Controlling eucalyptus
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.
The Biking Gardener 
Last year I planted a new variety E. ‘France Bleu’. Final estimated height 3-4m! Gorgeous dainty foliage. It’s grown well but I’m hoping it will stick to its estimated height!
Yes, I am very curious about this one. It is impossible to propagate eucalyptus from cuttings so, in the past, there were no cultivars. So I am not sure if this one is micropropped or if it is a seed strain. It is certainly a nice-looking plant and I will add one to the garden when I see one again!
I found a tiny seedling under it this spring which I have potted up. My neighbour has a huge E. gunii which sends debris here especially during storms. I fully expect it’s one of those, although I’ve never previously found seedlings. My new one did have a few flowers last year, so, here’s hoping.
Yes, large trees can be very messy, dropping bark and leaves all year round
Their size has always put me off planting one and I am happy to admire them in places like Kennedy Park. A small one, as Meriel mentions, sounds attractive.
I love them but dread it if neighbours plant them as most people let them get too big. I don’t think it is widely known that stooling will not kill the tree and it will keep it in check.
Actually . . . I coppice and polard eucalyptus annually; which are two techniques that arborists (here) do not condone. I do so because the juvenile foliage is what is desired, and also because I want the aromatic foliage of blue gum, which is a species that I do not want to grow into a full sized tree. Blue gum is what gives all eucalypti here a bad reputation. I recommend it ONLY to those who will do it properly and very regularly. If I ever stop doing it, the blue gum will need to be removed completely. It can not grow into a tree where it is at.