I put Epsom Salts around my roses this past week, but I never know when it's safe to begin fertilizing. I'm in zone 5 near the border of Idaho and Utah. Out here, we don't plant annuals until after Mother's Day, and last year we had snow on May 28, altho' it melted in a few hours. I don't know if fertilizing my roses too soon would result in killing the new shoots if the thermometer should take a nosedive for more than a day or so.