Creating a healthy bonsai depends on managing many different variables, from the physical environment through to feeding regimen, styling procedures & timing. I wanted to get a sense of which factors should take priority. In the end I have two tiers for you – 1/ must-haves which are necessary to keep a tree alive and 2/ should-haves which have a significant impact on tree vigour. While these apply to most trees, each species will have different needs across these factors so it’s important to work out the right level of inputs for the species you have (there isn’t really a one-size-fits-all answer for every tree).
Six Must-Have Factors for Healthy Bonsai
#1. Adequate Watering
It may seem obvious, but underwatering a tree can kill it through hydraulic failure. When there isn’t enough water available for the tree to maintain transpiration it suffers the plant equivalent of an embolism, where a bubble interrupts the water stream and stops water from moving from roots to leaves.ref (More information in my watering post). Underwatering can be avoided by checking the medium regularly and watering when it begins to dry (in summer at least once a day in the UK and sometimes twice if hot).
Similarly, overwatering can kill a tree by drowning the roots. Roots need access to oxygen, without it they cannot respire and the root tips die, then opportunistic pathogens (Phytophthora, Pythium) move into the damaged tissue.ref Overwatering can be avoided by ensuring good drainage of both medium and pot, and by simply watering less.
#2. Correct Temperature Range
Extreme temperatures kill trees – both too cold and too hot. The main cold-related problem is frost, which can cause a range of damage to your tree – there is more information (including suggestions for avoiding frost damage) in my post: What does frost do to bonsai trees?. By contrast a 30-minute exposure to 45–57 °C is irreversibly damaging to plants while indirect injury (reduced photosynthesis, impaired water/nutrient uptake, increased disease susceptibility) begins at 38–40 °C.ref A lack of humidity can worsen the damage from heat, more information is in my post Transpiration.
The correct temperature range can also include a certain amount of cumulative cold (usually 0-7°C) to release endodormancy and bud break properlyref. This is species-dependent and can usually be determined by working out the natural conditions of the tree’s origin or natural range.
#3. Suitable Substrate
The planting substrate, or bonsai medium, is a critical factor for keeping bonsai trees alive and healthy. As noted in #1, roots need oxygen to respire as well as physical space to extend and to allow for gas exchange to remove CO₂ and ethylene. A key concept here is Air-filled porosity (AFP) or the proportion of substrate volume occupied by air after free drainage. A more aerated substrate ensures oxygen availability (avoiding root drowning), enables gas exchange and produces denser, finer root systems, which is what you want for a bonsai.ref The substrate also plays a significant role in nutrient storage & uptake, water accessibility and improving the microbial environment in the pot. I have lots of information about bonsai medium in this post: Bonsai growing medium.
#4. Limiting Lethal Pathogens
Whilst a healthy, well-managed tree can often deal with pests in its stride, there are some pathogens which can be catastrophic across species, and others with similar impact for particular species.
Oomycete root rots are the primary must-avoid pathogens as they devastate a tree very quickly. They are water moulds which require water in the substrate to release zoospores, so creating conditions where excess water is not available is the best way to avoid them. This includes not overwatering, and ensuring good AFP in the substrate. A particularly devastating group of oomycete root rots is Phytophthora cinnamomi and Phytophthora plurivora which infect a documented host range of over 3,000 plant speciesref, including common bonsai species such as Rhododendron, Pieris, Taxus, Chamaecyparis, Quercus, Castanea and Pinus ref. The wider Phytophthora genus extends this to most temperate woody genera. There are fungicides which have some effectiveness against Phytophthora (mefenoxam and phosphonates) but they suppress rather than cure, resistance is widespread, and most are restricted to professional use. For a hobbyist, prevention is the only realistic option.ref
Other seriously damaging pathogens include some soilborne fungi like Fusarium oxysporum/Fusarium verticillioides which is a vascular wilt pathogenref and Verticillium dahliae another vascular wilt fungus which is particularly dangerous to Acer. These do not have any reliable fungicidal cure and the main approach for limitation is to avoid introducing contaminated plant material or substrate into your environment and to maintain strong tool disinfecting hygiene.
#5. Enough Light
Plants need light to photosynthesize, how much depends on the species and where they evolved. I wrote a detailed post about Photosynthesis, but the main takeaway is that bonsai trees need enough light, ideally sunlight. For example the photochemistry of acers is impaired at 20% or below of full sun, with optimum being 55–75%ref.
The challenge for bonsai practitioners usually arises when the tree they have isn’t native to their geography, which means accommodations are needed to enable the tree to receive the temperature and light combination they require. Many tropical species can’t be left outdoors outside of the subtropics due to the risk of frost, but even these may need dedicated lighting (see Artificial light for bonsai), a heated greenhouse or a well-lit position near a window.
A classic mistake made by beginners is to situate a bonsai tree in the wrong place – for example putting them indoors on a window ledge when the tree is an outdoor species like Juniper, Pine, Oak or any other tree you would see in a local forest or woodland. One interesting aspect of photosynthesis is how plants react when they get too much light – you can read about this in: Why do some conifer leaves go bronze in winter? In general it’s not going to be too much light which kills a bonsai tree, it is more likely to be too little.
#6. Critical Nutrients
Six macronutrients are required in large quantities for trees to survive and these are a structural and metabolic backbone. Nitrogen (N) for amino acids and chlorophyll, Phosphorus (P) for ATP and nucleic acids, Potassium (K) for osmotic regulation, Calcium (Ca) for cell walls, Magnesium (Mg) for the chlorophyll molecule itself, Sulphur (S) for amino acids. Deficiency in any of these is genuinely lethal because they cannot be substituted. Read more in What each nutrient does (x17). Providing these is usually achieved by the application of fertiliser and soil improvements of various forms. Standard chemical fertiliser will usually provide some ratio of NPK so the other elements need to come from elsewhere. A great source of these for bonsai is organic matter such as manure, chicken pellets, compost, and liquid seaweed.
Four Should-Have Factors for Healthy Bonsai
So assuming you have the top six sorted, there are a few others which are still important but won’t necessarily threaten the life of your tree in the short to intermediate term.
#7. Room to Grow
Obviously a big part of bonsai is keeping the tree constrained in a small pot, that’s the whole point! But we cannot ignore the fact that constraining a tree in a small pot also restricts its growth significantly, with studies showing that doubling pot size increases biomass by an average of 43%.ref The pot constraint will slow down growth, this is why we should develop trees in larger containers to get the trunk size needed before moving to a bonsai pot for refinement.
But once in a smaller pot, there are things which need to be done to make the best of that situation which mainly revolve around root pruning and encouraging root ramification. Root pruning prevents the roots from filling and crowding the pot, and allows the root tips to grow. Root ramification helps to maximise the root surface area given the constraint of the pot, and there are many different ways to do this – see Ramification of Roots (lateral root development).
Failing to root prune or encourage root ramification will end up penalising the tree and reducing its vigour over time, which also reduces its tolerance to other stressors like pathogens, or excesses in temperature or water.
#8. A Balanced Diet
Aside from the six critical nutrients in #6 above, there are eight additional elements also needed. These are the micronutrients including iron, manganese, zinc, boron, copper, molybdenum, chlorine and nickel. Since they are only required in tiny quantities and are usually present in trace amounts in substrate components and irrigation water they are less critical to replace with a fertilising regime. Deficiencies of these nutrients do occur and can damage your trees (for example iron chlorosis on alkaline-irrigated trees), but they cause disorders rather than death in the short to intermediate term. Dealing with #6 above usually tends to cover the elements in #8.
#9. Well-Timed & Sized Pruning & Repotting
Pruning and repotting both impose a carbohydrate cost on the tree as they remove tissue that contains stored non-structural carbohydrates (NSC), and the tree has to fund regrowth from what’s left. The conventional bonsai advice says repot just before bud break in spring, but the underlying NSC data suggests this may not be the optimal time for root work. Root carbohydrates are used for leaf growth, so removing roots just before bud break deprives the tree of those reserves at the moment it needs them most (unless you want small leaves, which you may want but acknowledging this will constrain leaf surface area for photosynthesis).
The chart I worked through in root food storage and pruning timing shows root NSC pools are lowest in late summer / early autumn for most deciduous species, which is also when peak root growth occurs naturally.ref So for root pruning, August is a defensible choice and arguably better than spring. The risk is frost damage to newly grown roots if you leave it too late, but since this is when most root growth happens anyway, the risk is smaller than it sounds. But this is definitely one for the endless bonsai debates!
Branch pruning is the inverse since branch NSC reserves are lowest just after bud break in spring, having funded the new leaves, so spring is the cheaper time to remove branches. The “amount removed matters more than exact timing” rule still applies so keep moderate pruning under about a third of foliage in any one sessionref, less on a weak tree. More detail on both in pruning and repotting tips.
The practical override is that within reason, timing matters less than the literature implies. A maxim I have is: the right time to do something is when you have time to do it. Unless you’re being aggressive (heavy root reduction, hard branch cutback), most temperate species tolerate work outside the theoretically optimal window if you compensate by reducing transpiration if you’ve removed a lot of roots, with foliage thinning, shade, or a humidity bag.
#10. A Healthy Microbiome
A bonsai tree has a microbiome, which is a community of fungi and bacteria living on the leaves, in the roots, and inside the plant tissue itself, likely started from when it was a seed. A tree and its microbiome together form what’s now called a holobiont. I’ve written more on the underlying biology in the microbiome and symbiotic microbes.
For bonsai practice, three things actually matter:
#1 Don’t kill what’s there. Systemic fungicides and insecticides which you might turn to for treating a pathogen or pest problem may have the unwanted side effect of damaging the microbial community in the substrate. The implication isn’t never to treat, but to weigh the cost since a healthy tree shouldn’t need routine chemical intervention, and prevention via the other Tier 1 factors (#1–6) avoids the choice altogether.
#2 Replenish after disturbance. When repotting, collecting Yamadori, seed-growing or air layering there may be minimal microbial community remaining in the substrate. Commercial dry mycorrhiza and bacterial inoculants could help here, though the evidence on whether they establish at useful densities is mixed and species-matching matters (different products contain different types of mycorrhiza which are relevant for different tree species). A handful of humus from a healthy, chemical-free woodland with similar species, keeping some of the previous substrate and limiting root pruning to retain enough microbes in the rootball are ways to provide a replenishment source. Also eventually the microbiome will reestablish by picking up local inoculum on its own.
#3 Create conditions microbes prefer. Good substrate structure with nooks for microbe colonisation (such as Biochar), reasonable moisture, drainage, light, stable temperatures, and not too much root disturbance.
It’s important to say that not all microbes are friendly! Improperly composted manure can carry Salmonella and E. coli. Legionella (the bacterium behind Legionnaires’ disease) is present in many composts made from wood, bark, green waste and peatref. So wash your hands and tools after working with substrate, and mix bonsai soils outside or under cover rather than indoors in a way that sends dust into the air. This won’t negatively impact the holobiont of your trees but will reduce the risk of you catching anything nasty.