It’s understandably alarming to a home gardener. A hibiscus or gardenia plant that looked fine a couple of weeks ago suddenly has many bright yellow leaves on it. What could it be? Nothing about the care of the plant changed, so what’s causing the yellowing?
A common cause of this type of leaf yellowing in late fall, winter and spring in our area is fluctuating winter temperatures. Just the recent (and somewhat unseasonable) dip of temperatures down into the 40’s was enough to trigger this yellowing on gardenias and hibiscus. This yellowing does no permanent damage to the plant and if it is otherwise healthy it will more than replace the lost leaves when vigorous growth resumes next spring.
Since we often experience a number of these warm-to-cool-and-back-again cycles until late spring, you may see repeated appearances of this type of yellowing.