Wild Violets
Violets are the most difficult weed to control. Period. They are perennial and grow in patches. Each plant is grafted to another and takes and it takes an aggressive control strategy to minimize them.
Fortunately, violets are not the ugliest weed we have. Many customers find them attractive and don’t mind their presence in their lawns, to a degree.
We often find them in older lawns, where violets were often used as a desirable ground cover in planting beds, but escaped onto lawns. The whole colony of violets can be grafted together so controlling them is akin to “whack-a-mole.” You kill a patch here and another shows up there.