I may be wrong, but I have been told that the pussy willows can deal with soil of average moisture, down to positively wet soil! Even so, if you can, put it in an area that gets a fair bit of natural moisture (runoff, etc.), rather than on the highest point of your yard. You WILL have to water it well (a gallon or more) when first planted out, every few days for the first 2 weeks, and then cut back to weekly (unless it's rained) for the next few months, until the end of Nov. If it's another mild winter, or if you are on sandy soil, I would keep on watering, lightly, in the winter, and then more heavily as it leafs out, through next summer, when the roots should have grown enough to cut back on watering to every few weeks. Newly planted trees SHOULD be watered weekly for the first 2 years, although most of us consider them lucky if the watering gets done at all after the first 6 months!! That said, I did water a bunch of my trees/shrubs that had been planted for more than 18 mo. over this summer, to stave off stress from the NASTY combination of very high heat and no rain for 3-4 months!

Mulching around the tree, out to 3-4' will help with the watering, keeping the soil evenly moist. Just DON'T put any mulch closer to the trunk than 3-6 inches, and don't pile it higher than 6 inches. Mulch next to the trunk invites fungus anad rotting, and deeper than 6" prevents the free exchange of gasses/oxygen through the soil. You want to encourage the roots to grw out as widely as possible.

Oh yeah, DON'T plant any willow next to your water lines or your septic system. They DO like water, and their roots will infiltrate the system.