Are Epsom salts good for outdoor garden plants (trees, perennials, evergreens, deciduous etc)? If so, how much & when do I use the Epsom salts? Please let me know which plants would benefit from this solution.
The best information I found on the use of Epsom salts in the garden comes from horticulture professor Linda Chalker-Scott of Washington State University. Epsom salts are also known as magnesium sulfate, and the theory (or the myth, according to Chalker-Scott) is that adding this substance to the soil provides necessary nutrients to plants, and improves their growth. The reality is more complicated, and she ultimately recommends that home gardeners not use Epsom salts, as they tend only to be useful in intensive crop production (such as farming) where there is a known lack of magnesium.
Rather than use Epsom salts, you might simply make a practice of amending your garden soil regularly with compost. WSU lists some of the many benefits of using compost.
Seattle Public Utilities also has information on growing healthy soil and composting.