Hardiness Zone: 10a
Kay from Florida
There are several reasons a hibiscus will start to drop leaves: over fertilizing, a build up of fertilizing salts, too much moisture, too little moisture, too long of a transit time between transplanting and insect attacks. You are the only one who knows plant's history, so you're going to have to try and use the process of elimination. There are some insects that will defoliate entire plants, so check closely for insect damage first-both around the leaves and near the base of the plant. If there are no visible signs of insects, consider root damage. When a plant loses its leaves from the bottom upward and very few new leaves are sprouting to take their place, it's often a sign the plant has sustained some type of damage to its roots. Sometimes it's easy to accidentally dig into hibiscus crowns when transplanting or weeding around nearby plants. If this is the case, there isn't much you can do but take a wait-and-see approach to see if the plant survives. If your remaining plant has new shoots coming off of the main stem, it says to me that it's trying to recover. Did you move them outside from indoors? If you took the cuttings late last summer and rooted them successfully indoors over winter, they may have succumbed to transplant shock.
Hope this gives you some ideas!
Check the soil - sounds like you may have a buildup of some salts or you're low in something it needs.
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