I am often asked how to keep eucalyptus within bounds. Most species, and certainly those that have a chance of being hardy here (and in the UK) are large trees. Eucalyptus regnans, the Australian (Tasmanian) mountain ash – not to be confused with ‘our’ mountain ash, (sorbus) – is the world’s tallest flowering plant (angiosperm) and possibly the tallest tree in the world, frequently 100m and exceptionally 130m high. Unlike many eucalyptus, it does not produce the characteristic ligotuber, a knobbly swelling at the base, that allows eucalyptus trees to regenerate after bush fires.
This capacity to produce shoots from low down is common to the genus and this means that they respond well to being stooled – or cut down to the base.
Eucalyptus are strange plants in so many ways. They do not form resting buds like most of our native trees and they have both juvenile and adult foliage. To most eyes the juvenile foliage is the most attractive and it is usually rounded, in pairs and silvery blue. When the tree reaches adolescence it starts to produce adult leaves which are alternate and usually sickle-shaped, and often greener. Regular pruning retains the juvenile foliage, no matter how old the plant is, but you lose the beautiful bark.
The most commonly cultivated species is E. gunnii, the cider gum. It is often planted as a shrub but, if not pruned every few years it will quickly become a tree. But fear not, even if it has got out of hand it can be pruned, even with a chain saw, and it will bounce back. The best time to do this is spring. This is so the new growth, which will be lush and vigorous, has time to mature and ‘toughen up’ before winter cold. Of course you do not have to leave things this long and it is better to hard prune in spring every year or two if you want a bushy shrub with juvenile foliage rather than a tree.