A couple of years ago, I planted three Wisteria frutescens 'Amethyst Falls' along my south-facing porch. They are now up about 15 feet and blooming well in the spring. In the summer, however, the leaves yellow and drop. We've had about 2 inches of rain each week with highs near 90 and lows near 70 most of the summer. The soil is sandy clay with neutral pH. I fertilized them last spring with a balanced, time-release mixture containing systemic insecticide. They are putting out new leaves in the past week (the weather has been cooler and dryer, with highs around 85 and lows around 60). Anything I can do to prevent the leaf drop?