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Most fully hardy species tolerate having their pots frozen solid for short periods at a time. Pines and spruces even seem to thrive on this treatment, preferring it to any form of protection at all, other than a temporary wind break. But the ravages of winter can take their toll on most other species in a number of ways.

All the effects below can damage a large bonsai but it should survive. Medium-size bonsai - say between ten and eighteen inches tall - can be severely damaged, possibly fatally, while small bonsai are very vulnerable indeed.

Constant freezing and thawing of the pot every day can devastate a root system, especially if the pot never entirely thaws before re-freezing. The root tips absorb moisture but cannot pass it through the frozen roots in the centre of the pot. On re-freezing they expand and burst, just like a water pipe. They can also be crushed by the pressure caused by the expansion of rapidly freezing soil in a container.

Remember a waterlogged soil expands more on freezing than a comfortably moist one. Trident maples and Chinese elms have thick, fleshy roots which are particularly at risk. Even large bonsai of these species should be placed in winter confinement.

Although roots are inactive most of the time during winter moisture is still lost through the hark. Under normal circumstances the tree would contain enough reserves and the roots could replenish the supply during mild weather. Expanding buds also demand a supply of moisture which frozen roots cannot provide. Sunny days can induce both transpiration and bud activity, but may not succeed in thawing the roots as well.

Cold, drying winds can desiccate fine twigs. Frozen roots cannot replenish the moisture so the twigs die back. Fine-twigged species like birch and Japanese maple are particularly vulnerable, and stiffer from the same problem in the wild.

Wind and sun can cause pots to dry out even in winter. Although this may not affect the tree directly it will kill the finer roots, putting the whole system at risk of decay and more vulnerable to damage by wet, freezing conditions.

Saturated soil not only increases the risk of frost damage, it also creates the ideal environment for root rot. There is not such a risk if it is raining every day, but if waterlogged soil is left to stand, the risk is increased dramatically.

Having filled you with dismay at the thought of all the dangers your bonsai have to face, let me assure you that by taking a few simple precautions vim can guarantee them a long ;mil happy life. Although the worst of the weather conies after Christmasin the northern hemisphere, it is as well to have the protection complete by the end of Noventher. karly damage compounded later.

If you are not sure about the hardiness of an unusual species then it is as well to keep it in a cool conservatory or unheated room through the winter.

This depends on the species and the size of the pot. (A three-foot-tall literati in a ten inch pot is a medium sized bonsai as far as climate is concerned). As a rough guide, a tree whose pot is less than eight inches diameter and two inches deep, or the equivalent volume, can be classed as small. Pots up to ten inches in diameter and three inches deep can qualify as medium size. Anything over this is large. The table provides a basic guide. By early spring you can begin to re-introduce your bonsai to their benches, tucking the smaller ones up at night during exceptionally cold weather.