Native to Australia, eucalyptus trees can form impressive landscape giants or perform as elegant backyard shrubs. The mature size of eucalyptus trees varies by type; the silver gum eucalyptus can reach a height of 50 feet, but can be grown in shrub form at 6 to 8 feet tall. However you want to prune your plant, there's only one suitable time frame for trimming.
You can prune your eucalyptus trees in the late winter or early spring while the trees are in their dormant stage. The exact time frame varies by your location. Any time before the tree resumes spring growth will be fine for pruning.
Since eucalyptus trees can grow very tall, the easiest time to prune them is when the trees are young and you can reach all the branches easily. At this time, you can use a range of pruning techniques designed to keep the tree manageable for home gardens or you can prune minimally to let the eucalyptus reach its maximum height if you have a large enough landscape to accommodate the tree. When the trees get too tall for you to prune, you'll need professional help.
Carry out this technique for young trees if you want a large tree. Identify the central leader or main trunk and clip off the offshoots on the bottom one-third of this branch. Cut the shoots on the next third in half, leaving short shoots, and leave the shoots at the top third of the central leader to grow, unless they're diseased or damaged. Next year, clip off the shoots on the middle third that you cut back the year before, since they're now on the lowest part of the central leader. Again cut the middle shoots in half and leave the top ones alone. Repeat this annually to train your tree to have a strong branch structure and lean trunk.
Coppicing and pollarding give you control over the tree's size. To coppice, allow your tree to grow all season, then cut all of the stems down to a height of 2 to 3 inches in spring, giving you a shrub rather than a tree. To pollard eucalyptus, leave the main trunk alone and cut all branches back to nubs coming off the main trunk. Repeat your chosen technique annually in the dormant season to maintain your tree.