Early spring is a great time of year to prune many types of deciduous shrubs. Knowing a few basic facts will make the job easier and will give your plants the care they need to thrive. Let’s start with some basic definitions.
Pruning Tools: The most useful tool will be a Fleco #2 hand pruner for branches up to ¾ inches in diameter. Additional tools include a lopping shears with a scissors-type cutting blade for branches up to 1½ inches in diameter and a hand saw with a tri-cut or razor tooth blade for branches over 1 inch in diameter. Keeping your tools clean, rust free and sharp will improve their performance.
Renewal Pruning: A technique that removes up to one-third of the oldest, thickest stems or trunks by cutting the stems down to the ground. Sometimes called thinning, it is the most common way to renew a shrub while preserving the overall plant shape. It can be done every year and works best in early spring before the leaves appear. This technique has an added benefit of reducing the height and width of a shrub without resorting to top shearing in the middle of the summer.
Rejuvenation Pruning: This technique cuts all the stems down to about 4-6 inches off the ground. It is a drastic technique that only works with certain plants; some plants never recover from this technique. This method works best with shrubs that have multiple stems, have become overgrown, neglected or require transplanting.
Crown Thinning: This technique selectively removes top branches using a hand pruners. The purpose of this method is to open the shrub to promote better light and air movement. This technique involves cutting only select branches back to a desired bud or lateral branch. If done correctly, it should not leave a stub; cut the branch on a slight angle to within ¼ inch above the bud.
Top Shearing: This technique involves the removal of the top layer of new growth across the entire shrub using a hedge shears. This method should only be used on formal evergreen hedges such as yews, boxwood, hemlock or arborvitae. It is important to shape the base wider than the top in order to insure adequate light is available to establish a dense layer of foliage on the sides of the hedge. Top shearing most deciduous shrubs is not recommended, as this method does not remove the dead, woody stems and will eventually reduce flower production. It will result in proliferation of new shoots that will make the shrub unsightly and top heavy with new growth that will lead to more maintenance.
Deadheading: This is the removal of dried flowers after they have faded and turned brown. Using a hand pruner cut the flower stem just above the last leaf on the stem. This technique will encourage a second flush of flowers on plants such as spirea and shrub roses.
Dead branch/winter damage pruning: Cutting branches below damaged or diseased areas would use the same technique as crown thinning. It is important to make your cuts on green wood and always clean your pruner between cuts with a disinfectant such as rubbing alcohol, a 10% solution of bleach to water, or a 25% Lysol to water solution. For additional ways to disinfect tools, see the following site: http://www.tnstate.edu/faculty/ablalock/documents/Disinfectants_for_Pruning_Tools.pdf
Spring flowering shrubs: The scientific definition of this category would be deciduous shrubs that develop flower buds on the previous year’s growth. Many guidebooks recommend pruning these type of shrubs within two to three weeks after they flower. Spring flowering shrubs that send up multiple shoots around the base, sometimes called suckering or multi-stem shrubs, benefit from the Renewal Pruning technique described above. Examples include lilacs, forsythia, viburnums, honeysuckle, burning bush, chokeberry, Japanese kerria, serviceberry, ninebark, red twig dogwood, and weigela. Pruning shrubs in March will remove some branches with flower buds and result in fewer flowers, but it will be easier than waiting until after the shrub flowers and the leaves get in the way.
Summer flowering shrubs: As a general rule of thumb, the upper Midwest temperate zone 4 lists summer flowering shrubs as those that flower after June 15. The scientific definition is that of a shrub that develops a flower bud on new growth or the current season’s growth. Examples include hydrangeas, roses, spirea, rose-of-Sharon, potentilla, and clethra. Pruning these type of shrubs in March using the Renewal method works well for most of the shrubs. Spirea and Annable hydrangea can also be pruned using the Rejuvenation method.
March is a great time of year to prune many shrubs. The lack of leaves in early spring will provide a clear view of a shrub’s branching structure. Renewal, rejuvenation, crown thinning, and top shearing are all different techniques used in pruning shrubs. Many shrubs require a certain amount of pruning to maintain their shape and form as they mature. The advantages to early spring pruning might focus on controlling the shape of the shrub, but pruning also benefits flower production, dead or diseased wood elimination, and the promotion of new stem growth.
Articles with more detailed information on pruning can be found at:
http://wimastergardener.org/?q=WinterPruning
http://www.garden.org/regional/report/arch/inmygarden/4258
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/pruning-trees-shrubs/
http://www.gardengatemagazine.com/pruning-shrubs/
https://www.hort.purdue.edu/ext/HO-4.pdf