My stand of Hibiscus is dying out, first 3 or 4 years they looked great, now they have mostly died out, and the ones that are left are small, Fist I have trees in the yard that have gotten bigger since I planted the Hibiscus and are shading the Hibiscus quite a bit, could this be the problem? Do I need to move some of what is left to a more sun lighted area, and if so, when should I do this? I also mow them off in the spring, I did not think this would hurt them. Thank You