Wild violet (Viola pratincola) does not stop with its pretty carpet of purple and white spring flowers, but continues through the summer, autumn, and mild winter weather, spreading rhizomes laterally underground, and developing a dense mat of glossy heart-shaped foliage which can choke out grass, annuals and even hardy perennials. Persistent application of appropriate herbicides accompanied by physical removal can bring wild violets under control.
Apply glysophate herbicide to the entire area of wild violets, according to manufacturer's directions, on a cool, damp evening (to aid herbicide in sticking to waxy violet leaves) in early September.
Apply triclopyr herbicide according to manufacturer's directions two weeks after the glysophate herbicide application--also on a cool, damp autumn evening.
Loosen soil in the area where violets are growing by turning it with a spading fork in the late spring, after the ground is dry enough to be worked and the spring crop of wild violets has had a chance to emerge. Hand-weed emerging violet plants from the loosened soil.
Place a hardware cloth square across a wheelbarrow, and shovel the top 6 inches of dirt from the area where the violets are growing onto the hardware cloth. Run your hands through the soil, removing violet plants and their rhizome root masses, allowing the clean soil to fall through into the wheelbarrow. Return the cleaned soil to the ground, and discard wild violet rhizomes and plant matter by burning or throwing them in the trash--do not compost.
Add 1 gallon of well-aged compost per square yard of surface area as you return soil from the wheelbarrow to the location where the violets were growing. Replant with desired seeds or plants.
Repeat herbicide applications the following fall as necessary.