Rose of Sharon (Hibiscus syriacus) is a deciduous shrub form of hibiscus, known for a profusion of beautiful flowers resembling those of hollyhock, which appear over a very long period—from early summer all the way to frost. This shrub is grown for its flowers, and without them, the plant is quite ordinary. But it is a common problem for rose of Sharon to produce lots of buds, but then to refuse to flower at all.
This is no reason to panic and discard the shrub. Although it's probably too late to correct the problem for the current growing season, there are easy steps to correct the situation and ensure plentiful blooms for next year' season.
The Normal Blooming Habit
The blooms of Rose of Sharon, also known as shrub althea, are most welcome since they come during the latter half of the summer and into fall when most flowering shrubs have finished flowering. In most regions where rose of Sharon thrives, the flowers are best in July and August. This makes it especially disappointing when the flowers don't come at all, or when the plants have numerous buds but they never bloom.
When a Rose of Sharon Doesn't Bloom
There are many possible answers as to why buds on your rose of Sharon flower do not open, but the most common causes are related to shade, rot, or overwatering, and drought:
- Excessive shade: Rose of Sharon will not bloom up to its full capabilities if given too much shade.
- Rot: During rainy summers, rot can set in and ruin the flower buds, although this may not be noticeable from the outside. The same effect can be produced by excessive watering if you water overhead. A rose of Sharon planted in dense, poor-draining soil can also develop rot.
- Drought: During dry summers, rose of Sharon flower buds can be damaged from drought and fail to open.
- Inadequate phosphorus. As a nutrient, phosphorus assists a plant in its ability to update nutrients from the soil. If the soil is deficient, the plant's buds may fail to open
- Inappropriate or inadequate pruning. Rose of Sharon blooms on new growth, and if the shrub is pruned at the wrong time, you may have removed the buds.
- Pests or disease. Aphids, in particular, can cause buds to soften and fail to open. In this case, the flower buds will likely feel soft and spongy and may rot. A number of other fungal diseases may also cause bud failure.
How to Deal With Problems
The problem of excessive shade is relatively easy to solve, especially if you are able to exercise some forethought. This is a plant for full sun. When you are planting the shrub, make sure that you choose a location that receives at least 6 hours of direct sunlight per day. If you have planted it in an area that is too shady, consider transplanting the shrub to a more suitable location. Alternately, other surrounding plants can be moved or trimmed to improve the sun exposure. Shade trees, for example, can be trimmed to remove lower branches that shade the plant.
When watering your rose of Sharon, always direct the water toward the soil around the plant rather than onto the plant itself. If you spray high, you will drench the flower buds, which invites rot. Of course, rain will soak the flower beds from overhead, and you have no control over that. But if your shrub is properly located in full sun the sunlight will help dry the water off the buds after a rainfall.
The problem of drought is the most straightforward issue to correct. Simply keep your plant adequately watered during the summertime, and make an extra effort during periods of drought. How much water should you give your rose of Sharon? The exact amount (and the precise frequency of watering) depends on the conditions. For this reason, rather than trying to put a number on it, just remember this: The soil around your plant should be kept evenly moist. Gently dig down 6 inches into the soil and feel it with your hand. The soil should feel neither dry nor soaked, but slightly moist. If it is dry to the touch, provide at least 1 inch of irrigation water, preferably apply directly to the soil rather than sprayed from overhead.
Rose of Sharon normally does not need much feeding, but if you notice the size or profusion of blooms dwindling, it may indicate a phosphorus deficiency in the soil, which is preventing the plant from properly converting the other available soil nutrients. A general-purpose fertilizer can correct mild deficiencies, or you can scratch in an organic bone meal fertilizer into the soil around the plant. Amounts you apply should be modest, as too much phosphorus can stunt the entire plant.
If you are not seeing even flower buds on your plant, it's possible you have pruned incorrectly. Because this plant flowers on new growth wood, if you pruned too close to flowering time, you will remove the new wood from which the buds and flowers appear. This shrub should be pruned immediately after flowering is finished in the fall, or in the very early spring before any new growth has appeared.
Pests and Diseases
Pests should be visible to close inspection. Aphids, the most common pest, will usually be visible on stems or the underside of leaves; buds may feel soft, and if peeled open, they may show rot. Neem oil or insecticidal soaps are safe ways to combat aphids and other insects. Fungal problems usually occur because of overhead watering or lack of air circulation. Pruning the shrub to improve airflow between the branches may help. Or, use fungicidal powder to combat fungal disease.