When and how to prune an evergreen

Janet K. Larkin
Frederick County Master Gardener Program

If you know the mature size of a plant before you plant it so pruning rarely becomes an issue. Many evergreen species have a large number of cultivars bred to reach a wide range of sizes. Shop carefully, ask questions and consult reference books or Internet sources.

Remember, all plants grow! Some very quickly, like the infamous leyland cypress, and others extremely slowly, like the venerable Japanese white pine.

For those of us with plants that need some reining in, there is a basic rule of thumb for ever-greens: Sun-loving conifers, those whose needles all need sun, can be pruned annually, but only gently within the living green foliage. This type of ever-green does not have green needles in the interior of the plant. Most do not even have growing buds on interior branches. As a result, if you prune that plant hard, into old wood, once it has become extremely over-grown, you will end up with an ugly brown bare spot in your tree or shrub. Most junipers and arborvitae fall into this category.

Some shade-tolerant ever-greens are hemlocks (tsuga), yews (taxus) and plum yews (cephalotaxus). These are readily prunable because they have interior growth buds and will produce new growth when severely cut back.

As for timing of pruning, there are a few factors to consider. First, diseased, dead or dam-aged branches should be pruned out at any time. Second, pruning stimulates soft new growth, which is easily damaged by cold temperatures and harsh winds. Therefore, if pruning is done too late in summer (before new growth has a chance to harden off) or too early in spring (when growth starts before the harsh winter ends) the tender new growth can be damaged. Third, some evergreens produce a large amount of sap when actively growing, typically in early spring and sometimes with another burst of growth in summer. Do not prune at these times because the plant will "bleed" profusely.

Now before you make your first cut, take note of how the plant grows. For instance, pines grow, from "candles" or bunches of new needles at the tips of branches. The best way to prune pines is to pinch back the candles by 1/2 to 1/3 in late spring (early June). This will keep the tree neat but natural looking and not give it a sheared look.

If you are growing an ever-green tree, also consider its mature size and shape before pruning. Many species look best with a skirt of foliage to the ground, so the lower branches should be retained. Others such as the lacebark pine (Pinus bungeana) have beautiful bark but would look strange, like giant lollipops, if limbed up too high, so prune out only the lowest branches to show off the bark without spoiling the mature form of the tree.

A common plant used for quick screening is the leyland cypress tree. The problem with them is they can eventually grow to over 100 feet. If kept in healthy condition, they can be pruned to keep them at a reason able height and width. Like most hedges, they will look better if pruned with hand pruners as opposed to electric hedge trimmers, which damage branches and leave more dying foliage exposed to view.

Don't prune your evergreens in late winter. Do some pruning now if you can stand the cold and use the trimmings for holiday decorations. For the remain der of your evergreens, wait until late spring. The arborvitae will probably never make a great topiary if it is already over-grown. If it is still small enough you may try pruning, but remember, cut only into the green areas of the shrub.

Read other articles about tree care

Read other articles by Janet Larken