Muhly doesn't need much in the way of soil prep, unless your soil is really heavy clay. It will grow in almost any other soil, but prefers fairly lean soil to very rich soil. It can grow in full sun to part shade.

It looks great before it flowers, in my opinion. It got very fine foliage. You can see photos of it before flowering here:

http://www.delange.org/MuhlyPink/MuhlyPink.htm

The flowers should be present in your zone for at least a month, but they will not maintain the color as time goes by. They dry and become beige. They also become very brittle, and will be scattered by the wind. Be aware muhly can self-sow in your zone.