I planted a willow hedge in early March. The willows were on average 6ft tall when planting. i can see now that they are about 9 ft or more but the leaves are starting to turn yellow and die. I have other willow trees and this is not happening to them and it much too early for them to be wilting for autumn. the ground is plenty wet and i can see the other willows thriving on the soil type. Are they too tall, should i prune them back now?