The leaves of my hibiscus are curling up, not yellow, but they are still flowering quite well.
Certified GKH Gardening Expert
Generally, this is a sign of an insect infestation. Spider mites, mealybugs, aphids, thrips, and whiteflies are common pests that affect hibiscus plants, sucking sap from them and resulting in anything from poor growth to leaf curl and drop. Thoroughly inspect the plant to see if this is indeed the problem and then treat the plant with neem oil. If you do not see any signs of pests, check the soil to ensure it is getting adequate moisture. These articles should help with identifying pests: https://www.gardeningknowhow.com/plant-problems/pests/spider-mite-detection-and-spider-mite-natural-control.htm, https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm, https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm, https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-thrips.htm, https://www.gardeningknowhow.com/plant-problems/pests/insects/whitefly-control.htm, https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm