Currently, I have a large Weeping Willow (50 ft. in height) in SE Pennsylvania. We had a very wet winter and spring, but a relatively dry summer. I understand that Weeping Willows are notoriously known for shedding--both leaves and and twigs. But recently, the tree seems to lose a lot of yellow leaves, and the top is getting very thin. Also, a river birch nearby is suffering from the same problem. Could it be a lack of water--should I water more often? Could it be as a reaction to lawn fertilizer (Scotts plus Halts)--I read somewhere that herbicides could cause this as well.