Last spring I purchased a cuphea plant and planted it in a large redwood planter in a partially shaded spot on my deck. Since then it has developed lots of new growth along the existing stems and some new stems as well. Recently, however, a lot of the leaves have started to turn brown/grey from the tip of the leaf back. Does anyone know why this is happening? Am I over/under watering (I use a moisture meter and try to keep the plant moist but not wet) or is this normal at this time of the year? There is no indication of any insect infestation.