It's really well known to be a good thing for them, and a fair number of people do it, so if it works for you, spread the word :-).

I'm lurking on the Bonsai forum. I potted a maple last summer and it is growing very well. I'm just looking for some tips. We used to show and judge seasonal flowers. The flower was 80 points and the leaves and stems were worth 20 points. The gorwers knew to add Epsom salts about 3-4 weeks before the shows to "green-up" the leaves and stems. It wasn't a secret, it was a well acknowledged fact. Epsom salts also promotes bud formation.

From good ol' Wikipedia, for some quick info:

Agricultural use

In agriculture and gardening, magnesium sulfate is used to correct magnesium deficiency in soil (magnesium is an essential element in the chlorophyll molecule). It is most commonly applied to potted plants, or to magnesium-hungry crops, such as potatoes, roses, and tomatoes. The advantage of magnesium sulfate over other magnesium soil amendments (such as dolomitic lime) is its high solubility.

-Audric

use htis all the time with my tomatoes, works good, just dont go over 1 T as it can burn them out quickly, effectively shortening thier life