I have two mature grey sally (willow) trees growing on the boundary of my rural garden. These shed lots of seeds during the late spring, which means lots of seedlings all over my flowerbeds. I also have an ash tree which is threatening to stifle a neighbouring beech tree. I plan to have these cut down and paint the stumps with SBK Brushkiller. Is this a sutiable time of year or should I wait until Spring?