Fall pruning of flowers

By George McVey, Ph.D.
Contributing Writer
Union County Post

Q. My hydrangeas last summer were abundantly covered with flowers. However, they became top heavy and fell over. Is it possible to prune white hydrangeas? If so, when is the best time of year to do this? Is it possible to cut them to the ground in the fall?
A. There are three species of hydrangea with white flowers, but I'll guess that you have Hydrangea arborescens 'Annabelle', the one with the least sturdy stems. This plant has large, round, white flowers that later turn green, and then brown through the season. It's not uncommon for the flowers to fall over after a drenching rain that soaks the flowers and weighs them down. Once down, they tend to stay down. This plant blooms on new wood, so pruning usually takes place in late winter or early spring; however, you can prune them in the fall, especially if they are lying on the ground and not providing any ornamental benefit through the winter. Prune plants to the ground; the stems are not hardy and the plant usually dies back to the ground anyway.
Pruning alone won't guarantee that plants will stay upright for the season. If plants are growing in full sun, the stems will tend to be sturdier, but if in shade or partial shade, the stems will likely be taller and less sturdy. A last option is to stake the plants; put your stakes in place as the stems are just emerging in spring and allow the plants to grow up among them. This might help.
Q. My spring-flowering shrubs just don't bloom like I think they should. I'm doing some pruning in late winter or early spring, just to shape them up - am I doing something wrong?
A. You may be pruning away flower buds, which accounts for your poor showing. If your plants bloom on "old" wood, then pruning in late fall or early spring removes some of the buds. A good rule of thumb is that for woody plants that normally bloom before mid-June, prune after flowering (this will depend a bit on the season's weather). This includes the following common shrubs; forsythia, honeysuckle, viburnums, serviceberry, mock orange, flowering quince, Itea, magnolias, lilac, weigela, firethorn, rhododendron and azaleas, a few of the spireas, and spicebush.
Small ornamental trees that bloom on old wood include redbud, fringetree, flowering, Kousa and Cornelian cherry dogwoods, smoketree, crabapples, flowering cherry and plum, and mountain ash. If you are in doubt about when to prune a plant, check a reference to find out whether the plant blooms on old or new wood.

This article was taken from the Ohio State University Plant Facts Web site.
The Master Gardener offers horticulture advice and tips, focusing on current issues, by volunteers who are with The Ohio State University Extension, Union County Master Gardener Program. For answers to gardening questions, call 937-644-8117 or e-mail mcvey.22@cfaes.osu.edu.