PRUNING EVERGREENS IN THE LANDSCAPE
Dr. Leonard Perry, Extension Professor
University of Vermont
Proper pruning is important for good plant health. Pruning at the wrong time, or the wrong way, can result in leaf and plant injury. A University of New Hampshire publication by this same name gives some excellent tips.
Thoughts of pruning really should begin when buying and placing plants. If you want a plant with a certain shape, try to find a selection that will grow into this shape. This will save you having to prune it at all, or pruning it minimally. Place plants with enough room to grow over the years. Often we don't, and so much pruning is needed to keep them within bounds. This is most common with foundation plantings, planted too close to buildings for future growth or that grow too tall and block windows.
Not all evergreen landscape plants are pruned the same way. Evergreens are those that keep their foliage year round. Narrowleaf species are just that--they have narrow needle-like leaves. Ones such as pine, spruce, and fir only grow in the spring and early summer, so don't need yearly pruning. Others such as arborvitae, juniper, yew, and hemlock grow through the season, so may need yearly pruning.
Broadleaf species have, as the name indicates, broad leaves. They
are more subject to winter injury than the narrowleaf species, so are less
common in the far
north. The most common is the rhododendron, while farther south one sees holly, mountain laurel, and boxwood. Broadleaf species may need little pruning if sited properly, or every three years at most.
Early spring is the best time to prune the species with needle leaves, after the ground has thawed, but before roughly the middle of June. Pines on the other hand ooze sap, or "bleed", when cut in early spring while the sap is flowing more. Prune pines in late spring to minimize such bleeding. Prune broadleaf evergreens in spring after bloom, but if this is not an issue you can prune in early spring. Avoid pruning any evergreens, except to remove injured branches, in late summer or fall. The wounds won't heal as readily, and new growth may be stimulated. Such tender growth may not harden properly before winter and be injured.
If you have overgrown established plants, do not try to correct years of neglect in one pruning or season. The plants will be unsightly, and may take years to recover if they do at all. Evergreens also should be pruned less than deciduous trees and shrubs, as they grow slower. Yews, hemlock, and arborvitae can be pruned harder (more growth taken off) than juniper, spruce, pine, and fir. This is because they can grow from dormant buds on older wood.
Especially with the narrowleaf evergreens, many prune yearly by trimming the outer edges, removing a uniform amount from all branches. This actually is not pruning but is termed "shearing", just as one would shear a sheep. It has several disadvantages including loss of the natural shape, reducing the total leaf area, preventing sunlight from reaching the inside and so creating a "dead" zone, creating a less structurally strong plant, removing new growth while keeping the older and less vigorous branches, and creating stale air inside the shrub which is conducive for insects.
Instead of shearing, thinning and renewal techniques should be used. These promote more internal growth, reduce winter injury, and promote the natural form of plants. Selectively removing certain branches will result in air and sunlight being able to reach the center of the plant.
To shape a shrub, first envision or mark with string a perimeter line.
This is the line branches shouldn't cross. The line should create
a shape wider at the bottom than the top. Then reach into the shrub
and remove at a node where branches join:
*any branches than extend beyond this perimeter line
* no more than one of every four shoots on average
*competing upright vertical shoots, or leaders (upright branches competing to be the main trunk), and crossing or rubbing branches.
*a shoot that has another nearby that can grow to fill the space
*branches that are growing sideways or towards the shrub center, and
*club-like growth that has resulted from years of shearing.
Finally when pruning, make sure to use sharp tools to avoid tearing
the bark. Sharp cuts will heal more quickly. Invest in a good
file for quickly and regularly sharpening the blades, available online
or at complete garden stores.
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