my pond has a dam around 2 sides and i planted willows on the 2 sides that did not have a dam.a willow is one of the few tree's you can cut branches off another tree and stick them in the ground and they will grow.i cut about 10 branches last year and about 7 of them took and grew well.it takes several months but they will pop up from the ground were you planted the branches.to answer your question i did this twice once in early march and once in about mid may.