try cuttings, willow is about the easiest tree to get rooted. on a fun note, willow leaves and twigs used to be steeped in a big jar and the liquid used as a rooting hormone. so one of my old gardening books says.
Cuttings of the stems in water, are an excellant rooting hormone solution, works on most of the plants that I have tried. I would preferably, plant cuttings in the ground in the early spring, just as you see them starting to break their buds, works better if you place your cuttings into water for a few days. Or anytime for that matter. You can use any sized, diameter cutting that you want to.