This is a good time to ask this question because late winter near the end of January or February is a great time to prune any willow. The willows or Salix are fast growing trees and shrubs and although they can handle pruning at any time of year if they are healthy, when you prune in late winter their is less of a chance of disease on the new growth. As long as the temp is above freezing, go outside and get snippy with your willows. One more tip. Willows contain a hormone that will help other plants form roots. They also have anti fungal properties so new cuttings will be less likely to rot. You can soak cut willow stems in a glass jar of hot water for 48 hours then discard the willow sections and store the water for using when you make hard wood cuttings for new plants. Just let the cut end of new cuttings sit in this willow water over night and you will find that the new roots form on the cuttings a few days to a week sooner than if you did not soak them in willow water. Gardening is an adventure so don't be shy about doing more research about using willow water or trying to make new trees and shrubs from cuttings.  Keep  growing, Marianne Binetti