Pruning Narrowleaf Evergreens
In general, evergreens should be pruned during the winter when they are dormant. An ideal and recommended time is at Christmas so the trimmings may be used for decorations. The exception to this rule is pine, which should be pruned when the candle growth develops in the spring.
Prune evergreens according to their growth habits. Allow these plants to assume their natural shape. Do not shape them into balls, birds or other formal habits. Pruning is a matter of cutting the branches to attain compact, controlled growth that results in a more desirable plant. This requires pruning individual stems rather than shearing. Shearing not only ruins the natural habit of growth, but the dense growth that results prevents light from penetrating the center of the plants and the foliage there drops. It also makes insect and disease control difficult because spray materials cannot penetrate to the center of the plants. Certain pruning guidelines apply for various types of narrowleaf evergreens.
Spreading Narrowleaf Evergreens
Some yews and junipers have a spreading growth habit. The proper pruning procedure is to cut back enough to prevent spindly growth. A common problem results from needles dropping off the lower branches because of shading by upper branches. To prevent this cut back the longer branches that develop on the top so the lower branches will be exposed to light.
It is best to cut back some each year to prevent the plant from getting out-of-bounds. It is not uncommon for a vigorous growing, spreading evergreen such as pfitzer juniper to produce 12 to 18 inches of growth or more each year. This should be reduced in sites where space is limited or even cut back into the previous year's wood to maintain the plant's size and shape. Reduce the height or width no more than 20 percent, however - the plant must have healthy, green foliage on the branches that remain to generate new growth. Spreading taxus - commonly called yews - may, if necessary, be cut back into two- or three-year-old wood. The cuts may be unsightly for one or two years, but new growth will eventually hide them.
Upright Narrowleaf Evergreens
Hick's yew, canaert juniper, pine and spruce may be either narrow or pyramidal in shape. In the case of narrow, upright plants, such as Hick's yew, the previous year's growth should be cut back about one-quarter to one-half to encourage a thick growing plant. Pyramidal plants, such as spruce or pine, should be cut back a little on the sides to maintain or develop a more compact shape. The central leader of spruce or pine should not be cut at all except when the plants are young to remove a multiple leader. Remove all but the straightest and strongest growing stem. Upright forms of arborvitae require very little, if any pruning.
When plants such as spruce or pine are young and growing vigorously, the top growing point may outdistance the rest of the plant. This results in an open space between the main body of the plant and the growing tip. To encourage the plant to branch and be more compact, cut the top back to a dormant bud located close to the main body of the plant. If this cutting back is done when plants are young, it will have little effect on the appearance of the plant.
Mature plants of tall growing narrowleaf evergreens - such as spruce, pine, upright arborvitae and some junipers - cannot be severely cut back or topped without damaging their shape. If one of these plants becomes too tall for the place it is growing, it may be necessary to remove the entire plant. Avoid this by properly selecting plants for the area. Hemlock may be allowed to grow in its natural pyramid shape or be cut back to form a hedge or screen.
Rounded Narrowleaf Evergreens
Brown's yew and globe arborvitae are normally globe shaped and should not be sheared into balls. Brown's yew can be maintained at about any height and shape desired by pruning the previous year's growth to keep it bushy and compact. This means removing about 1/4 to 1/2 of last year's growth. Thinning of individual branches, rather than shearing, will result in a more attractive, natural habit. In contrast to this plant, globe arborvitae requires little, if any, pruning because of its normal, formal habit of growth.
This article was prepared by students and professors at Michigan State University Extension, Home Horticulture.