Epsom salts contain magnesium and sulfur. Magnesium is a necessary element of chlorophyll and so, if your soil is deficient in magnesium then Epsom salts can help. Before you start applying anything to your soil or your roses, we recommend having your soil analyzed by your local extension service or a qualified soil analysis lab. Adding minerals or supplements to your plants without knowing what they really need, or if they need anything at all, can be detrimental to your plants.
To learn more about using Epsom salts for your roses, see these resources: