Epson Salts is used to provide Magnesium to the soil. Roses seem to be heavy feeders on this element. A soil test would be the only way to really know if your soil was lacking in magnesium, but most rose growers just apply a small amount per bush. You can sprinkle a couple tablespoons around the rose, or dissolve in in a watering can first and water in. If you are using a balance rose fertilizer that contains magensium, its probably not neccessary to ad more. WE tend to over-feed our roses at times. If your soil is light and sandy, feed more, if its rich or clay like, feed less..