Q.Why do my luna rose mallow buds turn black in summer?
Have had this plant for about 2 years, and it happened last year too. After blooming quite well and profusely at the beginning of spring, the following buds turn black and fall off, and the plant doesn’t have any more flowers through the summer or fall, just a lot of black buds,. It’s sad and frustrating – what can I do?

Certified GKH Gardening Expert
Do not allow them to dry out between waterings. They need regular, deep irrigation. Also, be sure to fertilize regularly during the growing season.
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm