Extension Educator, Horticulture
As you stare forlornly at your six-foot tall shrub, you reminisce about the plant label that said, "Grows to three feet." In our rich soils plants may exceed their usual height. Regular pruning can correct many problems. Pruning can promote new plant growth, maintain plant size, encourage flowering, remove diseased or dead limbs and help control insect and disease problems.
However, timing is important, especially when pruning spring blooming shrubs. Early blooming shrubs develop their flower buds during the summer and fall of the previous year. This is often called "blooming on old wood." Therefore as a general rule, shrubs that flower before June 15 should be pruned soon after flowering. Pruning these shrubs in late summer, fall or early spring will remove the flower buds and therefore the flowers.
Spring flowering shrubs are generally pruned by the renewal method. Each spring after flowering, prune out the largest stems to the ground to stimulate new growth from the crown and remaining stems. Remaining stems can then be shortened to shape.
Shrubs that should be pruned this spring soon after flowering include lilac, deutzia, kerria, mockorange, weigela, forsythia, viburnum, St. johnswort and redtwig and yellowtwig dogwood.
Shrubs that bloom after June 15 can be pruned in early spring. Summer and fall flowering shrubs bloom on new wood or stems that were produced in the same season as flowering. Many of these shrubs can be pruned by the scary rejuvenation method. Rejuvenation is the complete cutting of all stems down to 4 to 6 inch stubs generally done in February and March.
Rejuvenation is used when multistemmed plants become too large with too many stems to justify saving any one to two year old growth. In other words, the shrub is a tangled mess of stems.
However, don't wait too late in the year for rejuvenation pruning. To be cautious large old shrubs should not be rejuvenated in late spring or summer. With the early flowering shrubs that need rejuvenation, you may decide to just sacrifice the flowers one season and prune them to the ground in late winter. However I have rejuvenated Anthony Waterer spirea after bloom and they recovered well. I guess it depends on how much you mind risking their recovery.
The following respond well to rejuvenation pruning: Anthony Waterer spirea, abelia, honeysuckle, beautybush, snowberry, slender deutzia and privet. Potentilla is another common summer blooming shrub. However, it should be pruned by the renewal method (selectively removing some of the older growth) in early spring just as the green buds appear. Pruning at other times can cause dieback and sunscald, which could kill plants.
Before pruning, examine the plant thoroughly. If you are new to pruning, tie a colorful ribbon or spray paint the branches to be removed to help you visualize what will be remaining.
Shrub pruning requires a good set of pruning shears or loppers not hedge shears. At each cut of hedge shears, many new growing points are produced making the shrub very dense at the top. The dense growth causes the leaves in the interior of the plants to die.
Hedge shears should only be used on formal hedges. However, formal hedges must be pruned in the correct shape or the shrubs will lose their leaves at the base. Hedges should be pruned so the width at the top is less than the width at the bottom. This allows light to penetrate to the lower leaves. Deciduous shrubs should not be maintained at a permanent size during the growing season, but allowed to grow an inch or so at each shearing to maintain a good cover of new leaves.
For more info on pruning, contact your local U of I Extension for the circular U5040 Pruning and Care of Trees and Shrubs.