Rose of Sharon isn't fussy about how or when it's pruned

Question: We have a hedge of Rose of Sharon plants down the side of our driveway. They’re getting a little overgrown. They’re also very thick because they’ve spread by seed, too. What is the best time to prune them, and how do we do it?

Answer: Unlike most other trees and shrubs, Rose of Sharon (Hibiscus syriacus) isn’t fussy about how and when you prune it. There are, however, several things to consider before pruning your plants.

Rose of Sharon blooms on new wood. This means the flowers that appear in summer come from buds that develop on the plant just a few weeks prior to bloom-time.

In contrast, many spring-blooming shrubs, such as forsythia, lilacs, azaleas and saucer magnolias, form their flower buds the season prior to their bloom (on old wood). Improper pruning of those types of plants may mean that you accidentally cut off your flowers for the season.

However, summer-flowering shrubs that bloom on new wood, such as your Rose of Sharon, offer more flexibility in terms of pruning. And, since Rose of Sharons are notoriously tough and resilient plants, there is really only one time when you DON’T want to prune them — late spring.

Rose of Sharon pruning is best performed at one of these four times:

1. In late summer just after the plant finishes blooming

2. In autumn

3. In winter

4. In very early spring, before the plant leafs out.

Some people prefer to prune Rose of Sharon in winter, when there are no leaves on the plant, so they can readily see the shrub’s structure — and this is just fine. However, I prefer to prune my Rose of Sharon just after it blooms in late summer.

Not only does this keep the shrub from growing too large for its space, but it also keeps the plant from throwing thousands of seeds that could grow on to become weedy in the landscape.

If you choose to prune right after bloom time as I do, be sure you’re cutting the plant back far enough to remove all or most of the seed pods before they crack open and disburse the seeds. If you’re pruning in the fall, winter or early spring, the seeds will have already been shed.

A good rule of thumb when it comes to how far back to cut the plants is to never remove more than one-third of the total height or girth of a tree or shrub in any one year.

Don’t use a hedge trimmer to shear the shrub into a meatball shape. Instead, use a pair of hand pruners to judiciously thin out branches down to the point where they meet another branch. This retains the beautiful natural shape of a shrub while still keeping its growth contained and managed.

If your Rose of Sharon is extremely overgrown, take off one-third this year and one-third next year, rather than doing it all at once. This is a lot less visually dramatic and enables the plant to rebound a little in between major prunings.

And, as always, when pruning your Rose of Sharon, be sure to use a sharp pair of pruners or a hand saw. Prior to starting the job, sterilize the blade with a quick dip in a 10 percent bleach solution or a spray of disinfectant. This will kill any pathogens on the pruning equipment that could go on to cause problems for your shrubs.

Always prune on dry days and take your time to ensure the job is done correctly.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.