I have always had the most beautiful impatiens in window boxes, but this year they look terrible. I used Miracle Gro potting soil with moisture control. Because we have hot summers in Illinois, I thought this would help. I water only when the soil is beginning to get dry to the touch. All the foliage now has a white/gray coating that makes it feel almost like an african violet. They really are pretty bad looking. Any ideas how to salvage the remainder of the growing season?