Marsha,
I am trying Wilt-pruf for the first time this year so I have no first hand experience but I did go to their web site for info and e-mailed a specific question to them. They answered the same day (a Saturday). The general info I have is that it takes about four months for it to completely break down and need re-applying but and here is the big but for you I believe it needs to be applied on a day when the temps are 40 or above. Katy

I have tested wilt-pruf to see how long it lasts and the one - two week window that some people say that it has is pretty liberal according to my results. What happens is that the coating cracks and the water goes between the cracks and out into the air -- and your tree dries out. The wilt-pruf people can say that it takes a long time to break down because it does -- but those cracks just can't be stopped.