Bonsai

Q. I love bonsai and bought a little one but I have no clue how to grow them!

A. What follows is a general outline of the various procedures used by the staff of the Arnold Arboretum to maintain the Larz Anderson bonsai collection in a healthy condition.

REPOTTING: The smaller the pot the more frequently the plant needs repotting. This procedure is best done in early spring mid-to late March before the plant shows any signs of growth. The plant is removed from its container and approximately two to three centimeters (one inch more or less) of roots plus their attached soil are removed all around the sides and bottom of the root ball. Any roots thicker than a pencil are cut away to encourage the development of small feeder roots. This process effectively rejuvenates the root system of the plant and prevents lethal "girdling" roots from forming. After the root ball is trimmed the plant is returned to its original container surrounded by fresh soil. The large hinokis are repotted every four to five years while the smaller plants are repotted every two to three years.

SOIL MIXES: Plant roots are so intimately involved with soil particles that it is best to think of the soil as part of the plant itself. As such a great deal of time and care needs to go into its preparation. In general the potting mix should provide the plant with a balance of water retention and air circulation. Our repotting mixes consist of coarse sand (particle size 1-3 mm) peat moss or leaf mold and screened loam in various proportions depending upon the plant being grown. In general we use a mix that is one-half sand one-quarter loam and one-quarter peat for the conifers; and one-third sand one-third peat and one-third loam for deciduous trees. In either case small amounts of superphosphate and organic nitrogen fertilizer are added to the soil mix.

PRUNING: There are no universal rules about how much to prune a bonsai; the techniques vary according to the species being worked with. In general the best time to prune is when the plants are producing new growth-in early spring for deciduous plants such as the cherries and Japanese maples in mid-spring for pines and spruces and in early to midsummer for the junipers and the hinokis. Generally at least 50 percent of the new growth is removed at the time of pruning. If the plant produces a second flush of leaves later in the growing season these also require pruning.

With pines the number of candles is thinned out by one-half to two-thirds and those that remain are shortened.

With spruces and firs the newly flushing shoots are pinched back to half their length inducing replacement buds to form at the base of the new growth rather than at the tip.

With maples the new shoots are pinched back to a maximum of two pairs of leaves and sometimes only one pair. Any vertical-growing shoots are removed or are wired into a horizontal position.