When should I cut back damaged shrubs?
"Fielding Questions" columnist Don Kinzler also advises readers on tree trimming and cutting back ornamental grass.
Q: I'm planning to cut back my alpine currant hedge that was damaged in heavy snow from the previous two winters. Can I do the same with my two highbush cranberry shrubs that are overgrown and also damaged? If so, when should I cut them back? — Dan F.
A: Alpine currant and highbush cranberry viburnum rejuvenate beautifully, which removes old woody or damaged branches, and coaxes fresh new vigorous growth to emerge from near the ground level. The preferred time to prune these shrubs is late March or early April, before trees and shrubs begin to leaf out.
To rejuvenate these shrubs, prune all branches down to about 6 inches above ground level. Avoid being timid, because if the shrubs are cut back by only half or so, they branch out at that point, and a majority of the old branches remain, leaving a shorter version of a damaged, overgrown shrub.
Another method to rejuvenate shrubs is to prune back a third of the branches each year during a three-year period. But I’ve experienced even better success cutting back the entire shrub all at once, because I feel it pushes a healthier crop of new growth.
Shrubs that benefit greatly from rejuvenating every four or five years include viburnum, Alpine currant, ninebark, weigela, spirea, potentilla and dogwood. Lilacs rejuvenate beautifully, although they don’t bloom the spring of rejuvenation.
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Q: When is a good time to trim maple trees, Japanese tree lilac and mountain ash? All are in need of lower branches being removed. — Paul T.
A: The best time to prune trees is after the coldest temperatures of winter are likely past, but before the buds start to swell on the trees. Anytime from now through early April is good for pruning both trees and shrubs. Maples will tend to bleed sap from the pruning wounds once sap begins flowing, although it doesn’t harm the trees. If you prefer, maples could wait until after they produce leaves.
A common question is whether pruning cuts should be treated with anything. Extensive research has shown that cut surfaces protect themselves more effectively if the wounds are not covered with sealers, sprays, pruning paints, etc. The pruning cuts compartmentalize best when simply exposed to air.
It's also important to prune at just the right spot when removing lower branches back to the main trunk. It's important not to leave stubs, but it's also important not to cut perfectly flush with the trunk. There is a slight ridge, or swollen ring, where the branches meet the main trunk, called the branch "collar," that contains healing-type tissue. Make the pruning cuts just outside the branch ridge, preserving the collar.
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Q: I have several ornamental grass plants in our landscape. How soon can I cut the old, dried material back? I left the tops on over winter. — Cindy M.
A: Perennial ornamental grasses, including the popular Karl Foerster, add interest to snowy landscapes, and most people opt to leave them intact over winter. They should be one of the first perennials to clean up each spring, because new grass shoots begin to emerge from ground level very early, making it difficult to cut back the old grass without damaging the new growth.
In late March, as weather permits, cut the dried grass down to near ground level. By hand, or using a rake, gently comb out old, accumulated debris from the center of the clump.
If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at firstname.lastname@example.org. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.