Epsom Salt is Magnesium Sulfate, a salt of the elements Magnesium and Sulfur, MgSO4. It has medicinal uses as well as being used as a garden chemical. Most medical uses are best discussed with your doctor.
A soak in bath salts containing Epsom Salt after a hard day in the garden is soothing, eases sore muscles and aids sleep. Maybe it's the combination of warm water and minerals. I buy an over the counter soak that contains other minerals in addition to Epsom Salt. (Consult your doctor if you have medical conditions such as high blood pressure, heart disease or diabetes before using mineral soaks. My friend Ellie got a rash from using the OTC soak in greater than recommended amount.)
Epsom Salt is commonly used to encourage Gardenias. They need magnesium for increasing phosphorous uptake for 'strong bones.' The Sulfur makes the soil more acid. Sometimes gardeners will advise to use Epsom Salt when gardenia leaves turn yellow. I've learned from observation that yellowing may not indicate a systemic problem. At certain times of the year, gardenias will drop leaves and new leaves form. Drought may also cause yellow leaves.
Magnesium is part of the chlorophyll in all green plants and essential for photosynthesis. It also helps activate many plant enzymes needed for growth.
Soil minerals, organic material, fertilizers, and dolomitic limestone are sources of magnesium for plants.
Sulphur helps in chlorophyll formation, improves root growth and seed production, and helps with vigorous plant growth and resistance to cold.
Sulfur may be supplied to the soil from rainwater. It is also added in some fertilizers as an impurity, especially the lower grade fertilizers. The use of gypsum also increases soil sulfur levels.
Roses are going dormant now -- some of us may have to force dormancy later. Gardenias remain evergreen in winter in my climate but receive no fertilizer until spring.
Some Rosarians add Epsom Salt to a combination known as 'Hooker's Formula' to fertilize their roses for the first time in spring. The internet abounds with suggested amounts to apply to roses, evergreens, tomato plants and others. The Epsom salt carton even gives suggested dosing for plants. As with any other chemical, follow directions carefully and remember that extra is not better. Consulting with local rose growers and Extension agents is advised.
Peace Rose is more colorful in spring; late fall it blooms pale yellow.
Epsom Salt even has its own web site: Epsom Salt Council