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Care of outdoor bonsai

Bonsai training is an ancient art which has been practised for thousands of years mainly by the Chinese and Japanese and is now extremely popular with many people practising this ‘living’ art in its many forms.

Training bonsai takes many years and is achieved by pruning and wiring of roots and branches to maintain the bonsai in a ‘restricted’ condition, and to produce a tree which creates a feeling of great age and beauty.

General care

Outdoor bonsai have different cultivation requirements to indoor bonsai and do not require the same winter protection.  Nevertheless outdoor bonsai need shelter and protection from severe penetrating frosts and excessively drying winds or intense sun.

Certain outdoor bonsai benefit from shelter inside a cold glasshouse or frost-free place during the winter and include all the Japanese Maples, Trident Maple, Chinese Elm, Evergreen Oak, Holly and Box.

Japanese Maples and some other broad-leaved deciduous trees also require protection from intense sun, since leaf scorching can occur very easily.  Place these trees in some degree of shade during the summer months.


All outdoor bonsai require pruning or pinching out during the spring and summer to maintain and develop the crown shape and foliage pads.  This also has the effect of encouraging new shoots to develop and may be trained in as new branches over the next few years.

Broad-leaved deciduous trees require pruning after the first flush of new growth in about June, and again in July and August depending on vigour. Thinning of the whole crown can also be carried out at the same time.

Evergreens like Holly, Box and Rhododendrons usually require less pruning but still need some trimming and thinning.

Conifers like Larch require general trimming and thinning in June and July, Junipers require steady pinching out to develop the foliage pads and Pines require pruning of the most vigorous‘candles’ in June/July back by two-thirds.  This encourages new shoots to develop at the base of the shoot called back budding, and helps thicken up the foliage.


This operation is usually carried out in the spring before the leaves appear, and when the sap is rising and the branches are supple and easy to bend into the required positions. The wire is often left on for about six to eight months depending on vigour or until the branch swells.  The wire must then be removed before ‘scarring’ occurs.

When training three to four year-old trees, the trunk shape must be set before the side branches are wired.  This operation usually takes a year to complete, and once set the side branches can be wired in one go.  Always use one piece of aluminium or copper wire to set two branches at once.  This saves on wire and later it can be carefully removed and used again. Do not wire too much of the tree in one go because this will over-stress the tree and may kill it.

Once the branches have set remove all wire except where new branches are still being trained into position. If there is some ‘scarring remove all wire and rewire in the opposite direction about 9 months later to continue setting. Guying down of the branches can also be carried out to prevent scarring.

Watering and feeding

During spring the bonsai must be checked daily to see how dry the compost is and thoroughly soaked if very dry.

In about May start application of liquid feed through the hose system or by watering can and continue throughout the summer, once every two weeks.  Use a liquid feed like ‘MiracleGro’ or Levingtons feed, which have relatively low nitrogen analysis or a balance of NPK. Slow release fertilizer pellets can also be placed around the base of the tree and pushed into the compost. These break down slowly and release feed throughout the year. Organic fertilizers such as pelleted chicken manure can also be used and breaks down and feeds the tree slowly.

All bonsai are extremely susceptible to drying out in hot, dry and windy weather because of the small root system space, and must be watered daily or twice daily in summer. It is advisable to water in the late evening when there is no evaporation, and leaf scorch is kept to a minimum.

In September switch to a high potassium and phosphate and low nitrogen fertilizer such as Phostrogen or Chempak No.8 which will reduce any sappy growth and help to harden off the branches ready for the winter.  Continue to water daily as and when necessary.

In winter under normal conditions, there is usually enough rain and dampness in the air to render watering unnecessary. However, if the weather is exceptionally mild and dry, water should be applied to thoroughly wet the compost about once a week. Do not water in very frosty weather as this may cause root damage.

Potting and repotting

Established outdoor bonsai require repotting every three to five years to maintain health and vigour, and before this operation is carried out all the roots must be teased out and at least one third of the roots trimmed off.  This encourages the regeneration of fibrous roots which will eventually fill the pot completely.  No feeding will be necessary for the first year after repotting.

Newly trained bonsai must have an adequate fibrous root system before potting and at least one third of the roots may need to be trimmed so that the root system fits into the required pot.  If  being potted into an unglazed or glazed bonsai pot, the tree should be positioned correctly and wired in to give initial stability.

The plant at this stage can be raised up further in the pot if so desired to expose more of the roots.


For broad-leaved deciduous trees and evergreen trees (such as Beech, Hornbeam, English Oak, Maples, Holly, Box and Evergreen Oak):

  • Two parts leaf-mould or peat compost alternative
  • Two parts clay loam or Akedama
  • One part sharp sand or grit

For conifers and Ericaceous plants:

  • Three parts moss peat, or acidic peat alternative
  • Two parts sandy acidic loam or Kanuma
  • One part sharp washed sand or grit

A slow release fertilizer can also be added to the compost at this stage if so desired.


In the early stages of training, young bonsai can be grown in ‘trainer’ pots or large trays which help the root system develop in the right way.  At a later stage they can be repotted into a proper glazed or unglazed bonsai pot which must be frost-resistant.

It is important that the pot is of the right type and colour so the tree looks in the right proportion to the pot and it must also be located so that it is balanced.

All bonsai have a ‘front’ and a ‘back’ and the plant and pot must be viewed from the front as one (especially for exhibition purposes).

Pests and diseases

All bonsai are susceptible to pests and diseases like any other plants and will show the same symptoms on the leaves, flowers, stem or roots.

Bonsai plants must be inspected regularly for any signs of pests and diseases, and must be treated with the appropriate insecticide or fungicide as soon as it is detected.  In most cases, pest like aphids, woolly aphids, mealy bugs and scale insects can easily be seen and should either be removed physically by hand or sprayed with a soap solution, ‘Malathion’ or a systemic insecticide.  Repeat treatments every month.

Other problem pests include vine weevil which occurs in the soil as small white grubs and eat the roots of seedlings and established trees.  These must be controlled as soon as they are seen either with a root drench of Gamma HCH or ‘Provado’ once a month or a biological control such as Nemasys-H.  The adult vine weevils eat leaves by cutting out holes and semi-circles, and are best caught at night on the plants and destroyed.

Another soil living insect is the leather-jacket which is the larvae of the crane fly.  Again use a soil drench of Gamma HCH or ‘Provado’ once a month if seen in the soil, or pick out by hand when repotting.

Fungal diseases such as powdery and downy mildews, wilts and rusts tend to occur on the leaves of deciduous trees and occasionally on evergreens, and should be treated with a systemic fungicide as soon as it is detected.  Also maintain a humid atmosphere around the plant by regular watering and spraying which helps to control this disease.


The site provides information on the history of bonsai, different species and how to propagate them, and details of workshops on the art of bonsai growing and the care of bonsai, trees for sale, and of a re-potting service.
©2010 Colin Carpenter