When is the best time to prune evergreen shrubs? That depends on whether you mean needle-bearing or broadleaf types. Examples of the needle-bearing kinds include:
But another class of evergreen shrubs are those in the broadleaf group, which includes:
If you want to research the particular shrub that you grow before deciding on the best pruning time for it, find it in the Plants Database.
When to Prune Evergreen Shrubs
In general, prune needle-bearing evergreen shrubs in early spring, toward the end of dormancy and before the emergence of new growth. Pruning the evergreens at this time allows plenty of time for new growth to emerge, as well as plenty of time for these new shoots to harden off before the following winter.
But you will often wish to treat broadleaf evergreen shrubs (and some needle-bearing varieties) differently. While, technically speaking, you may treat broadleaf evergreens in the manner described above, there are often reasons not to treat them as you would their needle-bearing comrades.
For instance, if they are flowering shrubs (that is, plants valued for the display put on by their flowers), you will want to wait until after the flowering period to prune. Otherwise, you will miss out on at least some of that year's blooms (the more flower buds you remove in the process of pruning, the greater the loss will be). And what is the point in growing such a bush if you are not going to be able to enjoy the flowers? For example, pruning azaleas and rhododendrons after they are done flowering makes much more sense than pruning them beforehand.
With some broadleaf evergreen shrubs, you will have a difficult call to make. There may not be just one "right" answer. For example, when should you prune holly shrubs? During the dormant season (winter) is one acceptable answer, especially for holly bushes valued for their berries, since many of us tend to trim off some of the branches for use in indoor displays during the holidays. But this can reduce flowering —and consequently, berry production—for the next cycle. So a compromise may be for some varieties of holly: prune in December every other year.
It is essential to inform yourself as much as you can about the particular plant you have before you take that pair of pruners to it. For example, you may look at a mugo pine and, seeing a short, shrubby-looking plant with needles, conclude that it is one of the needled evergreen shrubs. In fact, despite its short stature, it is a type of pine tree, and the best time to prune it is after its candles have formed in spring, as with other types of pine trees.
Hedges: a Special Case
For evergreen shrubs (whether broadleaf or needle-bearing) that comprise hedges, you may want to prune after their new growth has emerged in spring. It is, after all, mainly the new growth that affords an opportunity for shaping (assuming, of course, that you have been maintaining the hedge all along). If the branches that you will be trimming are small enough, you can shear them off with a power tool known as a hedger. Among needle-bearing evergreen shrubs, hemlock is my top choice for hedges, because it likes being pruned and has nice foliage that can become quite dense (making it a good choice to create privacy in your yard).
How to Prune Evergreen Shrubs
How you do the actual pruning will depend partially on the bush in question and partially on your landscaping goals. Consequently, you should treat the following tips as general guidelines, to be modified on an as-needed basis:
As a general rule, you should not be pruning broadleaf evergreens heavily. Sometimes, they will need no pruning at all. Other times, pruning the tips of the branches lightly will be sufficient. Be careful to make your cut just above a leaf bud, so that you are not left with an unsightly stub.
Likewise, many types of needled evergreens will not generate new buds on old wood. That is, they do not produce so-called "dormant buds." So unless you leave some green on a branch when you prune it, you are signing that branch's death certificate. This is usually true of juniper, for example. Yew is one of the exceptions.
Shearing is often the order of the day for needled evergreen shrubs. Yew bushes, for example, you can shape and keep within certain dimensions by shearing it with a hedger.
Feel free to take off diseased (or outright dead) branches at any time, whether on a needled or a broadleaf evergreen.
When it is the right time to prune (see above), trim off any branch rubbing against another. Likewise, remove any branch "sticking out like a sore thumb"; such branches mar the appearance of the plant.