Observing the leaves of a hibiscus (Hibiscus rosa-sinensis) can tell you whether it’s getting all it needs to thrive. Hibiscus is hardy to U.S Department of Agriculture plant hardiness zones 9 through 11, where it’s a perennial. In the cooler zones it’s treated as an annual. Hibiscus grows dark green, glossy leaves and an abundance of flowers. Although yellowing hibiscus leaves can be an indication of the natural resting phase, it can also be a sign of a more serious matter, such as improper watering, lack of nutrients or sap-sucking pests.
The Resting Phase
Hibiscus won’t tolerate temperatures of 32 degrees Fahrenheit and lower. In cooler climates, the plant’s leaves will turn yellow and drop. This is an indication that it’s entering the dormant resting phase. To encourage it to regrow, grow the plant in a pot and bring it indoors, placing it in a cool room at 55 degrees Fahrenheit. Keep it here for six to eight weeks, watering it just enough to keep the soil from completely drying out. After this dormant period, use sterile pruning shears to cut back the plant to 6 inches from its base, place it in a sunny window at 68 to 75 degrees Fahrenheit, and increase your watering frequency to encourage new growth.
Hibiscus leaves that turn yellow and drop off the plant can be an indication that you’re watering it too much or too little. When the plant is actively growing, and during hot, dry weather, keep the soil consistently moist, not wet. Provide about 1 inch of water a week, adjusting your watering frequency after rainfall. Grow hibiscus in well-drained soil, and if you’re using pots, ensure they have drainage holes, because soggy soil and sitting water can encourage root rot.
Lack of Nutrients
A lack of nutrients can be to blame for leaves that partially turn yellow. Testing the soil pH before planting can give an indication of the available nutrients and will also provide instructions on how to amend the soil to bring it within the desired range. Hibiscus thrives in soil with a pH range between 6.0 and 8.0. If you notice yellowing as the plant grows, a fertilizer boost can be beneficial. Spread 1/2 a cup of a granular 15-5-10 or 15-5-15 fertilizer on the soil below the plant’s canopy, and water it in.
Sap-sucking pests, such as spider mites, mealy bugs and aphids, can rob nutrients from the hibiscus, turning it leaves yellow before they eventually drop off the plant. Regularly checking the undersides of the leaves can help you detect the tiny pests early on. If you decide to combat the pests with horticultural oil or pesticide, do so early in the morning or in the evening. You want to avoid spraying the hibiscus during hot weather, because this can also damage the leaves.
Some Like It Hot: Flowers That Thrive In Hot Humid Weather; Pamela J. Gartin
Alabama Cooperative Extension System: hibiscus: A Plant That Blooms All Year
North Dakota State University Extension Service: Questions On: Hibiscus
National Gardening Association: Hibiscus
Hidden Valley Hibiscus: Yellow Leaves on Hibiscus
Mississippi State University Extension Service: Soil pH For Landscape Plants
University of Florida IFAS Extension: Hibiscus In Florida