Here in Baltimore, in about mid-March I begin scattering my Violet plantings with epsom salt. This is just the regular epsom salt that one can find in the drug stores in 3lb. rectangular boxes. I continue scattering the espsom salt around the base of the Violet plantings once a month, until mid-October. Seems to help blooming and dissuade bugs/slugs. Attached is a photo of my Viola striata growing under a large white oak tree, in our back yard.

Here is a link that might be useful: Jill Ann's- Viola striata