Mom and Dad use epsom salts on their peonies - supposedly strengthens the stems. I haven't noticed it helpful for that, but it is a good source to correct magnesium deficiencies. Apparently a high potassium fertilizer can prevent plants from taking up magnesium in the soil, and the epsom salts can be used as a foliage spray.

I used it in my potted tomatoes last year as I thought I was seeing signs of magnesium deficiency. I apparently misdiagnosed the problem, because it did not help.