A few years ago I wrote about the challenge of pruning landscape plants. It is a complicated subject since one pruning style does not apply to all plants.
Each type of plant has a natural form that should be retained or enhanced. For example upright tall trees should not be topped, that’s especially true for conifers. An occasional limb reduction to remove diseased wood or a broken branch is enough to maintain their form and keep them healthy. Shorter, full shaped ornamental conifers can be tipped while training but it does change their natural form and can hurt their health.
Pruning conifers for size control is typically not a good practice. Just choose the right species of tree for each unique site to eliminate the need for harsh pruning.
To reduce the shock of conifer pruning, know the best time of the year for both trees and shrubs. Most conifers do not need much pruning but if you need to prune one do it when conifers are fully dormant. Mid to late fall in my opinion is the best time to prune conifers, since the new growth has hardened and trees are at their most dormant stage.
Pruning in the fall before steady rains begin will allow wounds to callus best. If you wait to prune conifers in the late winter or spring, the oozing sap from the pruning wound will attract many different types of boring insects. And if you prune conifers in late summer before dormancy, you will see dramatic needle dieback and browning where the pruning shears cut through needles and wood.
Each type of conifer requires different pruning practices. Scale foliage conifers like Incense cedar, Leyland cypress, juniper and arborvitae can take some light pruning to shape them but don’t prune into un-needled wood. If you prune into bare wood the branch will not make new foliage. Junipers are notorious for dying back if pruned too severely. Yew trees are the only species in this group that can be pruned severely and shaped.
Needle foliage conifers like Douglas fir, Grand fir, Noble fir, Blue spruce, Alberta spruce, White pine, Ponderosa pine and hemlock should not be pruned into old un-needled parts of branches or stems. To control the growth of these conifers when trees are young you simply need to pinch back the new shoot growth in spring.
These new shoots on pines and firs are referred to as candles. You can prune candles with your hands when they are about 4-5 inches long in spring, they break off quite easily. If you want to make the tree more compact simply break off most of the candle. If you want to make the tree just slightly more compact break off half of the candle.
Some conifers are small trees and may never need to be pruned. Examples would be Dwarf Alberta spruce, Birds Nest spruce, Norway spruce, Creeping juniper, Hinoki false cypress, Bald cypress, Mugo pine and Dwarf blue spruce. These trees are especially prized by people with small yards or gardens.
Our coastal area of the Pacific Northwest is a marvelous place to grow a wide variety of conifers. Take the time to learn how to train them and care for them. If you want to learn more about pruning conifers contact the OSU Master Gardeners.