With out seeing the plant and if you can't see any pest, I would guess this is normal leaf drop. This time of the year most hibiscus shed the older leaves for the new growth.

Well my hardy hibiscus blooms great now but the leaves still don't look that great. We get a LOT of wind and although it's staked, I'm thinking that the leaves are just getting windburned? Also, before I planted it almost a month ago, the stems were green. They're reddish now. What is that? J

Hi Gardengopher.... I am just getting re-started on Hardy Hibiscus... and still have two in pots... trying to figure out where to plant them.. lol I am in Tennessee also.. in Perry County... Carolyn

FWIW - applying an element or compound aimed at delivering a single nutrient (in this case, Mg) is FAR more likely to result in limitations than it is in improved growth ..... especially if you have no reason to suspect the plant is suffering from a deficiency of Mg. There really is no substitute for a nutrient supplementation program that is resultant of a measured approach to addressing the results of a soil test for plants growing in the ground. If your plants are in containers, regular supplementation of nutrients in the ratio at which the plant actually USES the nutrients is far more likely to yield better results than an approach based on the idea that it seemed to work before, so it's gotta work again. If Becky's plants aren't being limited by a Mg (or S) deficiency at the time the Epsom salts are applied, it's a certainty the excess will be a limiting factor. The goal for fertilizing containerized plants can easily be described. Ideally, we would work toward ensuring that all the nutrients plants normally secure from the soil are in the soil solution at all times, in the ratio at which the plant actually uses the nutrients, and at a concentration high enough to ensure no deficiencies yet low enough to ensure the plant isn't impeded in its ability to take up water and the nutrients dissolved in water. This goal is easily achievable using one water soluble synthetic fertilizer. You CAN use organic forms of nutrition, like fish/seaweed emulsions or various types of meal, but that makes it much more difficult to achieve the goal. Since you don't have reason to believe the result of your Epsom salts application will leave the plant with Mg in the soil solution at a favorable ratio, or if the ratio and therefore the o/a level of TDS (total dissolved solids) will be unnecessarily high due to the supplementation of unnecessary Mg and S, the probability is that following that plan will result in further unnecessary limitations. We can Keep It Super Simple by choosing a soil that allows us to water correctly, and using a fertilizer that provides nutrients in a ratio that ensures nutrient availability in a ratio that suits the plant, as opposed to one that virtually ensures limitations resultant of nutritional imbalances due to applications aimed at supplying a single element. Plants might look like simple organisms that sort of sit around not doing much of anything, but the truth is, plants carry out activities of amazing complexity every moment of their life, and react to cultural influences in their environment constantly in ways, some of which are understood, or in ways that yet still confound the efforts of researchers to isolate and describe. Approaching growing in ways that depend on a wing and a prayer will almost inevitably leave growers who embrace that plan standing in the slipstream of growers who take the time to put together a measured plan that takes what is known of the plant sciences into account. The more we learn about a plant's needs, the more likely it is our efforts will carry the day (from the plant's POV). Al