The methods explained below are for outdoor-grown bonsai. However, there is a section at the bottom of this page on indoor bonsai.
Most trees and shrubs can be grown as outdoor bonsai. The act of restricting growth causes the leaves to become reduced in size. Naturally small-leaved plants such as azaleas lend themselves most readily to bonsai, but bear in mind that their leaves will become smaller still. Scot's pine (Pinus nigra), larch (Larix sp.), Cercis sp, maidenhair tree (Ginkgo sp.), Lonicera nitida, yew (Taxus bacata) and Japanese maple (Acer sp.) are all good for starting your collection off.
How to care for outdoor bonsai
Branch pruning and training bonsai
The pruning of bonsai is very specific to individual species of tree. In the beginning, a young bonsai would require pruning about once a year in order to form your ultimate shape. Once established, plants require pruning and trimming several times a year to keep them balanced and in shape.
- Deciduous trees can be pruned at any time – as their style and shaping dictates – but major work is best undertaken from late winter to early spring
- Major work on conifers and other evergreens should be undertaken in autumn or winter, but they can be lightly pruned at any time of year. Pines are an exception: twist off two thirds to a half of their 'candles' (new shoots) as they lengthen in late spring
- Japanese maples should be hard-pruned in autumn or early winter, not spring as cuts may 'bleed' sap. Prune shoots as necessary during the growing season while they are still soft
Branch training and shaping is an art form in itself. Branches are coerced into many different arrangements by using wire wrapped along each branch and then moved into place.
Root pruning and repotting
Root pruning and potting on encourages the tree or shrub to produce fine roots, which take up water and nutrients more readily. It also allows the growing media to be replenished. These tasks should be carried out at two- or three-year intervals; with slow growing trees like pines needing root pruning every three-to-five years and rapid growing trees like birch requiring annual attention.
- Re-pot and root prune in spring, ideally before new growth resumes. However, autumn is suitable in mild and sheltered locations
- Remove the tree from its container. Be aware that the tree can be attached to the pot with wire for stability so this may need to be removed
- Loosen the compost and discard the spent compost. There are specialised small rakes that can be used for the task. Likewise, rinsing the soil off with a hose reveals the root-system for ease of pruning
- Untangle any long roots and then remove with sharp scissors
- You are looking to leave a neat and trimmed root-ball that is flat and circular
- Place back into its container and use a pencil or chopstick to gently work the compost in and around the roots
- Water well and keep in a shady, sheltered spot to reduce additional environmental stresses until it has firmly re-established
Some specialised books have this process set out with pictures that allow you to see what the finished pruned roots look like. See The complete practical encyclopedia of bonsai by Ken Norman (Lorenz Books 2009, ISBN 9780754821809). The book is made available through the RHS Lindley Library
There are different, specialised compost mixes available for bonsai. However, you can make your own mix:
- Two parts (by volume) peat-substitute potting compost
- Two parts sharp sand or lime-free alpine grit
- One part John Innes No 3 (omit this for eriaceous as it contains lime and use one part more of potting compost instead)
- Mix well and, if desired, add a slow-release fertiliser such as Osmocote at the manufacturers recommended rate
This mix was taken from an RHS Wisley Handbook, Bonsai by Jon Ardle (Cassell Illustrated/Royal Horticultural Society 2003, ISBN 1844030199). This book is available from the RHS Bookshop and the Lindley Library.
Watering and feeding
Bonsai are grown in small, often shallow containers, which restrict the quantity of nutrients and water available to the tree. The compost is also prone to drying out during warm and windy weather.
- Water as and when needed – in summer this can be once or twice daily. The overall aim is to keep the compost just moist
- Check throughout the year as a dry week in winter can catch you out
Ideally use rainwater as this reduces the build up of minerals in the compost. Use a small fine rose to apply the water gently and gradually so that the tree is well watered, as heavily applied water can run off.
Slow release balanced fertiliser can be added when re-potting. Then, once this is spent, apply a balanced liquid feed at the recommended low dose once every two weeks during the growing season.
Over feeding can result in excessive growth prone to pest and disease.
Bonsai is a vast and intricate subject full of history and art. With endless possibilities all you need is patience. To find out more there are many books on the subject and lots of societies small and large up and down the country (see the Federation of British Bonsai Societies (FOBBS) website). Likewise, bonsai nurseries not only offer all the plants and equipment you could need by they also offer demonstrations and talks on the art of bonsai.
Indoor bonsai are rather like houseplants. They are usually tropical or semi-tropical species and require lots of light, constant temperature and humidity, along with regular watering (as for outdoor bonsai). Depending on the species, some will lose a few old leaves in the spring to make way for some new ones. Plants regularly used are Chinese elm (Ulmus parvifolia), silver jade plant (Crassula arborescens), weeping fig (Ficus benjamina), celtis (Celtis bungeana) and olive (Olea europaea).