Late July means hot, sweltering temperatures, and one might think those would be perfect conditions for tropical plants like hibiscus. But one would be wrong.
While the hibiscus is tropical by definition, that doesn't mean they like 100-degree temperatures. Tropical plants in our humidity actually prefer 88-90 degrees. So, when the thermometer spikes, a myriad of problems can ensue in Gulf Coast gardens, which might normally be considered "Hibiscus Heaven."
I don’t have room here to detail all the problems hibiscus might see with high temperatures. Thankfully, I recently covered one of their bigger issues in my Mealy Bug Mayhem article. So, here, I will try to answer a question that has been dominating my email of late: "Why do many of my hibiscus buds turn yellow and drop before they ever open?"
As with most gardening issues, there isn’t one clear-cut answer. But there are a couple of dominant reasons for bud drop, especially in Houston. The first can be attributed to really hot weather. Remember that these tropical plants prefer 88-90 degrees with lots of humidity. Temperatures above 95 for long periods can stress out some hibiscus, especially hybrids, and they shed blooms as a defense mechanism.
Moving potted hibiscus could also cause premature bud drop. Much like the ficus - the "teenager" of the plant world 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 that hibiscus drop their buds are insect-related. In addition to mealy bugs, thrips and hibiscus midges cause dropped buds. However, the controls are different. So, it's critical to find out which one is the culprit.
Here's how I do it.
First, if you’re convinced it's not about extreme temperatures, you determine if it's thrips. Find an unopened bud (or one just on the verge of opening that is starting to turn yellow with burnt-looking edges) and tap it while holding a white piece of paper underneath. If you see tiny black flecks fall to the paper and start scampering, you have thrips.
You’ll know thrips are also the likely problem when the majority of the plant’s blooms look stunted, premature and slightly gnarled (below).
Thrips are easy to control with any liquid insecticide containing permethrin or bifenthrin. If you want to stay organic, try liquid pyrethrum. Spray the insecticide all over the remaining blooms. You may need two applications over two weeks, but I've found that one good soaking of the remaining buds is often all it takes.
If you find no thrips, and don’t believe the drop is from temperature stress, you can assume that it's hibiscus midge. They are hard to detect with the naked eye. Midges are from the “gall midge fly” family of pests, which lay eggs in the bud where microscopic larvae feed and cause the premature drop.
The treatment for thrips and mealy bugs will not phase midges. So, you’ll need to apply a liquid systemic insecticide that is safe for hibiscus. The systemic needs to work up through the plant and get to the buds internally since the midge larvae are so deeply inside them. The best controls are those with acephate or imidicloprid. For the record, I’m not a huge fan of systemic insecticides on flowering plants. Especially this close to hummingbird season. And because of the already existing threats to our bee populations. If you have the same concerns, my advice is to simply cut off every bud you have (or prune way back, period) and allow new growth to set new blooms for the remainder of the summer.
And there’s one other theory 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." There’s a need for consistency in moisture, consistency in food, and consistency in sunlight. If all these stay consistent, and if you can prevent the insects, there is little chance your hibiscus will be hit with premature bud drop.