Editor’s note: This is the fourth installment of a four-part series from the Dubuque County Master Gardeners about pruning trees and shrubs during the winter.
Each pruning cut is a wound that trees and shrubs can seal but never heal. If a tree or shrub requires frequent pruning to fit in a space, consider planting a replacement with the desired size and shape. Prune only when necessary, preferably in late winter (except for most spring-flowering plants) and at the plant’s growing points.
Pruning ornamental shrubs
• Natural forms require less pruning.
• Minimize shearing: Shape by pruning to growing points.
• Renovate overgrown shrubs.
• Understand the shrub growth characteristics.
• Do an overall inspection, then plan the pruning.
• Remove dead, diseased and damaged branches.
• Remove undesirable branches such as stubs, suckers and sprouts.
Bypass pruners are scissors-like and easily can cut live branches up to a half inch in diameter.
Anvil pruners have a crushing action that readily cuts the dead wood that often jams bypass pruners. Use anvil pruners for dead wood only. They crush living wood and slow wound sealing.
Bypass loppers have long handles and readily can cut live branches up to one inch.
Use a pruning saw for larger cuts. When making cuts at ground, powered reciprocating saws with a long carbide blade avoid the problem of soil dulling hand pruning saws.
Hedge shears are suitable only for shaping vigorous, dense surface growth. Do not use hedge shears for general pruning.
For winter kill, prune back to where the layer under the bark (cambium) is a fresh green. Then, trim to the next growing point. Make pruning cuts where they will seal fastest. Leave no stubs. Growing points, from the youngest growth back, are:
• Just above a bud.
• Just above a side branch.
• Just outside the branch collar.
• Ground level above the root crown (multi-stemmed shrubs only).
Stopping growth in one direction promotes growth in another. Removing a leader also removes branch hormones that had been inhibiting growth of side branches. Pruning to increase flowers and fruit is a balance between stimulating new growth and a reduction in the energy reserve of the plant.
Yews form buds on old or new wood. Arborvitae and juniper form new branches only where there is existing foliage. In spring part of the “candles” (new growth) of spruce, pine and fir can be headed back (pinched) to maintain overall plant size and produce denser branches.
Evergreens are best pruned in late winter or early spring, when they have high energy reserves. Pruning in fall can cause winter kill.
Renovation and rejuvenation
Many multi-stemmed shrubs allow a “redo” when they become overgrown. Most dogwood, forsythia, privet, mock orange, shrub roses, spirea, lilac and viburnum tolerate severe pruning.
Renovation, the most radical approach, cuts all the stems at once above ground level. The stubs then grow new stems.
After one growing season, select stems to be headed to increase branching and cut the remaining stems to ground.
Rejuvenation is more gradual. Remove the largest third of the stems annually for three years. Continuing to remove the largest stems annually can make shrubs an attractive part of the landscape for many decades.
— Pruning Ornamental Shrubs, Iowa State University Extension, PM 1958.