Category Archives: Keeping Bonsai Healthy

Nutrients for Trees

Before we dive into this subject, it’s important to know there are two aspects of ‘feeding’ trees and the one we are going to cover in this article relates to the nutrients that plants need in order to generate new cells and growth.

The other aspect relates to the nutrients which can contribute to a better growing environment for the plant – for example, increasing mycorrhizal fungi, reducing pathogens, and improving the community of bacteria interacting with the roots of the plant. You can read about those in Non-nutrient Additives.

In general true nutrients – as they are referred to in the literature – are elements – ie. they are not able to be decomposed into smaller components, and you will find them all listed on the periodic table of elements. There are 17 elements which are recognised as plant nutrients, and from these trees synthesise their own biomass as well as everything that’s needed to make it (like secondary metabolites, enzymes, carbohydrates, plants produce literally thousands of compounds).

Three of the required elements are derived from air and water (oxygen, carbon and hydrogen). Of the remaining 14, there are six macronutrients (present in higher volumes) and eight micronutrients (required in smaller amounts).

This is important for bonsai because trees usually obtain all of their nutrients from air, water or soil. Since bonsai are not planted in soil, they are vulnerable to nutrient deficiencies. To learn more about how trees absorb nutrients, check out how roots absorb water & nutrients. For a description of each nutrient, why it’s needed and how it’s obtained by plants when not in a pot, please read the post on what each nutrient does. If you don’t want the detail, below is the (relatively) short version…

There are two ways in which nutrients are used by a plant and these relate to what the tree is, and what it does.

What are trees made of?

The main structural components of trees are lignin and carbohydrates. Lignin is a polymer which combines three types of alcohol (p-coumaryl, coniferyl & sinapyl) in different ways, which in turn combines with cellulose (a carbohydrate) to form wood. Lignin makes up 25–35% in gymnospermsref and 20–25% in angiosperms, cellulose makes up most of the rest, with 4-10% of other components. Whole text books have been written just on the topic of lignin and it isn’t fully understood as a substance – it could be considered the ‘secret sauce’ to tree success. Feel free to spend $183ref to learn more…or buy another nice tree instead!

Both lignin and cellulose are made up of carbon, hydrogen and oxygen, all of which is obtained from air & water through photosynthesis.

What do trees need to function?

A tree can’t become a tree with just carbon, oxygen and hydrogen, even though those elements make up most of its structure. It needs chemical reactions to take place throughout its life to create the cellulose and lignin, and to manage all of the processes needed to maintain life. This is why it needs other elements in addition to C, O & H.

Key reactions within a tree include photosynthesis, nitrogen capture, cell division and defence against pathogens – but there are millions of chemical reactions going on inside a tree at any given time. Many of these depend on enzymes, a type of protein which acts as a catalyst – that is, it enables something to happen without being consumed by the reaction itself.

As a protein, enzymes are made up of amino acids, which all contain carbon, hydrogen, oxygen, nitrogen and (sometimes) sulphur. This is where the first two nutrients come in – nitrogen is needed in every enzyme, and sulphur is needed in some.

Enzymes are really interesting because they don’t just make reactions happen, they also speed them up in really clever ways (to learn more see Jim Al-Khalili’s book Life on the Edge: The Coming of Age of Quantum Biologyref). To do this, they use other elements, including metals like potassium, iron, copper, manganese, magnesium, nickel & zinc. Boron, chlorine and molybdenum are non-metal enzyme co-factors (elements which enable enzymes to function).

The remaining two nutrients are calcium and phosphorus. Calcium is used by plants in a similar way to humans, as calcium pectate it acts as a skeleton, strengthening cell walls. It also acts as a chemical messenger as part of processes related to root and bud growth and responding to stress. Phosphorus has multiple roles, as a component of the molecule ATP (adenosine triphosphate) which is used by all living things to store and transfer energy, as the structural framework for DNA & RNA and as part of the carbon fixation process.

How to obtain nutrients for trees?

So we know that plants need significant levels of nitrogen, phosphorus, sulphur, potassium, calcium and magnesium and they also need smaller amounts of boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. According to Hallé in his absolutely brilliant book ‘In Praise of Plants’, Nitrogen is a key limiting nutrient for plants. He says “Although they easily assimilate carbon, nitrogen remains a constant problem for plants. It is the reason that they are poor in proteins and rich in carbohydrates. The situation is the opposite in animals…”

Standard non-organic fertiliser does not contain all of these nutrients, so you need to make sure they are added somehow – either via an organic fertiliser or a combination of additives like liquid seaweed, compost or fermented/decomposed manure. Take care you understand what the definition of an organic fertiliser actually is. Unfortunately most fertilisers do not reveal their composition on their packaging, so look for one which does. My personal investment in fertiliser is my purchase of a Hotbin hot composting bin – this creates organic compost in 3 months and produces leachate which can be used as a liquid fertiliser. Also consider your carbon footprint. As mentioned in the what each nutrient does post, Ammonia production for chemical fertilizers is a major contributor to global warming as it uses fossil fuels as the main ingredient, as well as massive amounts of energy to produce, and contributes to ecosystem damage through nutrient runoff. Using compost with some manure is a more environmentally friendly way to provide nitrogen to your trees.

Consider your watering as well. Trees need *some* chlorine and grow better if they have a bit more than merely what they need – tap water can provide this nutrient if you use it for watering. You should check your local water company report as you may be able to obtain other nutrients from your water as well, including magnesium.

Whilst there are many research studies identifying the effects of nutrient deficiencies, there aren’t many with evidence for nutrient toxicity so it’s hard to work out if this is a myth or reality. Living things tend to have a system of homeostasis to manage levels of chemicals to avoid them getting too high (or low depending on the substance) so it may be that toxicity is rare due to homeostatic mechanisms removing excess nutrients.

Some bonsai practitioners are fans of foliar feeding. This can be useful in certain circumstances, but the better approach is to add nutrients to the soil.

How much is enough?

This is where the evidence starts to get very thin on the ground. Nutrient requirements vary between plant types and their ability to obtain nutrients is dynamic – it depends on the presence of other nutrients, temperature, pH, energy availability, the size of the plant – there are a lot of variables. Probably the best approach is to follow the guidance on the fertiliser you are using.

Reabsorption of Nutrients

A word of warning to those who like to remove those ‘messy’ dying leaves on deciduous trees at the end of the season. Towards the end of the growing season, once the tree has made enough wood and grown enough leaves, and, from the tree’s perspective, done everything it can to reproduce, it stops focusing on growth and goes into an orchestrated shutdown phase.

During this phase substances from the leaves are reabsorbed into the tree, to be stored in the rays and roots for use again next year – enzymes are again deployed to effect this absorption. This is why leaves change colour as different substances are reabsorbed. What is left in the leaf when it finally drops is actually quite low in nutrients. So let the tree drop its leaves as it wants, to optimise its health for next year.

Defoliation

There are quite a few research papers about tree defoliation because this can be caused by insects, creating a problem for the forestry industry. Defoliation is used on deciduous trees in bonsai to completely regrow a deciduous tree’s leaves, resulting in ramification and smaller leaves. This isn’t a practice for conifers, or at least, not for most of them, as many conifers simply can’t regenerate very easily and the effect will be weakening of the tree and not ramification. Although I must note here that my 2022 summer watering disaster caused a small larch forest of mine to defoliate and it looked fantastic after the foliage regrew!

Complete defoliation is a pretty drastic practice from the tree’s perspective and a double whammy – as not only does the tree have to use its stored energy reserves to regrow its leaves, it doesn’t have any energy coming in until those leaves are regrown. Defoliation significantly reduces the total stored carbon in a tree, and there is a point at which mortality occurs – one study found that once stored carbohydrates were less than 1.5% of the usual level, this will kill the tree.ref

As described in this article about the effect of grazing animals, “Plants adjust to conditions of chronic defoliation and the associated reduction in whole-plant photosynthetic rates by altering resource allocation patterns and reducing relative growth rates.”ref Although the article is focused on grasses, which are a different branch of the Plantae family to trees, it says that “root elongation essentially ceases within 24 hours after removal of approximately 50% or more of the shoot system…[and there is]…a rapid reduction in nutrient absorption”. So basically by defoliating 50% or more the roots will stop growing and nutrient absorption will reduce. Interestingly, several studies reported that photosynthetic capability of the remaining leaves on defoliated plants actually increases – perhaps a result of the resource allocation pattern change mentioned above.

The effect of defoliation is to force a deciduous tree to use the stored energy it has built up in the growing season straight away, instead of leaving it for the next season. Because of this, the tree doesn’t have the energy reserves to grow a full set of leaves at the same size it would normally, so it compensates by growing smaller leaves. Since this technique uses up stored energy, there isn’t much left for other types of growth, so it’s not a technique you would use if you were trying to thicken a trunk or grow branches.

This studyref found that a 50% defoliation of prunus saplings reduced their growth rates for the following 5 years and brought forward bud burst for a similar period, while this oneref found that larch recovered well from defoliation, but pinus did not. This oneref said that partial and complete spring defoliation reduced first-year diameter, height, and volume growth of 4-year-old loblolly and slash pines.

This article says that “scientists found that growth was reduced in both half and entirely defoliated trees in the short and long-term…both half and entirely defoliated trees had less leaf area than control plants. Defoliated trees also allocated more carbon for storage than control trees with no defoliation.”ref This suggests that defoliation in some way teaches your tree to divert resources to storage instead of foliage, not just once but into the future. Which means you really don’t want to do this while you are still establishing the branch structure and ramification because these will slow.

Interesting, Harry Harrington reports that some species don’t respond to complete defoliation by growing smaller leaves, instead they grow a small number of large leavesref. So overall a complete defoliation may be an unnecessarily unpredictable and heavy-handed way to achieve leaf reduction. One could hypothesise that defoliation of a tree which follows a fixed growth pattern (read more in Extending Shoots) might result in a greater leaf reduction effect, because buds and nascent leaves are not sitting there waiting to burst, they need to be completely regrown. But one could also hypothesise that this type of tree might struggle to regrow any leaves at all, depending on the weather conditions.

There are less drastic options than removing the entire foliage of a tree all at once – you can remove half of it for example, or do it in stages, so that new leaves can grow before the remove the next batch. It seems like you should be able to achieve a similar effect with constant low-level leaf pruning throughout the growing season, combined with bud pinching at the start of the season. A more gradual approach would allow photosynthesis and energy generation to continue, without stopping root extension and nutrient uptake, while still regrowing leaves and increasing ramification. It may be however that the shock of something more drastic is what’s needed to reduce leaf size significantly because the resources to regrow are shared more widely. An experiment for someone?

The timing of defoliation is really important. The tree needs to have had enough time with its new leaves to generate good energy stores for the next season and enough time to regrow and harden its leaves against frost. Somewhere in the middle of the growing season allows for both of these to happenref.

Should I remove flower buds or fruit?

That depends what tree you have and what you are trying to achieve. Obviously if you have satsuki azalea, you probably want to leave the flowers on the tree! If you have a crabapple, personally I don’t think there is much point if you don’t let a few fruit form. And I am really partial to rose-coloured larch cones. All trees form some kind of reproductive organs, whether they be conifers with their strobili (cones, either pollen or seed forming), ginkgo with their ovules, or angiosperms with their flowers and fruit. Some are almost unnoticeable and others are right in your face. Bonsai wisdom sometimes says these should be culled or removed entirely in order to avoid draining the tree of its energy.

When considering this question we need to understand the idea of resource ‘sources’ and ‘sinks’ in plants. A source is a material producer and exporter, and a sink is a material importer and consumer.ref See the below table for sources and sinks in trees. As you’d imagine, leaves are a major source of carbon and a sink of inorganic nitrogen (nitrogen as a macronutrient). Roots are a source of inorganic nitrogen and leaves are a sink. So what about fruit, seeds, and flowers, which supposedly drain the tree? As you can see they are major sink organs – but not only sink organs…they are also source organs!

https://academic.oup.com/jxb/article/68/16/4417/3002648

Let’s have an interesting little diversion – did you know that it’s not only leaves which photosynthesise? This fascinating studyref looked at the photosynthetic activity of (a) ears of wheat (b) sycamore seed pods (c) a green tomato (d) unripe and ripe strawberries (e) a greengage (f) unripe cherries; and (g) a green apple. The images below were taken using fluorescence imaging and anything with a colour indicates that there is photosynthesis taking place – with the red and orange areas the strongest. Check out the sycamore seed pods!

https://onlinelibrary.wiley.com/doi/full/10.1111/tpj.14633

How the heck can this happen – well there are various theories about the mechanism (including recycling CO2 from respiration, and the presence of stomata on fruit) but the point is that maybe seeds and fruit, particularly if they have periods when they are green, don’t act as such as sink as we might think, and for a period are acting as a source and not a sink.

This study states that “reproduction in Beech does not deplete stored carbohydrates, but it does change the amount of nitrogen stored” and this study found that “fruiting is independent from old carbon reserves in masting trees”ref which basically means that fruit uses current year photosynthates/energy and doesn’t actually deplete reserves.

On the other hand this study found that Douglas fir tree rings were narrower in years when they bore many seed-conesref and this one mentions that “experiments with apple trees have shown that roots can die from lack of carbohydrate supply when they are over cropped”ref

All living things have processes for managing and balancing resource allocationref and this is likely an evolutionary differentiator. In trees, resource availability limits the amount of fruit which is allowed to develop – even pollinated flowers may not develop into fruit if the tree does not have enough resources available – these could include energy, or nutrients.ref So to an extent the plant itself manages the resource allocation.

To complicate matters further many trees use a ‘masting’ strategy for reproduction, which means they have years where many more seeds are produced, often synchronised with other trees of the same species. One theory for how this happens is that the weather influences how pollen is distributed – in beech windy conditions lead to mast years whereas in oak short pollen seasons do.ref Temperature and precipitation also affect pollen production and distribution (high temperature increases pollen production but high precipitation washes it away).ref In this study on Japanese oak, “high seed production never occurred in two successive years, but successive years of low abundance were observed several times between 1980 and 2000.”ref

Overall there are a lot of factors interacting when it comes to reproduction. Photosynthetic seeds or fruit can contribute to carbon production, and may use only current year photosynthates, so the tax may not be as high as thought, but there is some evidence that reproduction can divert energy from roots and foliage.

If you are really focused on trunk growth, branch structure or foliage development on your bonsai tree, you might want to divert the energy from reproduction to these areas by removing some or all reproductive organs, until you are happy with the trunk/foliage. At this point then you could then let the tree reproduce (noting that removing cones one year will cause more cones to develop the following year)ref.