Why Do Hibiscus Flowers Turn Yellow?

Heat and Light

• Hibiscus flower color changes are sometimes natural and not necessarily an indication of a problem. Some hibiscus varieties, called carotenoids, naturally exhibit a range of flower colors, including yellow, orange and red. As temperatures and sunlight increase in summer, a flower's color intensifies from yellow or orange to red as if the bloom is ripening. The intensity change is a result of higher pigment production in hot, bright months. When temperatures and sunlight hours decrease in fall, bright flower colors soften into yellows and oranges. If weather becomes very cool or cloudy, some hibiscus flowers may turn pale yellow. If the color change appears problematic and is accompanied by flower buds dropping, then the temperature may be wrong for your particular hibiscus. A hibiscus plant produces its most buds when the daytime temperature is 65 to 75 degrees Fahrenheit. Flower buds also drop when light is too low.

Insect Problems

• Insect infestations are among the most common reasons flowers turn yellow and drop. If that happens to hibiscus flower buds rather than open flowers, then thrips probably are the cause. Thrips are about 1/20 inch and yellow, black or brown; they feed on buds. Midge larvae, small maggots that feed on hibiscus buds, can cause discolored buds and blooms. Both thrips and midges can be controlled by spraying the hibiscus with a ready-to-use insecticide containing bifenthrin. Spray the plant from a distance of 12 inches, covering upper sides and undersides of leaves. Reapply the insecticide every two weeks if necessary.

Dormancy

• A hibiscus plant kept indoors or outdoors in a pot begins to enter dormancy in fall and winter. During that time, its leaves may yellow and its flowers may become discolored. When the hibiscus is dormant, place it in a room that is 55 degrees Fahrenheit, and water the plant only when its soil feels almost completely dry when you check it. In early spring, place the hibiscus in a sunny location, increase how often you water it, and fertilize the plant with a fertilizer such as a granular, 18-6-12 fertilizer. Apply 4 teaspoons of the granular fertilizer for ever 1 gallon of the plant's pot size. Mix the granular fertilizer into the plant's soil so that it is evenly distributed, and water the soil thoroughly. Fertilize the hibiscus in that manner three or four times per year. The fertilizer should help the plant produce normally colored flowers.

Watering Issues

• Too little water can cause blooms to yellow and drop. If your hibiscus is in a pot, ensure the pot has sufficient drainage holes. Water an indoor hibiscus' soil only when the top 1 inch of soil feels dry to your touch, and water it deeply. A hibiscus growing outdoors in summer needs substantial water, and brightly colored flowers may fade and yellow if wilt from the lack of water is not dealt with quickly. Wilt also affects a hibiscus' ability to bloom in the future. Water your hibiscus deeply at least once each week when its soil is dry, wetting the soil to a depth of 12 to 18 inches.