Growing a Japanese Stewartia?

I planted a 6 foot Japanese Stewartia in Spring 09. I live in the Midwest, our soil is clay. Tree has a lot of buds, but there not opening? It is starting to loose some of its leaves. I did use high organic matter when I planted it. I am afraid to over water the tree. How wet am I suppose to keep the soil?

Hardiness Zone: 5b

By Fran from Palos Park, IL