Q: I have two hibiscus plants that are more than 3 years old. They have very few buds this year, and sometimes the unopened buds fall off the plants. I have noticed a yellowing of the leaves with small black speckles at the ends. I have sprayed them with Garden Safe Houseplant and Garden Insect Killer once. This does not seem to have gotten rid of the black speckles.
A: Hibiscus can fail to set flower buds or will abort unopened flower buds due to cultural problems or insect infestations.
Plants that are pruned at the wrong time can take months to flower again. Those that receive too much nitrogen fertilizer will produce a lot of vegetative growth at the expense of flowers. While it is always best to fertilize based on soil test results, fertilizers that are higher in phosphorous and lower in nitrogen are a good choice for flowering plants. All fertilizers should have an analysis written on the container -- a series of three numbers that denotes the percentage of nitrogen, phosphorous and potash in that fertilizer. The nutrients are always in the same order, so you would look for a fertilizer with a higher middle number, something like 5-10-5.
Another cultural issue is the hot, dry weather we have had for much of the summer. Heat stress can cause flowering plants to drop their flower buds before they open. It also may have been too dry for them if they were not watered when we were not receiving sufficient rainfall.
The black specks on the leaves could be evidence of insect infestation. A very tiny insect known as thrips (the term is plural and singular) is a common hibiscus pest that can cause flower buds to drop before opening. They feed inside the flower buds as well as on the leaves, and plants with active thrips populations often appear dirty and unkempt with dark spots visible on the leaves and buds. If flowers do open, they may be deformed or spoiled by irregular white streaks. To check for thrips, shake suspect-looking buds or leaves over a sheet of white paper. If they are present, you will notice small (1/16 to 1/8 inch), insects that are yellow, brown or black depending on their species and stage of development. A 10X magnifying glass or hand lens will help you to see this pest more clearly.
While the product you applied is labeled to control thrips, it probably is not the most effective one, and it would take repeated applications at intervals recommended on the label. Other products for controlling thrips include insecticidal soap (Safer's Insecticidal Soap), spinosad (Captain Jack's Deadbug Brew), imidacloprid (Bayer Advanced Tree & Shrub Insect Control) and lamba-cyhalthrin (Spectracide Triazicide Once & Done Insect Killer).
Hibiscus is also susceptible to spider mites. Symptoms include bleached out ("stippled") areas on the foliage where these very small spiders suck the chlorophyll out of the leaf. You can also see very fine webbing when two-spotted spider mite populations are high. The main miticides available to home gardeners are horticultural oil and insecticidal soap. Like thrips, spider mites are not clearly visible to the naked eye. Two-spotted spider mite adults are pale yellow to light red with a dark spot on either side of their abdomen.
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