I don't think I would worry about the rain. When I use neem oil I mix it with hot water and shake in order to get the oil mixed with the water to spray. So I don't think a spring rain will wash it off of the tree, the water won't break up the oil because it's cold. But also, when I use neem oil I have to reapply it every week or so in order to keep the bugs at bay. Hope this helps.