I'm attempted to propagate my dappled willow from cuttings. They root well, and mostly grow well until they reach a phase after a couple of weeks where the leaves turn dark brown/black along the edges, at which point much of the brach those leaves are on follows suit. I'm using a mix of peat moss and vermiculite for the potting medium and my goal is to move them outside either this year or next. How do get solid branches that don't wither before the end of the first month of growth?