I was discussing various horticultural topics with a group of gardeners recently when the topic of pruning came up. While most agreed that deciduous trees, including fruit trees, are best pruned during the trees' dormant period in winter, there was some misunderstanding about when to prune conifers such as cypress, juniper, pine and yew. Evergreen plant pruning has had less study than deciduous plant pruning, but we still know quite a bit about evergreen responses.

Like most deciduous plants, many conifers have only a spring or early summer growth flush. It's best to prune just before this spurt of growth in most cases. If you wait until after the growth flush to prune, you weaken the tree because there will be fewer leaves on the tree in summer, fall and winter. Leaves produce sugars (plant foods), and are therefore critical, especially for young, developing trees. Pruning after spring growth has a dwarfing effect on young conifers. Since the needles you remove would be producing some sugar during mild days in winter, the dwarfing process may even be greater than you anticipate. If your goal is to keep the conifer small, you may want to prune just after spring growth.

Another good reason to prune just before spring growth is to allow new growth to cover the newly pruned branches as soon as possible. In early spring, just a few weeks are needed to completely cover a pruned plant with new foliage. If you prune in late summer or fall, it will be several months before new growth covers the unsightly, leafless inner branches that pruning sometimes exposes. This is a mistake that is commonly made with shrubs such as junipers or arborvitae.

Heavy pruning may cause certain conifers, especially old trees, to decline and die. Many species, like pines, do not have living buds in old branches. In such cases, if you cut a branch back to old wood with no living shoots, no new shoots will grow from the part left and the branch will die. Except in species known to have live buds in old wood, like redwood and juniper, you should prune the branch off entirely, or cut back only to living leafy branches that can be invigorated by the pruning and grow. Be sure that the branches you are cutting back to have healthy terminal buds, or they may not grow either.

Monterey pine is a good example of a conifer that tolerates heavy pruning. In Christmas tree plantations, Monterey pines are sheared at least twice a year to achieve the proper shape and size. However, the shearing is done on the ends of new branches, and not on old wood. The Japanese black pine also tolerates pruning, and they are sometimes maintained as small to medium sized specimen trees by careful yearly pruning.