Rowly, cutting willows low and painting the cut surface of the stump with undiluted glyphosate (Roundup, Rodeo, etc.) is very effective if the application is made within an hour of making the cut. I always apply within minutes. As a spray for small willows or dense stands of small willows I use 3 fluid ounces of glyphosate and 1.5 fl. oz. of 2,4-D per gallon of water, with a surfactant. Either herbicide alone would probably work well but I find the combination especially effective. Of course, you know not to waste herbicide on dormant trees. Hope this helps you.
