Most gardeners have heard of the May rule: If the plant starts blooming before May, prune immediately after the blooms start to fade. These plants bloom on the previous year’s growth. By pruning as the flowers fade, the plant grows throughout the season and will set the flower buds for next year. Examples include azaleas, forsythia and oakleaf hydrangea.
If the plant starts blooming in May or later, prune before the start of spring growth, in late winter/early spring (late February to early March). These plants bloom on the current year’s growth. Examples include crapemyrtle, abelia and butterfly bush.
Remember that the May rule is a rule of thumb and for every rule there are exceptions. For instance, different hydrangeas have different pruning requirements. French and oakleaf hydrangeas bloom on old wood, and should be pruned soon after they bloom. Peegee and smooth hydrangeas bloom on new wood, and should be pruned before new growth or as it begins to stimulate more flowers. Variations in flowering and growth habits exist among roses as well. As a general rule, lightly prune varieties with weak growth and prune varieties with vigorous growth more severely. Plants grown primarily for their green foliage, such as hollies and boxwoods, can be pruned from January through mid-summer.
Knowing when to prune is important, but even more important is knowing why. Two of the most important reasons to prune shrubs in our landscape is to ensure the health of the plant as well as our own safety. Any dead, damaged or diseased wood should properly be removed from trees and shrubs. Hanging branches on a tree or shrub damaged by a storm pose a risk to those walking under them, and dead or damaged branches could possibly fall on nearby structures. Diseased limbs or branches should be removed to prevent diseases from spreading and improve the health of the plant. This type of pruning should be done any time of year when the damage is noticed.
Another reason for pruning is to open up the canopy of a plant to allow more sunlight and airflow. Allowing air to circulate within the canopy creates an environment less hospitable to insects and diseases. Branches rubbing up against one another can create wounds that provide entryways for disease and insects. A perfect example is crapemyrtles — some branches tend to grow inward and cross with other branches. Pruning out these branches will open up the middle of the plant and prevent branch wounds. “Training a plant” by pruning while the plant is still young will eliminate a lot of corrective pruning down the road.
Gardeners also prune to limit the size of plants in the landscape. We all want to be able to see out our windows — right? If plants need to be pruned yearly because of their size, odds are it’s planted in the wrong place. Just a quick reminder, pay attention to the mature sizes of plants and plan accordingly when planting this spring. Understand that when you do make a pruning cut, new growth is stimulated close to where the cut is made.
Landscape plants can weaken or become leggy over time. For some plants, renewal pruning is an option. This can be done on most broadleaf shrubs such as hollies, azaleas and camellias, and it’s basically cutting the entire plant down to 6-12 inches and letting it start over. Last year, I renewal pruned a relative’s 25-year-old azaleas that were heavily infested with azalea lace bugs and were overall unhealthy. By the time winter rolled around, they were about a foot tall and free of insect damage. If you have grafted plants like camellias and need to renewal prune, do not prune below the graft or you will stimulate growth from the rootstock (only cut back to about 16-18 inches). Junipers and boxwoods should never be renewal pruned. Junipers do not have buds on their main trunks, and boxwoods do not respond well to severe renewal pruning.
Our landscape plants behave and respond differently. Alabama Cooperative Extension System has a great publication with diagrams on pruning ornamental plants. The publication, Pruning Ornamental Plants, ANR – 0258, can be downloaded from online at www.aces.org.