What Causes Hibiscus Buds to Drop Prematurely?

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It's August, and that means hot, sweltering temperatures. One might think that would be just perfect for certain tropicals like Hibiscus, but one would be wrong. While Hibiscus are tropical by definition, that doesn't automatically mean they like 100 degree temperatures. Tropical plants by definition, prefer our humidity with temperatures around 88 to 90 degrees. That means that when the thermometer spikes, like it has lately, a myriad of problems ensue for what normally is the "Hibiscus Heaven" Gulf Coast gardens.

naUnfortunately, this is a weekly e-mail tip and I don't have time to detail all the problems that Hibiscus endure because of high temperatures. Luckily, we already covered one of the associated problems with last week's e-mail tip on Mealy Bugs. In fact, my next book may be all about tropicals for Houston and that would definitely include a major chunk dedicated to Hibiscus. But today, I will answer the question that has been dominating my email of late: "Randy, why do so many of my Hibiscus buds turn yellow and drop before they ever open?"

Like most situations in gardening, there is not one clear cut answer. But there are a couple of dominant reasons, especially in Houston. The first reason for bud drop can be attributed to the really hot weather. Remember, these are tropical plants that would much prefer 88-90 degrees with lots of humidity. Temperatures above 95 for long periods of time can stress out some Hibiscus hybrids to a point that they shed their blooms as a natural defense mechanism.

Moving potted Hibiscus could also cause premature bud drop. Much like the "teenager" of the plant world, the Ficus, which throws a fit by shedding its leaves when you move it, potted Hibiscus plants may also shed buds as a stress indicator.

naBut the most prevalent reasons that Hibiscus drop their buds are both insect related. While we discussed the onslaught of Mealy Bugs last week, and while they can cause a bit of bud drop, its Thrips and Hibiscus Midges that cause the most "dropped buds." Interestingly enough, the controls are different, which does not include the home made Mealy Bug control from last week.

So, that means it's helpful to find out which one is the culprit. Here's how I do that. First, you have to be convinced it's not about the extreme temperatures as noted above. Then, you determine if it's Thrips. The best way to detect Thrips is to take an unopened bud (or one just on the verge of opening, that is starting to turn yellow) and tap it on to a white piece of paper. And if you see tiny little black dandruff-like flecks fall to the paper and they start scampering, then you have Thrips.

Thrips are easy to control, and the best control is any liquid insecticide with Permethrin or Bifenthrin. If you must stay organic, try a liquid Pyrethrum. Spray one of these insecticides all over the remaining blooms. It may need two applications over two weeks, but I've often found that one good soaking of the remaining buds is all it takes.

But, if there's bud drop and there's no detection of Thrips, and you're convinced it's not temperature stress, then just assume away that it's the Hibiscus Midge, which is hard to detect to the naked eye. Midges come from a technical family of pests called a Gall Midge Fly, which lays it's eggs in the bud and the microscopic larvae feed on the inside of the bud, causing the premature drop.na

Unfortunately, the aforementioned treatment for Thrips and Mealy Bugs will not phase Midges. So, you have to apply a liquid systemic insecticide that is safe for Hibiscus. 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 control methods for Hibiscus Midges are anything with Acephate, anything with Disyston and anything with Imidicloprid.

There is also one other theory important to keep in mind that might lead to some bud drop, but that almost always leads to an excess of yellowing leaves too. I call it a "lack of consistency." That would mean a need for consistency in moisture, consistency of food and consistency of sunlight. If all these stay consistent, and if you can prevent the insects talked about earlier, there is little chance of premature bud drop on Hibiscus.

If you like the idea of a Tropical Handbook for Houston Gardening, as my next foray into the publishing world, drop me a line or even suggest a topic or two that you would like to see in such a book.

Randy Lemmon is the host of the GardenLine radio program on Newsradio 740 KTRH. Randy has been doing GardenLine in one capacity or another since December of 1995, for all three of the now Clear Channel AM stations - KTRH, KPRC & KBME. When Randy took over GardenLine, he replaced long-time Houston radio veteran and GardenLine originator, Bill Zak. For those who remember that far back, GardenLine was a weekly radio staple on KTRH from 10 a.m. to Noon Mondays through Fridays - along with a Saturday show as well. Now GardenLine is heard exclusively on Newsradio 740 KTRH on weekend mornings.