Answer: Wild Violets can be quite a problem to get rid of. The waxy coating on their leaves gives them extra resistance to many of the chemical controls (organic or otherwise) that work on other weeds. The best way to control them is to dig them up. Depending on how widespread they are, this may require several years to accomplish. For cool season lawns like yours, dig them up in the fall. You can reseed over any bare patches with a mixture of cool-season grass seed and compost. Make sure to water the new grass seed daily until it germinates.
In the meantime, it's worth noting that Wild Violets seem to show up and spread faster in acidic soils and in soils lacking in calcium. Adding lime can help correct pH problems and may slow down their spread, but you will need to have your soil tested to know how much (if any) you need to add.
Proper lawn care can help, too. Give your (cool-season) lawn a good feeding this fall with a slow release organic fertilizer or even better, by spreading one-inch of compost over it followed by a good watering.
Once established, Wild Violets are almost impossible to eradicate completely, so you may want to try to make peace with the fact that you're always likely to have a few around.
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