One might think that August's sweltering temperatures would be just perfect for tropicals like hibiscus. But one would be wrong.
While hibiscuses are tropical by definition, that doesn't necessarily mean they like 100-degree temperatures.
Tropical plants prefer our humidity with temperatures around 88-90 degrees. That means when the thermometer spikes, a myriad of problems ensue in what is normally "Hibiscus Heaven" — Gulf Coast gardens.
Unfortunately, there's not enough space here to detail all the problems that hibiscuses endure because of high temperatures. But here's an answer to the question that often dominates my summer e-mail: "Why do so many hibiscus buds turn yellow and drop before they ever open?"
As in most gardening situations, there is not one clear answer. But there are a couple of dominant reasons ... especially in Houston. The first can be attributed to really hot weather. Temperatures above 95 for long periods can stress some hibiscus hybrids to the point that they shed blooms as a natural defense mechanism.
Moving potted hibiscuses could also cause premature bud drop. Much like the ficus, which throws a fit by shedding its leaves when you move it, potted hibiscus plants may shed buds as a stress indicator.
But the most prevalent reasons for hibiscuses to drop their buds are insect-related. While mealy bugs can cause a bit of bud drop, it's thrips and hibiscus midges that cause the most drops. The controls are different, however — homemade mealy bug control only takes care of that pest. So, it's important to find out which one is the culprit.
First, you have to be convinced it's not about extreme temperatures. Then, you determine if it's thrips.
The best way to detect thrips is to tap an unopened bud (or one just on the verge of opening) that's starting to turn yellow above a white piece of paper. If you see tiny black dandruff-like flecks fall to the paper and start scampering, you have thrips.
Thrips are easy to control using any liquid insecticide with permethrin or bifenthrin. To stay organic, try a liquid pyrethrum. Spray all over the remaining blooms. You may need two applications over two weeks, however I've found that one good soaking is often all it takes.
But, if there's bud drop and no thrips ... and you're convinced it's not temperature stress ... then you can assume it's the hibiscus midge, which is hard to detect with the naked eye. Midges come from a family of pests called the gall midge fly. They lay their eggs in the bud, and the microscopic larvae's feeding causes the premature drop.
The treatment for thrips and mealy bugs will not phase midges. You'll have to apply a liquid systemic insecticide that is safe for hibiscuses. The systemics need to work their way up through the plant and get to the bud internally since the midge larvae are so embedded inside the buds. The best controls include anything with acephate, anything with disyston, and anything with imidicloprid.
There is also one other thing that might lead to bud drop. I call it a "lack of consistency," but that almost always leads to yellowing leaves, too. The hibiscus needs consistency in moisture, consistency in food, and consistency in sunlight. If all these stay consistent ... and you stave off insects ... there is little chance of premature bud drops.