This "hint" came from a gardening newsletter I get.
For anyone who, like me, winters Hibiscus rosa-sinensis in the garage or house," writes William Scheick, "there's hope in retarding low-light leaf-drop. In the April-June, 2009, issue of HortTechnology three researchers reported the benefit of adding a small amount of table sugar (3%) and a trace of citric acid (5 millimolar) to a standard plant fertilizer solution with an acidic pH of 5. About 60% of hibiscus leaves treated with this solution lasted 24 weeks, in contrast to untreated plants completely defoliating after a mere 12 weeks. The finding here might be applicable to other indoor plants inclined to shed leaves in low-light settings."
Certainly can't hurt to try this one.
