If the plant you are having problems with is the wild violet (Viola sororia Willd. var. sororia), then you have a battle on your hands. The wild violet has rhizomes, below ground horizontal stems, which can root. Therefore, if you physically remove the vegetative portions of the plant and do not get all the horizontal stems, the violet will keep producing shoots. An integrated approach using both mechanical and chemical means may be the best approach.

Pull up mulch and any weed barriers. Physically remove the violet using a trowel and get out as much of the roots and rhizomes as you can. Wait a week or two. If anything comes up, then spot treat with an herbicide. Make sure to direct the spray onto the violet foliage and keep it away from your roses or other desirable plants. It may take more than one application. After an herbicide treatment or two, replace the weed barrier and mulch. The best time to control perennial weeds through herbicide application is in the fall (before frost) when the plant is storing carborhydrates in the roots or in the early spring as the weed begins to green up. Use a herbicide such as triclopyr for violet control. Read and follow all label instructions when handling pesticides.