Extension Educator, Horticulture
A warmish, springish day in February turns every gardener into Edward Scissorhands. A good day for a gardener is measured by the thickness of our hand calluses and the height of our twig piles. Other than being a rite of spring, pruning can promote new plant growth, maintain plant size, encourage flowering, remove diseased or dead limbs, help control insect and disease problems and relieve cabin fever.
With some shrubs; however, pruning is best left until later in the season. Early blooming shrubs develop their flower buds during the summer and fall of the previous year. Therefore as a general rule, shrubs that flower before June 15 should be pruned immediately after flowering. Pruning these shrubs in winter or early spring will remove the flower buds and this season's flowers. Generally spring shrubs don't die after being pruned now, but we won't get flowers this year.
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. Shrubs that should be pruned immediately after flowering include lilac, deutzia, kerria, mockorange, weigela, forsythia, arrowwood viburnum, St. johnswort and redtwig dogwood.
Shrubs that bloom after June 15 can be pruned in early spring, generally February and March. 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.
Rejuvenation is used when multi-stemmed 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.
The following respond well to rejuvenation pruning: Anthony Waterer sprirea, abelia, honeysuckle, beauty bush, snowberry, slender deutzia and privet. Actually every 4-5 years I sacrifice the season's flowers and rejuvenate some spring bloomers such as redtwig dogwood by cutting them to the ground now. They look so much better with redder stems and healthier branches.
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.
Hydrangeas are a tough group to know how to prune since some bloom on old wood and some on new wood. Many of the new cultivars bloom on both, so the mystery has been removed. The hydrangeas with big white blooms (Hydrangea arborescens) sold as 'Annabelle' bloom on new wood. Some of the blue flowered ones bloom on old wood only. With hydrangeas, be sure to save the tag.
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. Formal hedges should be pruned with the base wider than the top in a keystone shape in order to keep leaves to the base of the shrub. If a hedge shows its bare legs in the summer, it can be rejuvenated by cutting back to 6 to 10 inch stubs in early spring and then reshaping the new growth.
Check out some great videos on pruning shrubs at http://www.youtube.com/user/UIExtension Sharpen your pruners and go for it!