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Care of indoor bonsai

Indoor bonsai have become extremely popular over recent years and have gradually developed from the original cultivation of outdoor trees collected from the wild.

Indoor bonsai are miniature versions of mainly sub-tropical and tropical trees and shrubs, whose native climate is hot and often very humid. Most species can be adapted to live inside a house or heated glasshouse, but a normal house environment though does not entirely correspond to the natural environment of the tree growing in the wild, even though they may initially look healthy.  Central heating is the worst culprit as it can produce a very dry atmosphere and most indoor bonsai find this too dry and not humid enough.

Many tropical plants are suited to an indoor climate because in their native countries there are scarcely any seasons and little variation in temperature.  Nevertheless, they still often require very moist and humid conditions.

Other plants like the Olive, Pomegranate and Myrtles grow in the Mediterranean regions and are classed as sub-tropical.  They require some humidity, frost protection and a dormant period in winter.

All plants with small leaves and branches that are not too woody are particularly good for training as bonsai.

General care

  • Most indoor bonsai require plenty of light, water and air during the summer.
  • Always shade the bonsai from direct scorching sunlight and feed once every two weeks.
  • Tropical indoor bonsai should be kept in a temperature of between 18°C - 30°C during the day and 16°C - 20°C at night. 
  • Sub-tropical indoor bonsai require cooler summer temperatures of 15°C - 26°C and winter temperatures between 8°C - 15°C.  Because they undergo a dormant period unlike tropical plants they often drop their leaves during the winter, and may look dead. They need to be kept just moist, but should not be allowed to dry out at any time.

Pruning

  • Sub-tropical indoor bonsai need regular pruning every month or so during the period March to October, especially when it is very hot and humid.
  • Tropical indoor bonsai, if kept in the house, will require pruning every month throughout the year.
  • All shoots need to be pruned back to two or three nodes or leaves which will encourage the plant to produce twiggy growth and back budding to produce new branches.
  • Older bonsai need only be pruned back after flowering, and to shorten back any long growths.

Wiring

  • Wiring of branches can be carried out at any time of the year but only wire shoots from late summer once they have become mature and slightly woody. Many indoor bonsai have very brittle and delicate trunks and branches. Great care must be exercised in wiring these and may be best done in the spring when a greater amount of sap is rising.
  • Wire should not be applied to flowering shoots.
  • DO NOT wire too much of the tree at any given time because this will over stress the tree and may kill it.

Location

  • All indoor bonsai require a cool airy position all year round with diffuse bright light facing north, east or west but if kept on a window sill, must not be left between the curtains and window in winter.
  • If light levels become too low in winter supplementary lighting may be required.
  • Most bonsai can be kept outdoors during the summer in semi-shade from late spring till early autumn, but keep a check on watering if it is very windy.

Watering

  • During spring, summer and autumn water demand will be a great deal more than in winter, and it is often beneficial to site the bonsai on a tray of pebbles or pea gravel in a dish beneath to help maintain humidity.
  • During the summer especially the compost can be allowed to dry out a little before watering, but it should never be allowed to dry out completely.
  • If bonsai become very dry the plant must be totally immersed in a tub or bowl of water until the air bubbles stop emanating from the compost, usually for up to 15 minutes at least. Remove and allow to drain.
  • If possible always use soft water (usually rainwater).
  • Always use water at room temperature and never water the bonsai out in the full sunshine as this may well produce leaf scorch.

Feeding

  • It is important to maintain nutrients to indoor bonsai and feeding should be carried out during spring to autumn using a liquid feed when watering every two weeks.
  • It is always better to use too little than too much fertilizer particularly when using inorganic feeds. DO NOT over feed with inorganic fertilizer as this may cause root damage.
  • Using organic fertilizers such as pelleted chicken manure is very beneficial to the tree as no scorching of the foliage and roots can occur.
  • Ensure the bonsai is well watered beforehand.
  • Young and fast-growing trees need more feeding than old and slow-growing species in order to thicken the trunk and build new branches up.
  • DO NOT use the same fertilizer all through the growing season - try to alternate, and use a phosphrogen based fertilizer in the autumn.
  • DO NOT feed bonsai shortly before and during flowering, after re potting, after root pruning, during the dormant period and if the tree is in an unhealthy condition.

Potting and repotting

  • Most indoor bonsai require a loam-based compost made up of a combination of loam, sieved peat free compost and sand or grit.  The mixture below is generally adequate for most species:

    Two parts loam, (John Innes potting No 2 or 4 is adequate)
    or Akedama
    Two parts peat free compost or leaf mould
    One part sand or grit
    Slow-release fertilizer granules (optional)

  • Acid loving species like Gardenia, Azaleas and Rhododendrons need an ericaceous compost which has a much higher ratio of peat to loam:

    One part acidic loam or pure Kanuma compost
    Four parts moss or sedge peat, (ericaceous compost).
    Two parts sand or grit
    Slow-release fertilizer granules (optional)

  • Generally these mixtures should be adequate for about two to three years but calcium- or rain-free water must be applied as much as possible when watering
  • Repotting of established indoor species should be carried out every two to three years and very young vigorous plants, every year.  If the roots are pot-bound they should be teased out carefully before trimming.  Trim off at least one-third to one-half of the roots only.
  • The same bonsai pots can be used for very old and mature indoor bonsai but a larger one may be required for young and fast-growing plants, which require further growth and training.
  • When choosing a new pot ensure that it has drainage holes in the base and that the tree looks to scale in the pot.  Also position the tree correctly in the pot.

Pests and diseases

  • A number of pests and diseases attack indoor bonsai and it is important that every plant is examined closely every week to ensure that no pest or disease is becoming established.
  • If the plants are well looked after and healthy they will be much more able to resist attack by most pests and diseases.
  • Indoor bonsai can often be more severely attacked than outdoor bonsai because they are kept in a much more protected environment.
  • Always use a biological control where possible instead of chemical treatments which may also kill many beneficial insects.  This is as effective and safer, but often takes a little longer to work than chemicals.
  • The most commonly found pests are:
Vine weevil (adults and larvae) Biological control Nemasys-H or 'Provado' for larvae
Leatherjackets (larvae of cranefly) Use soil drench of 'Provado'
Aphids (Greenfly and blackfly) Apply a soap solution or insecticide
Scale insects Scrape off with a knife and apply insecticide
Woolly aphids Spray with 'Provado' or Fenitrothion
Mealy bug  Use 'Provado' or Methylated spirits
Root aphids Use a 'Provado'soil drench
Red spider mite Spray with insecticide or use 'Provado'
Whitefly Biological control (Encarsia formosa).
  • For small infestations water spray or a solution of soapy water often controls aphids and red spider mite.
  • Diseases are just as damaging as pests and need to be identified early on so control measure can be taken.  The most common diseases are:
Root rot Remove all dead roots and pot plant into new soil and do not water for at least 2 days. Water with fungicide.
Powdery/Downy mildew Spray with Nimrod T or other systemic fungicide
Rusts Spray with Nimrod T, Copper fungicide or systemic
Botrytis mould Spray with Nimrod T, Copper fungicide or systemic
Chlorosis Evident on acid-loving plants where a lack of iron causes yellowing of the leaves. Treat with chelated iron.
Damping off of seedlings Soil drench compost with Chestnut compound, Dithane, Sythane, Copper fungicide or 'Filex'.
© 2011 Colin Carpenter (revised June 2011)
     

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