Q. My Rose-of-Sharon has been in the ground for three years and it's finally covered with beautiful, pink blossoms. I would like to know if it should be pruned and, if so, when? Also, how far back should it be pruned? It's planted about five feet from the corner of our house, so it should have plenty of room to grow.
A. For years I've turned up my nose at Rose-of-Sharon (Hibiscus syriacus). I suppose it's because it doesn't offer multiple seasons of interest (e.g., a combination of nice flowers, brightly-colored fruit and colorful fall foliage) like many of the shrubs I frequently recommend. And, though I'm embarrassed to admit it, I think it's also because I've felt it's a little too common for my more "discriminating" plant palette?
During the past couple of years, however, I've softened my view toward this shrub and have even added it to several of my landscape designs. After all, it's just plain hard to beat a low-maintenance shrub that blooms from late July all the way into September - a time of year when few other shrubs bloom.
Now, to answer your specific questions about pruning Rose-of-Sharon, the simple answer is that it never needs to be pruned, especially if it has plenty of room to grow. After all, in the wooded mountains of China and India where this plant grows naturally, there are no little elves that come around and prune these plants every year!
If, however, you decide that you absolutely have to prune your plant, the best time is from late March through early May, before the leaf buds open and begin to expand. Pruning in early spring will encourage additional side shoots to form. This should translate into more flowers in July and August, as flower buds will form on many of these new shoots.
As to your question about how far back Rose-of-Sharon can be pruned, you may be surprised to learn that it can be cut back to stubs no more than two or three feet tall without risk of injury. While you can use hedge shears to perform this major surgery, my recommendation would be to selectively remove individual shoots to different heights with hand pruners or a small, folding pruning saw. This technique will result in a smaller plant while preserving a more natural form.
A couple of other things you might like to know about Rose-of-Sharon is that they're adaptable to all but constantly wet soils and are very tolerant of drought once established. They also flower most heavily and tend to be stockier when grown in full sun, but will continue to produce at least some flowers even if they receive direct sun for only half a day. And, as a result of their prolific flower production, many Rose-of-Sharon produce large number of seed that often result in numerous seedlings that will need to be dug up and added to your compost pile, or given away to friends and family.
Finally, please remember that Rose-of-Sharon is not a small shrub. Most of the several dozen cultivated varieties can grow to between eight and twelve feet tall with a similar spread after ten years. Therefore, they get too big to be used as a foundation shrub, at least without annual pruning. And, should you choose to use them to create a summer-flowering hedge, they should be spaced six to eight feet apart so that they grow together comfortably, without crowding each other over time.