Wild violets spread underground by long rhizomes. Up to 4 feet long.
My idea is to hand dig the biggest plants in the infested area. You cannot dig out all the roots, but at least get the big ones. Then spray the remaining small plants and any that come up from the rhizomes in the next year or so.
Be sure to prevent wild violets from spreading into the yard from the woods or neglected gardens. Use deep plastic or steel edging.
Speedzone is very good. I would give a slight advantage to
T-Zone. Include some surfactant to give the maximum herbicide effect.
Just wondering--violets prefer shade and the thin grass resulting from shade.
I wonder if an exceptionally dense grass that is also shade tolerant would crowd out violets.
This would be a top rated chewings fescue, for instance. Compass is an example.
I deal with wv popping up in thick Tiff 2 bermuda full sun and it's been getting worse over the last few years (kinda like sedges but I digress). On my Zoysia lawns growth is tight enough it seems to crowd it out for the most part.