Ask Mr. Smarty Plants

The althea is also called Rose of Sharon, scientific name Hibiscus syriacus. It is a mallow, a member of the Malvaceae family, and native to China and India. At the Lady Bird Johnson Wildflower Center, we are focused on the use of plants native to North America and to the area in which they are being grown. So, this plant falls out of our area of expertise. However, we will see if we can find some information for you on the failure of the blooms on your bushes to open.

Among the things we learned about your plant is that the blooms last just one day, they open in the morning, close in the evening and the closed bud remains on the bush for a while until it drops off. They require ample moisture to flower best and avoid leaf drop. This has been such a hot, dry summer that might be a clue to the problem. However, you say they are 3-4 years old. Did the blooms on the plant ever open? Another thing, bud drop can be caused by too much or too little water or over-fertilization.

So, with these pieces of information, but no real answer to your problem, we are going to make some recommendations in the culture of your plants, and see if that helps. These are not an immediate fix, but over time could help. First, blooms are on new wood, so the plant needs regular pruning. It probably is too late in the blooming season for pruning to do any good for this year. But, when the weather cools off, give the bushes a good haircut.

Second, make sure that the blooms are really not opening at all. Is there any possibility that you only have time to go out in your garden in the evening, by which time the blooms have already closed? For now, stop fertilizing. You may have been over-fertilizing with a high nitrogen fertilizer, which will cause lush green leaves but not blooms. Next summer, about 6 weeks before the time that your bushes have tried to bloom in the past, put a fertilizer on them with a higher phosphate content, the middle number on fertilizer designations. This encourages bloom. Read the instructions on the package to determine how much and how often.

Now, let's address the water issue. We don't know what kind of soil you have, but are betting it's some version of an alkaline clay. Which is probably fine for the Hibiscus syriacus, but it can affect the way the plant gets water. About every other day, stick a hose down in the dirt around the roots and let a slow dribble go in until the water shows up on the surface. If the water stays on the surface more than about a half hour, you have poor drainage. Water can collect around the roots and just sit there. If that is the case, water them less but more frequently. This will be trial and error to see how well the drainage is working. If you can, try to work some organic materials, like compost, into the soil around the roots. This will improve the texture of the soil and definitely improve drainage. And mulch them all with a shredded bark mulch. This will not only help hold in the moisture and protect the plant from heat and cold but, as the mulch decomposes, will add more to the organic content of the soil.

This USDA Forest Service website on Hibiscus syriacus goes into more detail on the matters discussed above, and also mentions a possible blight on the blooms caused by a fungus. It doesn't say what should be done about that, but it would probably be better to try some of the other fixes first before you buy an anti-fungal spray. This site also notes that the plants could use some protection from afternoon sun, and you indicate that they are in full sun until 3 to 4 pm. You probably are not interested in moving them just on that account, but there again, the mulch and additional watering should help the bushes do better.