We hear this question a lot at the end of winter. We also see black spots on the leaves of the hibiscus in our own garden at this time of year. They look kinda scary! What's up with these spots?
Black Spots on Hibiscus Leaves
What are they?
As bad as they look, these spots are not symptoms of some terrible new disease that is about to destroy the hibiscus. These are bacterial or fungal growths that happen in winter when hibiscus leaves are wet for too long a period of time. Typically, in areas where hibiscus will survive outdoors, the dew point is reached on many winter and spring nights. Dew forms on the leaves and does not evaporate until the sun is well up and temperatures have warmed up enough. During these hours of cool and wet, bacteria or fungus find the conditions just right to multiply. Cool, rainy days and lack of air circulation can also contribute to the growth of these small pockets of fungi or bacteria.
As bad as they look though, these leaf fungi and bacteria rarely become systemic in hibiscus. They are confined to the spot on the leaf where you see them growing. They do not cause any other symptoms in hibiscus and are not a threat to the life of the plant. They do look bad, but once winter passes and the nights are warm enough so that the leaves remain dry, or at least dry out more quickly, these bacteria and fungus of wintertime are no longer able to colonize hibiscus leaves, and they die a natural death. The old, black spotted leaves will eventually fall off the plants, and new, fresh, clean green leaves will grow in to replace them.
Black Spots on Hibiscus Leaves Go on Their Own
As Soon as the Weather Warms Up
You can hurry this process of new leaves growing in by removing badly affected leaves or even by pruning branches that are out of balance with the plant. Fertilize with good quality hibiscus fertilizer and water without getting the leaves wet. If you're worried about the spots, you can spray the plants with Bayer Advanced 3-in-1 that contains a fungicide to make sure the fungal infection doesn't spread. Spraying won't remove the existing black spots, and in most areas is not necessary to prevent new spots from forming in summer, but it can speed up the process of clearing away the fungus in cooler, damper locations.
This is one way that hibiscus differ from roses - hibiscus leaf problems are cosmetic only, and almost never harmful to the plant!