Master Gardener - hydrangea&Rose4162.JPG

Hydrangeas (front right) when improperly pruned fail to bloom. Shrub roses (background left) present few pruning problems.

Wow, it truly is a jungle out there! This past week I was pruning shrubs and herbaceous (soft-stemmed) perennials. By hacking off my overgrown Forsythia bushes after July first, I was going against time-honored pruning advice. However, even overly officious pruning police who happened to catch me in the act of chopping back white shrub roses or trimming Knock Out roses, wouldn’t have a word to say about bad timing. Why is it okay to prune some but not other woody-stemmed flowering plants in late July?

Flower power is the reason for this distinction. Experts say gardeners should prune spring-blossoming trees and shrubs soon after their flowers fade but quit trimming them back around the beginning of this month. This is because woody-stemmed plants which bloom once a season in springtime develop flower buds for the following year on shoot growth from this spring and summer. These cultivars are said to “bloom on old wood”. Most plants that bloom on old wood set flower buds by early July. Removing branches from those varieties right now will cut away the flower buds, too. That means fewer blossoms next year.

On the other hand, most woodies that flower only in  mid– to late-summer are said to “bloom on new wood”. Their flower buds form on new shoots from earlier in the same season. The third category are trees, shrubs and vines that bloom on both old and new wood. They develop flower buds for the current and the next season on shoots which grow this year. Trimming them now won’t leave you without blossoms come spring.

When wielding loppers in late July or early August, knowing the specimen you are dealing with and when it blooms can be critical. Shrub roses are easy to care for. They bloom on both old and new wood. You get flowers with them whatever the pruning time table. Species like clematis and hydrangeas can be a lot of trouble. There are many different varieties of each. A specimen could be in any of the three pruning classifications– those that bloom on old wood, those that bloom on new wood and those that bloom on both old and new wood. The proper method and timing for pruning one kind of clematis (or hydrangea) can yield disastrous results if applied to another. Whether a particular hydrangea or clematis flowers, can also be influenced by the amount of available light, water and fertilizer so it’s hard to know exactly what causes a failure to bloom.

Despite knowing forsythias bloom on old wood and that pruning them now means some of their bright yellow blossoms will be missing next spring, I decided to cut them back anyway. They’ve become wildly out of control this summer. These shrubs are crowding and shading neighboring  plants. Those forsythia had to be cut back to a manageable size to preserve the health of plants around them. 

Experts also say, “Don’t prune woodies after mid-August.” I do pay close attention to that advice. This late-season pruning prohibition helps keep plants healthy. Woody perennials in climates where winter temperatures dip below freezing, go through changes at summer’s end to help them survive the cold. The process is called “hardening”. Pruning encourages new shoots just below the cut. New growth is always very tender and full of moisture. That late in the season there may not be enough time for young shoots to harden sufficiently to overwinter without harm. The only exception to the ban on late summer and early autumn pruning is a need to repair damaged wood. In other situations, wait until late winter to make those cuts.  

Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region.  UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (931-484-6743) has answers for horticulture questions, free publications and details on how to become a Master Gardener. Send email comments or yard & garden inquiries to Master Gardener Rae, mgardenerrae@frontiernet.net.

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