Hibiscus is a tropical shrub that can be grown outdoors in the warmest planting zones in the south--9 and 10--where it can grow as tall as 15 feet. In other parts of the country, it can be grown in containers and brought inside during the winter. There are hundreds of types of hibiscus and all can fall prey to a variety of problems.
Stress can be caused by over-fertilization, by the soil being too wet, insecticide that was not applied according to directions, transplanting and disturbing the roots or even a change in the amount of light. Symptoms include yellow leaves and will disappear when the source of the stress is corrected. Stress can also be caused by frost. In this case, the plant will be slow to bloom.
Phytophthora occurs when soils dry in the summer and get wet in the winter. It can kill the plant, but can be prevented by providing good drainage. Collar rot is a slimy, brown discoloration that appears where the root joins the plant and attacks in cool damp winters when the plant has poor drainage. Sclerotina rot attacks the stems as they emerge from the ground. Leaf spot is a fungal disease that attacks mostly container grown plants. It appears as dark brown or black spots on the leaves.
Different insects can cause different levels of damage to the hibiscus plant. Butterfly grubs do just visual damage. Snails and slugs eat the leaves of young plants. Spider mites cause white spots and/or chlorosis of the leaves, aphids destroy the buds and mealy bugs and aphids cause a disease called sooty mold. Thirps will attack the plant in dry areas. The larvae of the gall midge live in the buds and eventually the buds drop off. The Japanese beetle will eat both the buds and the flowers.
Sparse foliage is caused by the plant not getting enough sun. Deformed and sick looking leaves can be cause by using systemic insecticides. Yellow leaves at the bottom of the branches is normal. Higher up, where the new leaves are indicates a magnesium or iron deficiency or over feeding with chemical fertilizers. It the yellow leaves have green veins, it is an iron deficiency. Brown around the edges of the leaves indicate windburn or toxic reaction to salt. If they turn brown at the tip, but, purple around the edges and become brittle, is indicates a potassium deficiency. Frost damage can be fatal.