. You don’t want to remove so much of the tree’s foliage that it struggles to stay alive or develop the areas that you do want to grow out.

When you are creating your branch structure, often you will need to reposition branches – this is done with a wide range of different tools and techniques. A more advanced technique for adding new branch structure is grafting.

Sometimes the trunk itself or larger branches need a rework, to make them more interesting or to make them look more like old trees – for example adding deadwood or hollowing out the trunk. Usually this is achieved through carving.

2B. Creating a Strong Root System

The trunk thickening and branch structure phases both work best when the tree has lots of energy and so letting it grow in the ground or in a decent sized pot during these phases will get you there quickest. This also allows the roots to keep growing, but you want to understand about the role of roots, and root structure & architecture even if you still have your bonsai in a training pot. Particularly in this case, knowing about how to foster the the rhizosphere will help your tree stay vigorous. To maximise the roots’ exposure to nutrients and water you want to encourage Ramification of Roots (lateral root development).

Eventually it’s time to move the tree into a bonsai pot. This requires cutting back the roots, but as long as the roots are balanced with the foliage in terms of biomass, the tree should be OK. Root growth is usually prioritised outside of times of stem/foliage growth, and above 6-9 degrees C. So repotting might be best conducted at times that meet this criteria. Your growing substrate/medium is an important consideration.

3. Ramifying Branches & Foliage

Ramification is when branches subdivide and branch, giving the impression of age and a full canopy – and a well-ramified tree is a bonsai enthusiast’s goal. There are some techniques for increasing the ramification of branches and foliage. But not as many as there are for root ramification.

This stage also involves ongoing branch selection and reshaping (see 2A above). Another consideration is whether to keep or remove flower buds.

4. Reducing Leaf Size

An end stage in the journey to bonsai perfection is leaf size reduction. In nature, leaf sizes reduce relative to the biomass of the tree as it ages but since bonsai are small this effect doesn’t translate since the biomass never gets large enough. The tried and tested method for reducing deciduous tree leaf size is actually to practice one of the various methods of defoliation. A couple of others are covered here in reducing leaf size.

When to conduct these various activities depends on when the tree can best recover from them – which is a function of the Tree Phenology (or Seasonal Cycles).

5. Evolving Branches

Trees are not static organisms – they obviously continue to grow which is what we harness in the above steps. Part of this is that eventually branches may become too large for the design, or they may fall off (Peter Warren notes that Mulberry are known for this). As bonsai artists we want to have this in mind so that branches are being developed which can take their place in the future. This is an ongoing version of step 2A.

Foliar Feeding

Some products advise spraying them on the leaves of your trees – a process known as foliar feeding. At first glance this makes no sense, as plants synthesise everything they need from nutrients obtained from the soil and air and these nutrients come up with water through the roots and xylem. And leaves haven’t evolved for nutrient uptake, they have evolved for photosynthesis.

But could this actually work? Well, in order for the nutrients in foliar feed to be useful to plant cells, they would need to both penetrate the leaf and enter the cell.

Can substances on a leaf surface enter the leaf itself? For the most part they can’t as leaves are covered by a protective waxy layer known as the cuticle (described in the post Leaf Structure). One of the main roles of the cuticle is to stop pathogens and other environmental stressors entering the leaf.

But as so often happens with systems of mind-bending complexity like plants – it’s not that simple. For one thing, we know leaves have stomata which allow gas to enter the leaf. But it also turns out that the rest of the cuticle isn’t completely impregnable. The cuticle has tiny pores in it at the base of trichomes (hairy projections) and glands – these range from 0.45 to 1.18nm in diameterref. One study did indeed find that dissolved nutrients can enter the cell through these pores in the cuticle: “penetration of ionic compounds can be fairly rapid, and ions with molecular weights of up to 800 g mol(-1) can penetrate cuticles that possess aqueous pores.” The key term here is ‘aqueous’ – the pores need to be wet in order for nutrients to enter through them. For example carnivorous plants use this process to bring nutrients in from their traps via the pores in glandsref.

A great article summarising the physics of nutrients entering a leaf is here – they conclude that it’s easier for positively charged ions (calcium, magnesium, potassium, ammonium-form nitrogen) to enter via the cuticle pores whilst it’s not as easy for negatively charged ions (phosphorous, sulfur, nitrate-form nitrogen). Similarly smaller molecules or those with a smaller positive charge are easier to translocate around the plant – including ammonium, potassium, and urea. Larger molecules will stay close to their point of entry, including calcium, iron, manganese , zinc and copper. Another study states that younger leaves are less able to transport nutrients out and so applying foliar feed to developing leaves may result in the nutrients staying within the leaf (which perhaps is an effect one might want to achieve?)ref

So it seems that some amount of foliar feed may be able to enter via the cuticle’s aqueous pores, and a subset of this may be able to move around the plant.

But what about the stomata? Previous studies have said that “the combination of cuticular hydrophobicity, water surface tension and stomatal geometry should prevent water droplets from infiltrating the stomata”.ref (ie. water can’t get through stomata) but apparently dissolved ions can in some circumstances, because the ions change the surface tension properties of the liquid. This study ‘confirmed the stomatal uptake of aqueous solutions’ref; but also said this depended on whether the aqueous solution was chaotropic (reducing water tension) or kosmotropic (increasing water tension). So it’s easier for the ions on the left to enter via the stomata, and harder for those on the right.

from: https://water.lsbu.ac.uk/water/kosmotropes_chaotropes.html

But once in the leaf, can nutrients be used by plant cells? It seems so, in some cases, but the evidence is extremely varied and there are many different variables to untangle.

A research study was conducted by ‘Christmas Tree Specialist’ Chad Landgren for the Oregon Department of Agriculture in 2009 comparing foliar feeding to other forms of nutrient applicationref. They tested a range of approaches on blue spruce, Atlas cedar and four varieties of fir (abies), in pots and in the ground, using application methods including “helicopters, mist blowers and various backpack sprayers”. Their conclusions were: “Each conifer species and site are potentially different with regard to nutrient needs and response. Blue spruce appears rather “immune” to foliar application… Nordmann fir appeared to pick-up some of the foliar fertilizer… on other sites, no treatment (soil or foliar) appeared to move the foliar nutrient content levels.”

In another paperref the author concludes that “foliar application of particular nutrients can be useful in crop production situations where soil conditions limit nutrient availability.” and that fruit can benefit from direct sprays, but also that “applying fertilizers to leaves (or the soil) without regard to actual mineral needs wastes time and money, can injure plant roots and soil organisms, and contributes to the increasing problem of environmental pollution.”

And then of course it’s not just the leaves themselves. We now know that there is a phyllosphere – a symbiotic community of microbes in and on the leaves which perform a whole range of functions for their hosts, one of which includes producing cytokinins
that can be bioactive within the plant. If foliar feeding increases these bacteria, there may be effects throughout the plant not just on the leaf.ref

The message from all of these seems to be that foliar feeding may work for leaves or fruit with specific mineral deficiencies which need to be corrected in-situ, if the nutrient in question can get through the cuticle or stomata. Or for plants which have environmental reasons for not being able to access nutrients through their roots (like pH?). But there needs to be a specific requirement in a specific location on the tree for it to make a difference – and it will be dependent on the species, environment, nutrient etc. In most cases I would say it would be better to provide the roots with the requisite nutrients instead.

Phloem

The word ‘phloem’ comes from the word for bark in ancient Greek. It is a parallel system to the xylem which transports water and nutrients up from the roots, but instead transports the products of photosynthesis (‘photosynthates’) from the leaves to the rest of the tree. A big callout to The International Association of Wood Anatomists for the images in this post, contained in this open-access publication.

One of the main photosynthates produced by trees’ leaves is sucrose (maple syrup anyone?), but others found in phloem include fructose and glucose, sugar alcohols and the raffinose family of oligosaccharides (RFOs). A sugar alcohol known as ‘D-pinitol’ has been found in substantial amounts in gymnospermsref and is believed to be the main carbon transport molecule for Scots pine. In addition to sugars, the phloem system is used for signalling and defence throughout the tree (as is the xylem), so plant growth regulators (including auxin, cytokinin and salicylic acid), proteins, minerals and RNA travel in the phloem sap as well. If a foliar insecticide/herbicide/fungicide has been applied and is able to penetrate the pores or stomata (see foliar feeding), and is able to get into the phloem vs staying inside adjacent cells, it will translocate throughout the plant.ref As a result I would not be eating non-organic maple syrup (previously paraformaldehyde was used to reduce microbial attacks on maple trees for syrup product, but this was banned by 1989).ref

There still seems to be quite a bit that’s unknown about how phloem actually works – an article published in 2014 said “Because of the difficulties in measuring phloem function, particularly in trees, we lack a basic natural history and phenomenology of tree phloem”ref and another published as recently as 2021 said “phloem loading strategies in gymnosperm trees have been only tested in three species: P. sylvestris , Pinus mugo and Ginkgo biloba.”ref

But the basic principle is that sugars are created by the process of photosynthesis, ‘loaded’ into the phloem cells (with assistance from adjacent cells) and transported to places in the plant where they are needed, then ‘unloaded’ (but even the mechanism for transportation of sugars in phloem is debated – a famous theory involving ‘osmotically generated pressure gradients’ has dominated but many recent articles point out the lack of data to support it.ref) According to one account, sugars are loaded from leaves into phloem companion cells by active transport (a process which consumes energy) and then diffuse into the sieve tube elements through the plasmodesmata (cytoplasm which is shared between cells via small pores between them). Water then moves by osmosis into these cells (creating the phloem sap), and sugars translocate (move) when sinks (areas of the plant consuming energy) remove sugar and reduce its concentration in the phloem sap.ref

Phloem is also believed to translocate (move from one place in the plant to another) sugars even when photosynthesis is not taking place – eg. in winter in deciduous species.ref In this case the sugars are coming from storage tissues in the rays and roots.

The cells which make up the phloem system in gymnosperms are different to those in angiosperms (similarly to the difference in xylem), but the basic structure for both is that tubular cells, known as sieve cells (gymnosperms) or sieve tube elements (angiosperms), are connected together via pores in their end walls, and the phloem sap ‘flows’ through these sieve cells/tubes.ref

Below is an image of pine sieve cells. The side and end walls are structurally similar, unlike the sieve tubes of angiosperms. The phloem sap flows from cell to cell downwards, through the pores. Many studies reference the fact that sieve cells & tubes contain material which would appear to create a barrier to flow, which calls into question the abovementioned ‘osmotically generated pressure gradients6’ theory.ref

https://search.library.wisc.edu/digital/AVCQSJHVTUYFUP9D

If you’ve read the post about the cambium, you’ll know that there is a constant process of creating new xylem and phloem cells, and in the case of phloem, the most recent does the conducting.ref The conducting phloem usually lasts for one season, but can remain ‘functional’ for one-two years (ie. the cell is still alive, even if it’s not conducting phloem any more). Like xylem, phloem rings are created – see the image to the right of pinus strobus – all of the dark cells are the annual phloem sieve cells which are now non-conducting. The conducting cells are in the lower purple region.

https://scholarlypublications.universiteitleiden.nl/access/item%3A2951200/view

A key difference between xylem and phloem is that phloem cells are living cells. This means that phloem sap must pass through living cells and their membranes in order to flow and this articleref suggests that this mechanism provides a high degree of control for the plant in managing what gets into and out of the phloem system. The phloem passes through holes in the sieve cells known as sieve plates (see pics below both of ficus species, the left hand side shows a transverse section and the right hand side a lateral section).)

https://scholarlypublications.universiteitleiden.nl/access/item%3A2951200/view

In order to create the space for the phloem sap, sieve cells and tubes are missing quite a bit of the normal cell machinery, including a nucleus, vacuole and ribosomes – so they can’t control their metabolism or make proteins. Although they still have some specific proteins (P-proteins – apparently previously known as ‘slime’!ref), mitochondria, endoplasmic reticulum, and sieve element plastids.ref Both types of sieve cells have helper cells alongside which metabolise on their behalf – companion cells in angiosperms and Strasburger cells in gymnosperms.

Since phloem is full of delicious sugar-rich fluid, it can be a magnet for insects, which in turn introduce microbial pathogens including bacteria and viruses.ref Plants produce metabolites to defend themselves against these pathogens, and also induce sieve plate occlusion – basically blocking up the sieve cell or tube where the pathogen is located to avoid it spreading.ref

Both the active phloem and the old phloem which no longer transports photosynthates are together known as the inner bark. Outside these phloem layers is the ritidome or outer bark. You can read more about bark here.

For bonsai there’s really not a lot you need to worry about with respect to phloem, unless you are wiring super tight and cutting off the phloem (but by then your wire will be well embedded in the outer bark).

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 articleref 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 studyref “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:

https://www.researchgate.net/figure/Scanning-electron-microscopy-of-stomata-in-leaves-of-Paphiopedilum-and-Cypripedium-a-P_fig1_45651528

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 cellsref.

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 agoref, 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 triangularref, 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 stomataref. 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.

Leaf Structure

It’s useful for bonsai enthusiasts to understand how a leaf is structured, as this answers some questions about how water/air/nutrients/sugars get in and out of the leaf and therefore also the rest of the tree. Of course there are many different leaf types belonging to different trees in different environments, so there will be many differences between them. What’s important to know is the main structures which are common to most leaves. Below is a diagram of a leaf cross-section:

http://Scaling Functional Traits from Leaves to Canopies – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/The-internal-structure-and-biochemistry-of-leaves-within-a-canopy-control-the-optical_fig2_342371888

The outside of the leaf is covered by the ‘cuticle’, which is the first line of cells between the leaf and the outside environment. This is not a passive line of cells, but instead “waxes, fatty acids, and aromatic components build chemically and structurally diverse layers with different functionality.”ref Not only that, the cuticle changes as the leaf develops – building up its layers and constituents over time until the leaf has fully extended.ref The above diagram only shows a cuticle on one side of the leaf – but apparently a cuticle “covers the outer epidermal surface of most above-ground tissues, such as leaves, fruit, and floral organs.”ref

The main function of the cuticle is as a barrier. It protects the tissue beneath from mechanical damage by the elements, or from insects, and acts as the primary defence against pathogens.ref It is composed of “the polyester cutin, containing oxygenated and unsubstituted fatty acids, glycerol, and phenolic acids, that is impregnated by waxes of very-long chain fatty acids (VLCFAs) and their derivatives.”ref In another study the top layer of the cuticle was found to contain Kaempferol. This is a flavonol which is known to be an antifungal, antibacterial and antioxodant (see this article about HB-101).

Since the waxy cuticle is impermeable to water and CO2, leaves have specially controlled holes distributed across itref – these are known as stomata (described below).

Underneath the cuticle is the epidermis – the upper epidermis at the top of the leaf and the lower epidermis at the bottom. Humans have an epidermis too – it’s the top layer of skin. Everything you could want to know about the plant epidermis is covered in an excellent article in ‘The Plant Cell’ journal from January 2022. The authors say “the epidermis plays many important roles including regulating the exchange of gases, water, and nutrients with the surroundings, responding to external threats such as pathogens, herbivores and abiotic stresses, resisting mechanical strain, detoxifying xenobiotics, and contributing to mechanical strength while allowing the flat and flexible shape necessary for maximum light capture.”

There are three main types of cells in the epidermis, and these develop into their final form starting from the leaf tip and gradually moving back towards the petiole until all of the cells are formed.

The first cell types are ‘pavement’ cells – so named because they interlock with one another and look like paving (sometimes it’s crazy paving – other times it’s very neat). Among the paving are stomatal guard cells – these “form microscopic valves in the leaf surface” so that gas can get in and out for photosynthesis. You may have heard of stomata – this is the name for the hole that is created and controlled by the stomatal guard cells. Basically plants breathe through their stomata – air comes in, oxygen from photosynthesis and CO2 from respiration come out, and water vapour comes in and out as well. It’s actually reasonably easy to ‘see’ the stomata even with the naked eye – depending on the species of tree and the shape of the leaf. If you put adhesive tape on a leaf, and pull it off, you pull off some of the cuticle which shows the outlines of the stomata. There is quite a lot to say about these guys – see this post: Stomata.

Aside from the stomatal guard cells and the pavement cells, the epidermis can also have ‘trichomes’ which are hair-like protrusions from the surface. These can appear in lots of different forms, and can be ‘glandular’ (or not). If a trichome is glandular, it can “biosynthesize, store and secrete a large diversity of specialized metabolites including terpenoids, alkaloids, polysaccharides, and polyphenols” – such as the terpenoids that conifers exude to defend against insectsref. This image from the journal nature shows the trichomes on white spruce:

https://www.nature.com/articles/s41598-020-69373-5

Anyway moving on past the cuticle and the epidermis, you come to the mesophyll. The mesophyll is “the parenchyma between the epidermal layers of a foliage leaf”ref – OK great Merriam-Webster dictionary, now what is ‘parenchyma’? Parenchyma is “the essential and distinctive tissue of an organ”ref which in the case of leaves means the cells which photosynthesise and store the products of photosynthesis. So the mesophyll is the engine room of the leaf.

Referring to the image at the top of this post, you will see there are two types of cell in the mesophyll – palisade cells and spongy mesophyll cells. The palisade cells face the light, and are located on the top of the leafref. They are columnar cells (with the end of the column facing the light) and they are supposed to contain the majority of chloroplasts, which are the organelles responsible for photosynthesis. The spongy mesophyll cells are arranged in a lattice, with air gaps (like a sponge) to allow for the absorption of CO2 – they also contain chloroplasts, but apparently not as many. Good luck trying to find a research paper which actually counts them! The best I could find was this dataref looking at five species living in different light conditions, and the number of spongy mesophyll cells ranged from 40-50% of the total chloroplast count. Which isn’t exactly a minority.

The shape of these cells has evolved to improve photosynthesis. The palisade cells which are long and columnar, “act as light conduits”ref distributing collimated (parallel) light to chloroplasts within the leaf. Internal light scattering also takes place, allowing photons of light to reach the chloroplasts in the spongy mesophyll cells. When a leaf has a different (usually lighter) colour on one side, this can keep light inside the leaf by reflection.

The final part of the leaf structure is the vascular bundle – this contains the water-transporting xylem and the sugar transporting phloem. See xylem & phloem.

Not to forget, leaves have their own microbiome, just like the roots. This is called the phyllosphere and contains many bacterial and fungal species in symbiotic relationships with the host plant.

You can dive even deeper into the structure of leaves by going into the plant cells themselves, looking at mitochondria, chloroplasts, vacuoles and the thousands of chemical reactions going on, but that’s a post for another day.

Photosynthesis

Another epic topic which has occupied scientists for the best part of 400 years, the equation for photosynthesis itself was not understood until the 1930s.ref

Photosynthesis is the process of turning energy from sunlight into chemical energy, a process which famously is performed by plants, specifically by plant cells containing ‘chloroplasts’. Chloroplasts contain a substance called chlorophyll. Chlorophyll is often described as a ‘pigment’ – but this makes it sound like its only role is to colour things green! Which of course it isn’t – being green is just a result of its real function which is to absorb sunlight of a certain spectrum. Chlorophyll absorbs visible light in two regions, a blue band at around 430 nanometers and a red band around 680 – according to Vogel. Other sources state the bands are 680 nm and 700 nmref. Everything else is reflected, which looks green. Chloroplasts are believed to have originally been cyanobacteria which were incorporated into plant cells to provide photosynthetic capability (Lane, 2005). To see where chloroplasts and chlorophyll are present within a leaf, check out this post on leaf structure.

The sequence of reactions which happen during photosynthesis are known as the Calvin-Benson-Bassham cycle, after the scientists who discovered it. The simplified equation describing photosynthesis is as follows:
6CO2 + 6H2O –light energy–> C6H12O6 + 6O2.

This suggests that photosynthesis is the exact opposite of respiration, creating glucose instead of consuming it. But actually this equation is incorrect as photosynthesis does not create glucose. The Calvin et al cycle produces a molecule called G3P which is a 3 carbon sugar which can be used to create other molecules. Anyway, that’s probably not super-relevant for bonsai! The main insight from the equation above is that light energy is needed for plant survival and growth – this is why trees do not like being inside unless they have a suitable artificial light source.

Another point to note is that plant cells, like all living cells, respire. Plant cell respiration is like animal respiration, that is, cells consume oxygen and glucose to produce energy, while emitting carbon dioxide and water. Cells use respiration to generate the energy they need for metabolism (basically, keeping the cell alive and functioning). All cells in a tree respire, including the leaves, roots and stems, 24 hours a day. This means trees offset some of their photosynthesis by respiring – in particular at night when no photosynthesis occurs.

On balance, leaves produce a lot more sugars and oxygen through photosynthesis than they use during respiration – and this provides the energy they need to maintain themselves and grow. In fact the point at which they *don’t* produce more than they use through respiration is as low as 1% of full sunlight (Vogel). Note that this low level can be reached if a leaf is entirely shaded by 2 other leaves, since the typical light transmission through a leaf is only 5%.

Another interesting fact about photosynthesis is that leaves can only use about 20% of full sunlight before the photosynthetic system saturates (Vogel). This number actually varies depending on the species and leaf type. So leaves below the top layer (likely to be ‘shade leaves’ described in this post: Leaves) can still get enough light from partial exposure, if they are slightly shaded or lit for only part of the day, to saturate and max out their photosynthetic capability. The majority (99%) of the energy absorbed is used to maintain the leaf itself, so only 1% is released for growth.

But – back to the photosynthesis equation – which is an extremely simplified version of what is actually happening. Photosynthesis requires 150 discrete steps involving a similar number of genesref. A good reference is this Nature article.

Photosynthesis takes place in two stages. First the ‘light dependent’ reactions happen. Light energizes electrons within the chlorophyll, and these electrons are harnessed to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) which are molecules used by cells for energy and as an electron source. The chlorophyll replaces its lost electrons by taking some from water – effectively they ‘burn’ or oxidise water, leaving the oxygen behind as a waste product (Al-Khalili).

After this the ‘light independent’ reactions happen. These are facilitated (sped up) by an enzyme known as RuBisCO. Using the energy sources created in the first step (ATP and NADPH), RuBisCO ‘bolts’ together the hydrogen from the water, and the carbon and oxygens atom from the carbon dioxide, along with phosphorus, to make the 3 carbon sugar glyceraldehyde-3-phosphate (C3H7O7P) (Lane, 2005). This is known as carbon fixation.

In reality there is also a third stage where the ingredients for the cycle are regenerated so that it can continue running.

The full equation (the Calvin-Benson-Bassham cycle) – is:
3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → C3H7O7P + 6 NADP+ + 9 ADP + 8 Pi   
(Pi = inorganic phosphate, C3H7O7P = glyceraldehyde-3-phosphate or G3P)

Sorry for geeking out for a minute there!

An important point is that photosynthesis doesn’t just go up with increasing sunlight, it has a set of limiting factors which were described by Blackman in 1905 in his articleref “Optima and Limiting Factors”. He said “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest”. The limiting factors he described for photosynthesis included light intensity, carbon dioxide and water availability, the amount of chlorophyll and the temperature in the chloroplast.

The amount of sun needed to max out photosynthesis from a light intensity point of view is not as high as you would imagine. Vogel says a leaf “absorbs about 1000W per square metre from an overhead sun shining through a clear sky” and photosynthesis only consumes 5% of this – 50W per square metre. Obviously this is going to vary depending on the species and whether a leaf is a sun leaf or a shade leaf.

But what does it mean for bonsai? Well – you might have spotted some other elements in the photosynthesis equation. NADPH (formula C21H29N7O17P3) contains nitrogen and phosphorus, as does ATP (formula C10H16N5O13P3). RuBisCO relies on a magnesium ion to perform its role as a catalyst. There are a bunch of other enzymes and co-factors which are required to support photosynthetic reactions as well – which explains why certain nutrients (including Nitrogen, Phosphorus, Magnesium, Potassium, Chlorine, Copper, Manganese and Zinc) are critical for trees in varying amounts – more here: Nutrients for Trees.

And if you’re concerned about whether your trees have enough light for photosynthesis, you can see exactly how much energy is arriving on your outside bonsai at this site. Find your location and download the PDF report and you’ll see the irradiation levels – which can be useful in understanding how your bonsai trees will fare. As an example, the max in my location is 328Wh/m2 in April which explains why the olive trees in London are so unhappy looking – if you look at irradiance in Greece, where they thrive, it barely ever goes under 328Wh/m2 even in winter! Trees in Greece receive nearly double the amount of irradiance than those in South West London.

One final point of interest with admittedly little relevance to bonsai – some plants (nineteen different plant families, independently of each other) evolved an improved photosynthetic process which is known as C4 photosynthesis. This concentrates CO2 nearer to the RuBisCO enzyme, reducing its error rate. Unfortunately most trees use C3 photosynthesis with its associated inefficiency – other than Euphorbia, which are apparently “exceptional in how they have circumvented every potential barrier to the rare C4 tree lifeform”ref.

Leaves

Rather naively, when setting up this website, I made a note to create a post on the topic of ‘leaves’. Several days of research later it’s clear, there can’t just be one post about leaves! Why? Well, leaves are the reason that complex life on earth exists. Their ability to photosynthesise – to turn energy from the sun into energy that living things can use – is fundamental to our planet. So – there is a lot going on inside a leaf. My starting point for this post is the wonderful book ‘The Life of a Leaf’ by Steven Vogel, and I am going to cover the purpose of leaves, respiration, transpiration, and how leaves deal with environmental challenges like heat, wind and cold. There is a lot more to know about leaves, to read this check out all the other posts tagged with ‘leaves’, or start with leaf structure.

So, what is the purpose of leaves? Simply, they are the energy generating mechanism for plants. Through the process of photosynthesis they use energy from the sun to ‘burn’ water, removing its oxygen atom and bolting the remaining hydrogen onto carbon dioxide to create sugars. These sugars are used to power the life and growth of the plant. And as a side-product, oxygen is created, for animals and humans to breathe. Photosynthesis only happens when a leaf is illuminated by the right kind of light – in nature, this is sunlight during the day. If you want to geek out with more detailed information about how photosynthesis actually works, try this post: photosynthesis

A key concept when it comes to leaves is transpiration. It turns out that up to 97% of the water used by plants is actually evaporated back into the air – a process known as transpiration. The reason for this is because plants need to get CO2 into their leaves in order to supply it to the chloroplasts (organelles which perform photosynthesis) and to make this possible they need to open holes on the leaf surface to allow CO2 molecules to enter – these holes are known as stomata. When the stomata are opened, water naturally evaporates from within the leaf – it is the negative pressure in the xylem caused by this evaporation which pulls the water (and nutrients) up from the roots. What this means for bonsai is that your trees need a lot more water than you might think based on their size – remember up to 97% of it will be evaporated out. But if you water on a sunny day, will you burn the leaves? Find out here.

Did you realise that individual trees have leaves of different shapes and sizes, depending on their position on the tree? Shade leaves are larger, thinner, darker green (containing more chlorophyll) and less lobed, with fewer stomata than sun leaves; conversely sun leaves are smaller, thicker, lighter yellowish green, more lobed, and have more stomata – each leaf is more efficient in their niche of light exposure (Thomas). In her book The Arbonaut, Meg Lowman describes a coachwood tree with each leaf having its own unique formula for success – the lower leaves are long-lived and better structured to harvest low amounts of filtered light, the upper leaves are short-lived “extremely high-powered chlorophyll factories…[which]…produce sugars that keep the entire tree alive, healthy and growing”. And between these, variations to suit the different light conditions. All of this is called ‘photomorphogenesis’, or a “developmental process in plants in which the incident light determines the growth of the plant”ref. One research team even discovered a gene which could force the growth of sun leavesref and resulted in 30% greater photosynthesis.

What this means for bonsai is unclear as it’s not obvious from the research at what scale these differences can manifest. In theory there will be a layer of leaves around the canopy of the bonsai tree which could be sun leaves, and leaves below and within it which could be shade leaves. If it is literally only the sun exposure on a bud which determines what type of leaf is grown, then theoretically these two different leaf types could appear on a bonsai. If so, then removing the top layer of leaves – as is often recommended in bonsai, to ‘let light into the tree’ may not be the best course of action. If the leaves below have grown as shade leaves they may struggle to tolerate their new position in full sun. Anyone noticed sun & shade leaves on their bonsai? Let me know at info@bonsai-science.com.

‘Leaf morphology’ – or the shapes and sizes of leaves – is another rabbit hole you can joyfully enter via Google if you so desire, but to sum things up, leaves have differing physical attributes, each lending the leaves different properties – “leaf traits may reflect the adaptation mechanisms of plants to the environment.”ref

A key measure related to leaves is the ‘specific leaf area’ – the ratio of total leaf area (ie. the total amount of leaf surface) to total leaf dry mass (the total weight of the leaves without water). SLA is used as a measure of the overall health of a plant, as it reflects the efficiency of carbon gain relative to water lossref.

What is observed is that different trees have different strategies for optimising specific leaf area – in addition to photosynthesis mentioned above. Three interrelated attributes include leaf size & shape, leaf venation (how the xylem & phloem ‘veins’ are structured) and stomatal conductance (the number and distribution of stomata on the leaf).ref The model in this article suggests several predictions related to these attributes.

One of these is that a leaf with a larger width to length ratio can support higher carbon production factors, stomatal conductance, and leaf area than a leaf with smaller width to length ratio. So long, narrow leaves are less efficient and have higher ‘stomatal resistance’ – ie. they are less efficient at transporting gas and water vapour in and out of their leaves. This also applies for thick leaves, which have higher stomatal resistance than thin leaves.

That’s not all bad though, because higher stomatal resistance is correlated with smaller xylem, which are more resistant to embolism. These tradeoffs help different trees thrive in different conditions and drive evolutionary adaptation.

The implications of this model that on average narrow leaved species will do better in more arid conditions with less available water, since they transpire more slowly and are less at risk of embolism, and as water becomes more available the broadleaved trees will do better. So conifers may do better on a mountain top, and angiosperms such as beech will do better in the valleys. Translated to bonsai medium, this suggests that a narrow leaved tree (including most conifers) will prefer a more well-drained medium (hence the advice that conifers don’t tend to like getting ‘their feet wet’). Your tropical trees on the other hand – like a Ficus – may be perfectly happy with extremely wet conditions (as long as they don’t become anoxic – ie. so wet they prevent any oxygen from entering).

Leaves have more to contend with than ‘just’ photosynthesising and transpiring – they also have to deal with extremely large temperature variations – “on windless days, ordinary leaves on ordinary trees quite commonly run around 10°C, nearly 20°F, above the air that surrounds them. Under exceptional (exceptionally bad) circumstances they can reach twice that.” So explains Vogel, who then describes in detail the different strategies used by leaves to cool down – starting with evaporation but also including reradiation (radiating heat back into the air) and convection (transferring heat to the air from leaf surfaces and edges). He suggests that smaller, more lobed leaves are better at cooling than large wide leaves – and that large wide leaves avoid horizontal positioning in order to reduce overheating. What this means for bonsai is probably just that the natural leaf shape and positioning of a particular species helps it achieve a temperature balance (as well as photosynthesis) – and also perhaps that highly ramified trees with lots of smaller leaves may have improved temperature regulation (due to improved convection).

Another environmental challenge faced by leaves is freezing temperatures through the winter. One obvious strategy for avoiding these is to be deciduous, but there are plenty of evergreen trees out there and many of them thrive in extremely cold conditions (eg. the boreal forest where snow cover lasts for months and “gymnosperms such as Abies, Larix, Pinus, and Picea dominate”ref). Of the various strategies deployed to avoid damage from freezing (four at least are described in detail by Vogel, as well as in this paperref), the strategy most relevant to bonsai is that the tree generates substances within its cells and organs – such as dissolved sugars, resins and anti-freeze proteins – which reduce the freezing temperature in cells, reduce ice crystal formation and prevent ice crystals from growing within cells. The reason this is relevant is because one of these mechanisms – the creation of anti-freeze proteins – requires a gradual acclimation to the cold. This allows the tree to turn on the genes which produce the anti-freeze proteins, and gives it enough time to accumulate them. So it is a bad idea to move a conifer from a warm/protected area directly into the freezing cold. But if that tree has been in the same place as the seasons change, it will likely have time to build its defences against freezing temperatures.

The final environmental challenge described in fascinating detail by Vogel is the wind. Leaves are adapted to deal with wind so that they can continue to photosynthesise but also not tear, or build up enough resistance to pull the tree over. They do this by bending with the wind into aerodynamic shapes which minimise drag – Vogel’s research includes a great photo illustrating how four different groups of leaves move into cone shapes as the wind increases. One key point for the bonsai enthusiast though – leaves are more resistant to the wind once they have hardened off – if they are new spring leaves they are more prone to being wind damaged.

It’s not just the leaves which contend with all these environmental challenges though, it’s all the microbes on the leaves, in its phyllosphere.

I could write pages and pages more on this fascinating aspect of tree biology but for now I am going to leave it there.

Repairing damage (or not)

When a tree is damaged or injured in some way, various responses happen, but none of these would be characterised as ‘repair’ in the same way one sees the human body repair itself. Trees create new growth to compensate for damage, and seal off damaged areas to prevent infection or further damage occurring. I like the way Wayne K. Clutterbuck put it in his article about tree wounds – “trees don’t heal, they seal”.ref

If leaves detect high wind, excess UV or frost, they furl up which protects them from damage. Similarly, they can respond to insects or other invaders by producing defensive compounds or thickening their leaves; defence is an important part of plant survival. But if eaten, ripped, scorched or frostbitten, leaves have no repair mechanism, as they do not have a meristem with active stem cells which could initiate new growth. Instead a tree will rely on other leaves, or grow new ones to replace the damaged ones. Deciduous trees simply drop their leaves every year, along with any damage they have incurred, and grow a new set in the spring.

If a stem or shoot is removed, the tree grows another one from a bud, it cannot replace the one which was removed in exactly the same place. The same principle applies to roots. As outlined in ramification of roots the act of cutting roots causes more lateral roots to grow to compensate.

The wounding of a tree’s trunk or major branches has more important consequences for the tree than just a leaf or stem.ref The tree detects that it has been injured because pressure changes within its cells, and the normal flow of hormones through its phloem and cells is interruptedref. This articleref (admittedly from 1985 but has some nice illustrations) explains what happens – first the cells nearest to the wound adjust their biochemistry to become antimicrobial, then a barrier zone is formed around the wound which prevents microorganisms from breaching the zone. The tissue around the wound is discoloured by these compounds – a good illustration is below. The tree has been damaged by drying cracks in the bark and boring insects. It has reacted by creating a sealed-off dead zone indicated by the darker wood, to repel and prevent further ingress by insects. You can also see that the cambium has generated new xylem and phloem annually which has curled over the edge of the wounded area.

https://www.nrs.fs.usda.gov/pubs/gtr/gtr_nrs97.pdf

Cut paste is a product which is sometimes advocated by bonsai enthusiasts, but there isn’t much to be found in the way of evidence for its effectiveness. Most research papers on the topic come from the 1930s or before, but there are a few – seemingly all from Korean researchers – which identify positive effects from a fungicide called thiophanate-methyl which was found to improve wound closure on Acer palmatumref. The mechanism wasn’t detailed in the study but presumably it worked by protecting the wound from fungal pathogens. I couldn’t recommend this though, partly because you risk dripping it into the soil and onto your your friendly mycorrhizal fungi but also because this substance is toxic to inhale, carcinogenic and causes birth defects.ref

Research shows that wounds are easier for a tree to respond to in warmer weather – in one study at 15 degrees C wound response was strong but at 5 degrees C during dormancy, wound response was minimal.ref

What all of this means for us bonsai practitioners is that when we do major carving or trunk/branch chopping on live wood, we should give the tree the best chance of sealing the damage off and preventing pathogens from entering. To do this we can do it in warmer weather, when the tree is in active growth.

Tree Phenology (or Seasonal Cycles)

The term phenology is used to describe the life cycle of a biological organism like a tree. Phenological events for trees include bud development, bud break, flowering, fruiting and leaf & fruit drop, as well as other unseen changes such as sap rising, seed development, root growth, cambial activity or hardening off of tissues for winter.ref

Tree phenology is entwined with the environment in which the tree lives. As there are a very large number of different climates and micro-climates within them, there are accordingly many different nuances in tree phenology, according to the location and environment. Even the same species can show widely different phenology between two different places (at least from a timing point of view).

So to really understand how phenology would play out for your own trees, you need to understand the species phenology and how it varies based on location. You’ll often find bonsai articles are specific to the location of the author which won’t always be relevant to you.

The main phenological events relate to a tree’s growth and reproduction. For example, roots stop growing below 6°C, buds break when the tree detects a low chance of frost in the future (which might damage the tender buds and shoots), photosynthesis, energy production and growth is highest when there is the most sun, and reproduction happens in conditions which most favour seed survival.

  • In the boreal forests – “high-latitude environments where freezing temperatures occur for 6 to 8 month”ref phenology is mainly driven by temperature, affecting the timing of the start of the growing season and thereby its durationref
  • Temperate-zone forests are located between the tropics and the boreal forest zone – they have hot summers and cold winters with high temperature variationref, and their phenology is also mainly driven by temperatureref
  • Mediterranean coniferous forests are mainly driven by water availabilityref
  • Australian ecosystems are extremely diverse and also subject to irregular events such as fire, drought, cyclones and flooding, which can affect phenological events, but a key driver is water availability.ref Where evergreens dominate in this ecosystem, flowering is the main phenological event.
  • In tropical forests which have less variation in temperature and usually high water availability, leaf shedding and growth is continuous, but reproduction (flowering and fruiting) demonstrates ‘mast’ timing effects associated with drier than normal conditionsref (ie. all trees fruiting at the same time every seven years)

In boreal and temperate areas the phenology is described in this article and summarised in the images below. But if you’re keen to understand the specific phenology for your tree in your area, you could consult google scholar.

The chart below shows the proportion of Eucalyptus loxophleba flowering at any given time in a seed orchard in the southwest of Western Australia. The highest proportion of flowering happened in spring (Sept-Nov in Australia) but a significant portion also happened in winter (June-Aug). Flowering fell to zero in the hot, dry summer (Dec-Feb).

https://www.nature.com/articles/s41598-020-72346-3/figures/2

This all seems a bit confusing given how many different variables there are, but there are some basic principles you can use from a bonsai perspective:

  • Trees in their growth phase (usually when there is plenty of sun and water) will be able to recover more easily from significant damage (such as large trunk chops or carving wounds) and fight any pathogens which might seek to take advantage of these.
  • Similarly leaf pruning during active growth will result in more buds activating.
  • Trees which are in a strong vegetative growth phase (growing leaves and stems) deprioritise root growth. Root growth gets a turn after the leaves establish.
  • Trees which have set buds but haven’t flowered yet – if you prune indiscriminately – you will lose flowers! There is a way to identify flower buds on your tree but it involves a bit of effort. Flower buds differentiate from vegetative buds at a certain point prior to flowering/leafing out. You can identify different looking buds on your tree, then remove one example of each. Cut it open and look at it under a loupe or microscope and you will be able to see which one was the flower vs the leaf or shoot. Or if you’re both patient and organised, take a picture of some your tree with buds and then with flowers – and you should be able to see what the different bud shapes are.
  • Storage of carbohydrates to storage tissues will take place during growth phases, and these will be used in turn when less photosynthesis is happening, to drive respiration and other processes requiring energy. Read more about how storage varies in roots here: Root Food Storage (or, can I root prune before bud break?)
  • If you’re a fan of wiring, doing this before a stem hardens off will allow you more bendability (although watch out for growth around the wire)
  • Depriving a tree of resources (water, nutrients) will mimic ‘hard times’ and cause it to respond accordingly phenologically – drop its leaves earlier, produce less flowers/fruit or not flower at all, or push out emergency growth (like adventitious buds/suckers)
  • I think it’s important to say that although the term ‘dormant’ gets used in relation to trees, this is a little misleading. Trees are living organisms and still need to maintain their metabolism even during winter. This includes respiring (using oxygen and stored energy to maintain metabolism), photosynthesising (for any tree with green areas remaining including evergreen trees but also deciduous trees with green stems), transpiring (even deciduous trees still transpire during winter, although a lot less than when they have leaves and in particular they take up water to swell the buds prior to bud breakref), and taking up nutrients through the roots. As I’ve written elsewhere in this site, root growth can happen above 6 degrees C, so your tree may well be more ‘alive’ than you think during winter.

I know there will be people saying at this point – just tell me what happens when!! For those people here are some general guidelines for temperate zones.

You can expect conifers to cease xylem production in autumn and root growth in winter, and to pick these up again between 2-7 degrees C (cambium) and 6-9 degrees C (roots). Buds will burst from early spring onwards depending on the species and latitude and pollen cones will release their pollen. Seed cones will start maturing, which can take just one summer (Picea, Tsuga) or one or more years plus the summer (Pinus, Cedrus). Next year’s buds and future years’ seed cones will form in late summer, and old needles (2+ years depending on species) will drop in late autumn. Mature seed cones will drop or release seed from late autumn onwards. ref1 ref2 ref3 Hardening leaves for the winter also happens in late autumn.

The main differences for angiosperms in temperate zones revolve around xylem production, leaf growth and senescence within the season, and flowers & fruit. In spring xylem creation will commence – in diffuse porous trees buds can break earlier but ring porous trees need to create the new season’s xylem layer before budding. Some trees will burst bud based on temperature and others on photoperiod (or a combination of the two).ref Whether flowers or leaves come first depends on the species, and the timing of flowers is hugely variable (Frank P Matthews has a list of flowering times for ornamental trees in the UK). The leaves of deciduous trees start a structured senescence process in the autumn, when they remove cholophyll and other molecules from the leaves for storage and recycling (hence the colour changes). After this has been completed the tree creates a cork layer at the base of the leaf causing it to drop off. Fruit develops throughout the growing season and depending on the species will drop off from early summer through to winter.

There’s one more phenological domain which I haven’t covered in this article – the phenology of the microbiome. This is a whole other kettle of…microbes…and might be the subject for a future post.

Finally, the fabulous In ‘Defense of Plants’ podcast has covered phenology in this podcast episode.