My husband built a planter that is about 25 inches X 11 inches and 10 inches deep and planted lantana in it in a dirt/mulch planting mix. While everything else that was planted is doing well, the lantana looks sick. The leaves have brown edges and it is not growing new ones. Is it getting too much water? Does lantana need a sandier soil? He did put drain holes and rocks in the bottom of the planter.