Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
Most evergreen shrubs are better off pruned in early spring just before new growth begins. When pruned toward the end of dormancy, evergreens will sprout growth to hide pruning scars and new shoots will harden off by fall.
Two basic pruning cuts are useful in pruning evergreens - heading and thinning. Heading refers to shortening a branch by cutting it back to just above a bud or smaller branch. Thinning involves removing a branch at its point of origin, which could be a trunk or the ground. Heading causes a proliferation of new shoots below each cut, and gives plants a stiff, formal appearance in the long run. Thinning creates gaps for light to penetrate thick foliage, and it results in a more open, informal effect that reflects an evergreen's natural growth habit. Use both techniques to control plant size and maintain attractive features.
Broadleaf and needleleaf evergreens differ in their response to pruning. Broadleafs generally respond well to pruning, while needleleaf evergreens are less tolerant of pruning.
The various species of shrubs also vary in their responses to pruning. The small round leaves of Japanese holly, a shrub commonly planted around homes, tolerate shearing (an extreme form of heading), though this practice produces a plant more at home in a formal boxwood garden than a suburban yard. A better way to prune Japanese holly is using hand pruners to snip individual branches back to a reasonable length. Head back each branch by six to 12 inches and then thin out crowded stems. Vary the amount cut with each stroke to avoid an unnaturally flat outer surface.
Helleri and dwarf Chinese holly grow in dense mounds that remain short and rarely require heading. Occasional thinning eases crowded branches and encourages leaf growth on inner stems.
Burford holly can tolerate light or severe pruning. Prune it as you would Japanese holly, cutting one branch at a time with hand pruners. Hedge clippers take less time and effort, but they lead to a thick layer of leaves around the outside and bare stems on the inside. They also spoil a holly's natural form and branching habit. The resultant cut leaf edges are brown and unattractive as well.
Boxwoods have finer stems than hollies. Prune them by simply snapping out 6-8 inch long twigs by hand over the canopy of the plant to increase light penetration. Thinning, good for all evergreens, is essential for English boxwood. These shrubs grow a canopy of leaves so dense that damp, dark conditions develop inside. Since fungus diseases thrive in such places, an annual thinning operation will help keep English boxwood healthy.
Any holly or boxwood too large for its location may be pruned until nothing but bare branches remains. New leaf buds, though hard to see, are numerous enough to give abundant fresh growth in summer. It will take several years for either of these plants to fully regain their beauty, but such heavy pruning is possible, if necessary. Another alternative for oversized Burford holly and boxwood is to prune them into a tree shape by removing their lower branches. The bare trunks often have interesting shapes and are much less bulky than the bush form.
Greater care needs to be taken with needleleaf evergreens. Tall growing junipers withstand moderately heavy pruning if done just before growth starts. Use hand pruners to head back branches grown too long, and thin out crowded stems. Ground cover junipers only require heading of new growth at the tips and thinning the oldest runners.
Never shear junipers with hedge clippers. This practice removes their characteristic pointed twigs, resulting in highly unnatural shapes. Once you shear too much, cutting back to leafless wood or dead zone, the juniper will stay brown and leafless. Always allow some tufts of foliage to remain below each cut.
Arborvitae also contain a dead zone where heavy shade causes inner needles to wither away. As with juniper, all cutting should be above this zone. Neglected plants are difficult or impossible to bring down to size without pruning into the dead zone, and they are best replaced. A light yearly pruning starting when the plant is small will help keep its size within bounds, though a better solution is to choose a variety whose mature size is correct for its location.
There is more margin for error with yews. These shade-loving needleleaf evergreens will sprout fresh growth when pruned as much as 50 percent. Hemlocks are also very tolerant of pruning and are one of the few needleleaf evergreens recommended for use in sheared hedges. However, do not prune into the dead zone of a hemlock or the plant will die.
Pruning of evergreen hedges involves a few special features. While most evergreens should be pruned in very early spring, hedges should be pruned after most of the new growth has been made. This allows shaping of the new growth. Whether shearing the plants into a formal hedge or heading back and thinning individual branches to maintain an informal hedge, be certain that the base of the hedge is wider than the top. This allows sunlight to reach all parts of the plant, preventing a top-heavy hedge whose lower foliage has died away for lack of light.
You should also delay pruning flowering evergreens until later in the spring, after they have bloomed. Earlier pruning removes the flower buds and eliminates the floral display for the year. When pruning rhododendrons, be aware that a branch pruned back below all buds, either dormant or growing, will die back to the trunk. Either remove the entire branch or leave a few leaves and buds at the tip of the branch.
(Prepared by Ellen Silva, Program Support Technician, Environmental Horticulture, Virginia Tech, Blacksburg, VA 24061-0327, from Virginia Cooperative Extension materials.)
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