Bonsai Growing: Caring for Bonsai Trees

Bonsai plants and trees are not just another kind of indoor or balcony plant. They demand constant and repeated attention – which is the price you pay to maintain these magnificent collector’s plants.

If you are unable to look after your bonsai plant sometimes, do not worry: a fellow enthusiast is sure to help out or a professional bonsai nursery may well be happy to take in your plants and care for them in your absence: but you can be sure that they will still recognize you when you return!

caring for bonsai treesRepotting Bonsai


Good quality compost is essential, since it has a direct effect on the health of the tree. Opinions on composition are divided, but as a general rule, a bonsai will thrive in compost made up of equal parts of good quality loam, peat and either sand or rotted turf. Some plants, such as rhododendrons and azaleas, need a lime-free or acid growing medium. The experts generally agree that the ideal mixture differs for deciduous trees, conifers and flowering trees. It is important to stick to the compost mixture in which the bonsai plant developed, so you should ask the grower about this when you buy your bonsai. The compost should always be sieved to eliminate any risk of damaging the roots.

Why repot?

This is an important matter, which is frequently neglected. Unlike a normal indoor or balcony plant, the bonsai needs a container in complete harmony with its size and style. Let us recall that in Japanese, the word bonsai means tree in a tray. Ignore this at your peril: you may end up with an aesthetic catastrophe and a spoiled bonsai.

There is an extensive range of bonsai containers, usually offered by the growers themselves. Most of these trays are of Japanese or Chinese origin. Flat trays are best suited to trees with a spreading or trailing habit, with slightly deeper trays for upright trees and even deeper trays for tall, slender or cascading plants. Forest or grove arrangements are most attractive displayed in very flat trays or even sometimes simply on stone slabs decorated with some rocks.

This group planting badly needs repotting. The large, flat tray shown in the bottom right hand corner is perfect. When the roots reach these proportions, urgent action is needed

Most bonsai trays are made of stoneware, either glazed or unglazed. They are seldom decorated and the most usual colours are blue, pale green and brown.

Preparing the container

Bonsai trays have large drainage holes, through which any excess water from the frequent watering can drain, so that it does not stagnate and set up root rot. Do not cover these holes with a stone or piece of clay pot, as in normal plant pots.

1. This Acer palmatum is due for repotting. In the foreground are the necessary tools. 2. Plastic mesh should he placed over the drainage holes to prevent any soil leaking out.Use plastic mesh (which will not rot or rust), fixed in place with plastic coated wire hooked on to the outside of the tray. In this way, none of the finely sieved potting compost will leak out, even after generous watering. The plastic mesh also has the advantage of stopping unwanted visitors, such as the wood louse, from getting into the pot.

Removing it from its pot

It becomes necessary to change the pot or tray when the plant becomes ‘pot-bound’ by the growth of its roots. To remove the bonsai from its pot, stop watering until the compost is relatively dry, but at the same time make sure the plant does not suffer. Then gently lift the bonsai by its trunk: if the compost is dry, the plant should come out quite easily.

Check the soil

It is a good idea to check the soil for the sake of the tree’s health. Check that the compost is not harbouring any undesirable creatures, such as ants, woodlice or insect larvae. You will also be able to monitor the root development and decide when to repot.

bonsai root pruning

Root pruning

This is thought to be vital to the art of bonsai, root pruning directly helping to dwarf the tree. However, root pruning will also help to rejuvenate the tree by bringing the feeding roots a little nearer to the trunk.

Start by removing most of the earth from the old root ball by gently scraping with a special bonsai rake. This will also untangle the roots. It should be done as gently as possible, avoiding damage to the roots (especially the larger ones) so far as practicable. Take a pair of wide-handled scissors and snip the roots down to about half their length. Remove completely any roots which do not seem healthy or which were damaged when the tree’s roots were combed. Your tree is now ready for repotting.

Root pruning is a testing time for the bonsai tree: this is why it is vital that it takes place at the start of spring, when the tree is at its most vigorous. Water generously after repotting, then keep fairly dry until the tree is re-established.


We have seen how important it is to choose a container of the right size. When you have prepared the tray as described, spread a drainage layer of gravel or pebbles in the bottom. The potting compost should be sifted several times, with the coarsest material placed on top of the drainage layer, followed by successive layers of compost with the finest on top. Then plant your bonsai and add the very finest soil. If dry enough, the fine soil will easily filter down through the roots. Firm lightly before watering.

You may find it difficult to keep some of the larger trees in place unassisted. Avoid using any kind of stake. Instead try keeping the tree’s root ball in place with metal wire passed over the base and through the drainage holes. This is an efficient, invisible method. Do remember to remove the wire once the tree has taken root. The tray should be about three-quarters filled with compost, with a layer of very finely sifted soil to finish off the surface. Firm it down with the spatula at the other end of the bonsai rake or use tweezers. One can also cover the soil with a layer of moss which is decorative and serves to keep the soil moist after each watering.

Watering should be done slowly, continuing until excess water runs out of the drainage holes. This job can take a long time, since the compost used for repotting should have been very dry. To prevent moisture evaporating too quickly, put the bonsai where it is sheltered from wind and sun for several weeks. It should start to flourish in no time, overcoming the shock of being repotted. Never forget that repotting is always traumatic for the plant and act accordingly. It is also imperative to choose the right tray for transplanting the tree. If you change your mind after repotting is completed, it could do your bonsai irreparable harm to subject it to a second repotting after such a short time.

How to Water Bonsai

A tree developing in its natural surroundings needs a considerable amount of water, which it obtains by developing deep roots which stretch far into the ground. A bonsai has similar requirements in proportion to its size. In other words, most bonsai need frequent and copious watering, particularly in summer. The fact that most bonsai are kept outside and exposed to the rain does not mean that one can dispense with watering since the moisture held by the root ball is always inadequate.


Like other trees, bonsai need fresh water, free from toxic physical or chemical elements. Of course, the best water is rainwater, even though it may be polluted in towns. The ideal solution would be to collect rainwater in a water butt, but this is not always practical.

Well water is also suitable, provided it is not hard (ie. limy), in which case water softening equipment should be used. Alternatively -water softening tablets which are offered for sale can be added according to the instructions on the packaging to ‘soften’ the water by reducing the pH.

Tap water should only be used if relatively free from chemicals used in the purification process. The main danger is from chlorine, which is highly toxic to plants. If there is no alternative, keep the water outside in the open for several days, in which time much of the chlorine should evaporate. Whatever water you use – particularly if it is well water -make sure it is not too cold or it could give the plant a shock. It is a good idea to fill your watering can several hours before use, so the water can warm to the temperature of its surroundings.

Don't leave your bonsai to a critical stage of dehydration - if yours has reached this stage, abundant watering is necessary

How to water

Always water bonsai with a fine rose so you do not damage the delicate leaves or wash away the surface compost. If you only have a few plants a watering can with a fine rose will do. If you have a collection of some size, watering may become a major chore and you will need sprinklers, operated mechanically by a tap or automatically by an electronically controlled valve.

As well as a watering can, you will need a sprayer, preferably a pressure sprayer, for misting over the leaves during warm weather. However, a sprinkler device will do this for you automatically.

It is impossible to say exactly how often bonsai should be watered, since each species has different requirements and climatic conditions may also vary. As a general rule of thumb, water often and sparingly, rather than occasionally and generously. The compost should be moist but not waterlogged. Any retention of water may set up root rot.

It is important to realize that bonsai require most water when growing most vigorously and in hot weather, and that less water is required in the weeks following root pruning, and clipping of shoots and leaves. This is because the reduced root system draws in less water and less is evaporated from the leaves.

In general terms, trees should be watered about once a week in winter. Although this will depend on the overall humidity, and several times a day in midsummer. If the root ball dries right out. The tray should immediately be immersed in a basin of water. If you have to do this, be careful that the tree does not become uprooted. You should also damp over the foliage. It is especially important to look at the tree on a daily basis as the spring – and hence warmer weather – approaches.

Too much water can also be detrimental to the tree. For outdoor bonsai the heavy autumn and spring rains can be just as damaging as the hot summer sun. To stop the soil becoming waterlogged, we recommend placing a sloping shelter. Such as a sheet of clear plastic or polythene. Over the trees, or angling the trays slightly, so that excess water runs away on its own.

If the tray is left on a slant, watch that this does not result in the tree growing out of shape. Apart from water-loving trees such as wisteria and willow, do not stand the container in a tray or saucer, as this could stop water draining away and cause root rot.

Watering indoor bonsai poses fewer problems, since the indoor climate re-mains more or less stable. Water them at least once or twice a week, depending on the size of the pot (the smaller it is, the more often you will have to water it).

The leaves should be misted over with water frequently as the atmosphere can be very dry indoors. Central heating aggravates the problem still further, and you will find it necessary to spray even more frequently, especially in spring. When the growing season starts and often the heating is still on.

A practical precaution which is almost a necessity, is to place humidifiers on your radiators. This will prevent the air becoming too dry.

Feeding Bonsai

Besides taking water from the soil, the roots also absorb the nutrients needed to feed the plant. It is quite obvious that the small amount of compost in a bonsai tray is inadequate to supply enough nutrients to ensure the development and survival of the plant. So regular feeding with fertilizer is vital.

Which fertilizer?

feeding bonsai with organic pelletsIt is hardly surprising that the bonsai tradition frowns upon the use of synthetic fertilizers, particularly liquid fertilizers and mineral-based powders. Apart from any consideration of dis-missing them as “modern”, it should be noted that bonsai trees derive most benefit from slow-acting fertilizer, which means an organic fertilizer with a slow decomposition rate.

An ideal bonsai fertilizer contains about 50% nitrates, 30% phosphate. And 20% potash. It may be based on bonemeal, fishmeal, powdered horn or dried blood.

It may come in powder form and be sprinkled over the surface of the soil and raked in, or in the form of pellets, which are simply placed on the surface and absorbed by capillary action. The traditional preference is for pellets, but these cannot be used if the surface of the soil has a moss covering, which the pellets may “scorch”.

There is no single rule about fertilizing. It is a little like watering, as amount and frequency depend entirely on the species of plant and size of tray. You should bear in mind that too much fertilizer does more damage to the plant than too little. Fertilizers are not intended to make bonsai grow, but to ensure their survival. Too much fertilizer may counteract the effort to dwarf the tree, besides which it may “burn” the roots and lead to the death of the plant.

Feed your plants during the tree’s growing season, that is, from spring to autumn. Do not give flowering or fruiting trees any fertilizer until after they have flowered. For deciduous trees, continue feeding until the leaves drop, but conifers should not be given fertilizer after mid-autumn. If you use powdered fertilizer, two doses (two teaspoonfuls) per month should suffice. If you fertilize with pellets, wait until they have dissolved before replacing them and do not put the new pellets in the same places as the old ones. Always place the pellets about half way between the edge of the container and the trunk. Make sure, however, that they are slightly nearer the trunk, as they can damage the roots of the tree. Do not bother to feed in winter, when the roots absorb very little nutrients.

A foliar feed could be added to the water used to mist over the foliage. However, always use your discretion when tempted to use products advertised commercially for house plant care. Some products, such as leaf polishes, can prove harmful to certain types of plant. Do not confuse bonsai with indoor plants. Even though they are grown in rather similar ways, they need different care. Finally, note that indoor bonsai should not be given any fertilizer for three months after they have been repotted. With bonsai, you must be patient.







Promotes leaf and branch development, and growth in general.

Phosphoric acid


Enhances root and cell tissue development: regulates the reproductive activities (flowering and fruiting).



Promotes production and circulation of sap, flowering and fruiting.

24. March 2012 by Dave Pinkney
Categories: Bonsai, Bonsai Care | Tags: , , , , | Comments Off on Bonsai Growing: Caring for Bonsai Trees


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