Pruning Needled Evergreens
Figure 1. Beautiful foliage of Thuja occidentalis 'Steeplechase' by Susan Marquesen, Penn State Master Gardener
Pruning is a very important maintenance task for gardeners. Correct pruning can help maintain plant health, direct and control plant growth, reduce the risk of dangerous situations like branch failure, and improve a plant's flowering, fruiting, and vigor. Pruning is both a science and an art, as it requires that the pruner understand the manner in which the plant will grow from the cut being made as the pruner visualizes the future long-term ideal form of the plant. When a plant is young, formative pruning is especially important to develop a strong and healthy mature structure. Properly pruned plants are an asset to the landscape. Improperly pruned plants can be unsightly, unhealthy, short-lived, and even dangerous.
Depending on the plant's size and the necessary pruning cuts, a few tools are necessary to do the pruning work. Gather bypass hand pruners, also known as secateurs, used for small cuts; loppers for smaller diameter branches; pruning saws for larger branches; a pole pruner for out-of-reach cuts; and a hedge trimmer for shearing. I leave larger cuts that require a chainsaw to professionals. Tools must be sharp for clean cuts. Dull tools create messy cuts that may allow entry for pests and diseases. Keep the tools clean to avoid transmitting problems from one area or plant to another. Spraying before and between cuts with a diluted bleach solution of 1 part chlorine bleach to 9 parts water or with 70% strength rubbing, grain, or wood alcohol will do the job.
Before starting, acquaint yourself with the different types of cuts and how plants react to each of them.
- A thinning cut removes a branch back to a larger branch or the trunk. The cut should be made just beyond, but not into the branch collar, and at a proper angle to allow for the area to heal well. Proper thinning cuts must keep the branch collar intact so the wound can heal with callus tissue. No stubs will remain.
- A reduction cut removes a larger branch where it intersects with a smaller branch and is commonly used in training young trees. Reduction cuts leave the branch with a weak defense against decay, which is not a major concern for young, actively growing branches. However, reduction cuts are discouraged on mature trees and limbs larger than 2 inches in diameter.
- A heading cut refers to cutting a branch tip or the trunk off somewhere along its length. This type of “topping” is often done to reduce the size of a shrub or tree. Heading cuts encourage branching and result in too-vigorous growth. While useful in creating hedges and in fruit tree pruning, it is not a desirable type of cut for most trees and shrubs.
There are many categories of plants, and each one deserves separate attention with regard to the specifics of proper pruning. This article focuses on needled evergreen trees and shrubs. These are plants that retain their leaves throughout the winter, and their leaves are needles. Needled evergreens, common in Pennsylvania, include several species of fir (Abies), spruce (Picea), juniper (Juniperus), yew (Taxus), hemlock (Tsuga), pine (Pinus), arborvitae (Thuja), and false cypress (Chamaecyparis). All of the needled evergreens on this list are also conifers: they are cone-bearing. These species vary in size, from growing only a few inches tall to growing very large trees. Shapes include groundcovers, horizontal spreaders, upright pyramidal forms, and some with very odd forms and even weeping shapes. You can find many of these needled evergreens as dwarf shrubs and trees. Foliage color ranges from a gold and cream variegation to all shades of green, gray-green, and blue-green. There exists a needled evergreen for every landscape.
Some of these needled evergreens will need formative pruning to shape them when they are young. Upright trees produce a strong central leader, and if the tree develops a double leader, then that extra leader must be removed with a thinning cut to the main trunk. Pruning is also recommended to retain a balanced form. Natural pyramidal shapes can be maintained by selectively shortening higher branches so that lower ones get the sunshine needed to survive. Once lower branches are lost, they will not regrow. With the exception of yews, cutting back to only brown wood will not encourage growth on that branch. It will be dead. Only yews among this group have dormant buds on old wood and can be rejuvenated through drastic heading cuts.
As these evergreens age and when they are fully mature, most pruning tasks will be devoted to the plant’s health. Remove dead, dying, diseased, deformed, desiccated, or scorched parts and any accumulated debris within the tree’s canopy.
Timing is important. Species differ in the best time to prune them, depending on purpose. Conifers are very resinous, so pruning when their sap is running is discouraged for some species. The best time is usually late winter to early spring before new growth develops. One exception, of course, is for safety. Prune dangerous branches anytime.
A few of these conifers—arborvitae, hemlocks, and yews, can withstand shearing and can be trained into hedges. Start when they are young. Hedges should take on an inverted "V" shape—wider at the bottom than the top to allow sunlight to reach the lower branches. Otherwise, the shaded branches will thin and perhaps even die. The top can be flat, rounded, or pointed. Note that the berries and foliage of yews are poisonous to pets and humans, but not deer. Consequently, dispose of the trimmings, clean your tools, and wash your hands and gloves. Sheared plants should be pruned in early spring, with another optional light pruning in the summer. For health, closely cropped yews will need to be thinned out every few years to open up the center to more sunlight and air.
A Few Special Cases
Upright juniper trees that are small to mid-size require light structural pruning when they are developing and for general health when they are mature. There are many spreading juniper shrubs and prostrate groundcover cultivars. When the foliage of the groundcover reaches the sidewalk or outer limit of where it’s wanted, do not just lop off the branch at the point where it meets the sidewalk. It’s unsightly, and the heading cut is unhealthy for the shrub. Instead, follow the offending branch back to a lateral branch or the parent stem and make a thinning or reduction cut.
Dwarf Alberta spruce (Picea glauca ‘Conica’) is found in many gardens due to its attractive characteristics, such as dwarf size, dense foliage, and its conical form. Sometimes, the gardener will notice a branch with a different needle structure that has pushed through the outer canopy of the tree. That branch is said to have "reverted" and will continue to grow at a greater speed and size than the rest of the tree. It will need to be removed. Follow the branch to its origin and remove it there with a thinning cut. Other dwarf varieties, such as bird’s nest spruce, require only occasional cuts for health reasons.
Our native eastern white pine (Pinus strobus) becomes an enormous tree. Always follow the adage of "selecting the right plant for the right place." It cannot be kept small through pruning. Dwarf mugo pine (Pinus mugo 'Compacta') is an attractive shrub that can fit into many home landscapes. All pines can be pruned to become denser. Wait until the annual growth has been completed but before it fully hardens off, usually from June to early July. Pines produce “candles” at the tip of branch growth. Remove ½ to ⅔ of the candle by pinching it off with your fingers or by using a bypass hand pruner.
Eastern arborvitae (Thuja occidentalis) and western arborvitae (Thuja plicata) are naturally large, upright trees. Many dwarf varieties fit more easily into most gardens. Some can become screens and tall hedges as arborvitae take well to gentle shearing. Selecting a dwarf variety (Thuja occidentalis ‘Holmstrup’) whose mature size matches your needs will require much less maintenance pruning over time, but will require a little patience. Just be sure to plant them using the recommended spacing to allow for unrestricted root growth.
