If you live in the Midwest, you are likely familiar with the summer-blooming shrub commonly called Rose–of-Sharon, but you may not know it by its other common name – shrub althea. You may not be aware that it is a Hibiscus, that its scientific name is Hibiscus syriacus or that it belongs to the Mallow family, Malvaceae.
Rose-of-Sharon is a large shrub, reaching up to 12 feet in height and nearly that in spread. The plant adapts well to most soil conditions, except extremely wet or dry, and is generally hardy throughout Indiana. It will perform best in moist but well-drained soil in full sun. The foliage is late to leaf out in spring, remains green through late autumn and has little, if any, display of fall color.
The primary attraction is its large flowers of white, red, purple or blue, beginning in late June to early July and often continuing through August and perhaps September. When all goes well, the plants are loaded with blooms, virtually covering the entire shrub.
However, failure to bloom and bud drop seem to be common problems with Rose-of-Sharon, and, yet, we don’t know exactly why. It flowers on the new growth each year, so even if the plant experiences winter injury, it is still able to produce flower buds. But many are frustrated when the plant puts on lots of buds that fail to open. Sometimes the plant may start out blooming normally but, as summer wears on, the buds start to drop prematurely.
Individual flowers of this plant are not particularly long lasting, so it is difficult to say what is premature blossom drop. Hot temperatures, heavy rain, wind, etc. will hasten drop of mature blooms. But, if buds and immature blossoms are falling, it may be caused by plant stress, such as too little or too much moisture and/or fertilizer. There is a fungal disease called Botrytis that infects flower buds and causes them to turn brown and drop, often before or just after they open. Thrips are an insect pest that feed on flower buds and can cause the buds to drop. It is possible that a combination of these factors is to blame.
But I do wonder whether some bud drop is just ‘normal” for this species. After all, the shrub does tend to produce huge numbers of flower buds, so maybe this is nature’s way of thinning out the load so the plant’s resources are not overwhelmed.
Since it flowers on new growth, you can prune Rose-of-Sharon in late winter or early spring. It can be pruned back hard to keep the plant more compact. If fewer, but larger, blooms are desired, you can trim back again in late spring to reduce the number of flower buds per stem. Some authors recommend pruning back to 2 or 3 flower buds per stem. I wonder if this would reduce the blossom drop as well?