The only plant that isn't fond of ES is sage. It's fine for your hydrangeas.
Add some to the water when you soak the new roses as they arrive and use that water in the hole for them.
ES were discovered by a farmer in Epsom, England and he noticed his cows' sores healed faster but they didn't like the taste of the water - due to the Epsom Salt. It's been used for soaking feet, added to bathtubs to improve levels of magnesium (helps muscles relax) and in the garden.
