I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
When the Hibiscus tree loses its foliage it is usually due to inadequate soil moisture. Both dry conditions and soggy wet conditions can cause loss of roots followed by loss of leaves. I suggest you try to maintain even moisture without overwatering and start fertilizing the hibiscus weekly with a soluble plant food such as a Miracle Grow.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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