A Practical Guide To Fertilising Your Bonsai

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This article was originally published in Bonsai Focus magazine May/June 2019

bonsai fertilizer

Applying slow-release fertilisers to my bonsai in the garden in Spring.

Bonsai need water, air and sunlight in order to photosynthesis and grow.
The combination of these 3 elements is enough for a tree to manufacture the sugars and starches it requires from its leaves.
As the keepers of these trees, we can govern how a tree grows, to an extent, with the use of fertilisers. Fertilisers are to our bonsai what vitamins and nutrients are to our children. The better the quality of nutrients they absorb, the better they grow!

We can influence strong vegetative growth using high nitrogen feed, or tight, fine growth using low nitrogen. Using high phosphorous levels in a fertiliser, we can encourage flowering at the expense of vegetative growth.

It is however very easy to get bogged down with complex feeding regimes and variations of fertiliser components. My personal opinion is that, in terms of regularly feeding an entire bonsai collection on a season-long basis, there is too little difference between many fertiliser products and it is easier and only necessary to follow a simple and straightforward fertilising regime. Particularly when one then considers that any slight advantage in one product over another can easily be lost to the weather, heavy rainfall or poor watering practises.

The intention of this article is not to provide an exhaustive text on the chemical and biological pro's and con's of any given preparation, nor is it intended as a primer for fertiliser as these are widely available in books and online. Rather, I hope, a practical guide to getting the best out of your trees by giving them a solid fertilising regime.

Types of Fertilisers for bonsai

Slow-Release Organic Fertilisers

Slow-release organic fertilisers are available as pellets that are laid on the surface of the soil. As the tree is watered, the pellets break up and are absorbed into the soil itself. This means that they are not left on the soil surface attracting the local wildlife and looking an unsightly mess!
Although slow-release organic fertilisers are relatively weak, with a NPK of around 3-3-3, they are composed of natural ingredients that are broken down by bacterial and fungal activity in the soil, slowly releasing their nutrients on a continual basis and this means that nutrients are always available to your trees. Compare this to liquid chemical feeds that maybe as strong as 20-20-20 at their peak, but are rapidly dispersed from our inorganic soils during rain and watering.
Organic fertilisers are also just that, they add an organic, microbial element to our largely inorganic modern soils and this helps create a healthier ecosystem within a bonsai pot.

Unless temperatures are reliably above 12C, plants do not start absorbing Nitrogen. This fact leads to a number of interesting conclusions. Firstly, fertilisers can be applied to the soil at the time of leaf-burst, typically when daytime temperatures just reach double figures, but they are still not utilised until the tree is ready.
Secondly, organic fertiliser still present in the soil as temperatures drop back down to single figures in the Autumn after the growing season is left unused, and remains locked up in the soil until the following Spring. There is no requirement to feed a low Nitrogen 0-10-10 fertiliser at the end of the growing season as the tree stops absorbing Nitrogen anyway.

slow release fertiliser

Slow-release fertiliser pellets.

Fast-Release Liquid Chemical Fertilisers

Fast-Release Liquid Chemical Fertilisers are diluted with water and applied to the soil as one would when watering their trees. I typically use a regular brand high nitrogen fertiliser intended for trees and plants and available in supermarkets and garden centres.
Liquid fertilisers are applied at regular intervals according to the manufacturers' recommendations on the product (typically 10-12 days), and should be considered a short-lived 'boost' to your trees that require strong and vigorous growth and where coarse growth is not an issue. Within hours of application, the levels of the fertiliser in a modern inorganic soil will have started to deplete. This is particularly true after watering and during rainy weather. It is possible that within days the level of chemical fertiliser can become very low and nutrients are no longer available to the tree until the next application.
Note that while it is very very difficult to over-feed a tree with slow-release organic fertilisers, mixing fast-release chemicals at a higher strength than recommended can cause fertiliser-burn.

Feeding Regimes For Bonsai

How you feed your bonsai and bonsai collection as a whole is dependent on the types of growth you require, and how much time you are willing to invest every couple of weeks throughout the growing season, actively feeding your trees.

The most straightforward, all-encompassing fertilising regime is to apply an organic, slow-release fertiliser to the soil surface every 6-8 weeks throughout the growing season. That is, when your trees are actively growing. This will make a healthy mixture of Nitrogen, Phosphorus and Potassium plus trace elements available to your trees from Spring until Autumn.

For a very basic feeding regime, an application of fertiliser to all trees in Spring, and a second at midsummer will keep them adequately fertilised for an entire growing season. This basic feeding regime should be considered essential to keep any bonsai collection healthy and vigorous.

However...........you can start to encourage different types of growth for different bonsai in different stages of growth by using combinations of fertiliser at different times.
For the sake of simplicity, I will divide the typical range of trees in a bonsai collection into 5 types. Coniferous species (Pines and others), deciduous species and flowering species. Trees in development, from raw material upwards, and fully developed trees in need of refinement with short internodes and no coarse growth.

Deciduous Trees In Development

If a deciduous tree still requires growth to build either the trunk, or the primary and secondary branches, then a heavy feeding regime with plenty of Nitrogen can be employed.
I use a base feed of 3-3-3 organic fertiliser, applied to the soil surface every 6 weeks from leaf-break to early Autumn.
A high nitrogen 20-20-20 or even 24-8-16 liquid feed is then applied every 12-14 days (in addition to the slow-release fertiliser). This is applied from leaf-break until late Summer when growth on deciduous trees typically slows. This “super feeding” will encourage very rapid growth, vigour and development in healthy trees.

For species that typically exhibit very coarse growth in Spring, I will wait until the first flush of growth has hardened off before beginning liquid feeds. This discourages unusably-coarse elongation in particularly vigorous trees that would simply need to be removed at a later date, particularly when developing secondary branches. Once the Spring flush has occurred, further flushes are naturally shorter and repeated fertilising with high nitrogen liquid fertilisers is started.

Deciduous Trees In Refinement

Finished deciduous tree require much more refined growth with short internodes. However, we ideally want them to still be vigorous and offer repeated flushes of growth each year in response to pruning and defoliation.

For naturally vigorous species I hold back from using any fertiliser until the first flushes of Spring growth have finished, then an organic fertiliser is applied every 6 weeks for the remainder of the season. Once midsummer has been reached and any (partial) defoliation has taken place, I then start to additionally fertilise with a high nitrogen fertiliser to encourage as many repeat flushes of growth as possible.

If you feel that elongation on any particular tree is too long, withdraw the liquid fertiliser.

Flowering Species

Some flowering trees will flower regardless of our fertilising regime. Some such as hawthorn (crataegus monogyna), pyracantha and blackthorn (Prunus spinosa) are much more reluctant to flower. Care should be taken to avoid feeding these species with high nitrogen liquid fertilisers from late Summer onwards as nitrogen will encourage vegetative growth at the expense of flowers.

Coniferous Species Excluding Pines

Feed with organic slow-release fertiliser from early Spring, indicated by deciduous trees coming into leaf and the soil of Junipers beginning to dry out rapidly for a 2-3 week period. It is especially important that coniferous species have an added organic element to their soils due to their reliance on microbial activity.
Additional liquid fertilisers can be applied from late Spring onwards to encourage elongation, as necessary. Be aware that high nitrogen will encourage juvenile growth in Juniper types that are susceptible to it.

Pines/Pinus species

This feeding regime is based on the fact that in the UK and Northern Europe, all pine species can be considered to be one-flush pines. We have neither the temperatures or length of growing season for pines to have two true flushes of growth.

Fertilising a pine as the buds elongate into candles in Spring will encourage longer candles followed by longer needles. If short needles and refined growth are required, withdraw fertiliser until the needle-size is set, around midsummer.
If strong and vigorous growth is required at the expense of having longer needles, for instance when developing trunks, fertilise with organic and/or liquid fertiliser from early Spring onwards.

Typically, Pines are worked on from late July onwards, being pruned, needle-plucked and wired. At this point the needle-size of the current year is set and we want to prompt a vigorous response to late summer pruning and also to fuel production of new buds for the following year. For all pines, an application of organic fertiliser should be applied from late Summer, and for those requiring maximum development, liquid feeds can be applied through until early Autumn when ambient temperatures drop and Nitrogen is no longer required by the tree.

 

Sick and weak trees including newly collected yamadori.
Fertilisers are not 'plant medicine' and feeding a weak or sickly tree with fertiliser will not make it healthy. Applying fertiliser during this time can have the reverse effect and damage the tree further.
For a similar reason, only start fertilising a newly collected tree when it has actively recovered from the trauma. Fertilising too early will not help the tree survive, rather it may kill it.

Too much fertiliser/fertiliser burn.
Always follow the manufacturers recommended dilution rates and feeding intervals. By doing this you avoid the possibility of scorching the roots of your tree.
Where trees that have too much unused nitrogen locked up in the soil and are in danger of being overfed, the soil surface will develop a dark-green colour (not to be confused with moss!). This can occur with species and individual trees that require less Nitrogen than normal, such as Elms (Ulmus) and Legumes (members of the pea family). In these cases, simply withdraw repeated high nitrogen liquid fertilisers.

Slow-release organic fertiliser pellets

bonsai fertliser

The slow-release fertiliser pellets applied to the surface of the soil. It is not necessary to use many pellets as they will swell to 2-3 times their size once watered. The pellets can be spread evenly throughout the soil as shown here. Or located in specific areas to encourage root growth from the nebari/trunk base or towards the edges of the pot.

bonsai fertilizer

Water-in the pellets well and then leave them for 10-15 minutes while they absorb moisture and swell.

bonsai fertilizer

Once swollen, the pellets can be watered again to encourage them to enter the soil itself.

juniper fertilizer

Some tree species such as this Sabina Juniper require a very large-grained, open, fast draining soil that cannot be routinely changed. For trees such as these, the fertiliser pellets can be inserted into empty tea-bags and placed on the soil surface. When the tea-bags of fertiliser are watered, they take the nutrients into the soil.