Wild violets are plentiful in my lawn even though I have a company that has sprayed for weeds a few times a couple of months ago. I also have weeds that look like clover but with tiny yellow flowers. What needs to be used to kill these weeds?
The clover-type weed is oxalis. Look at life cycle and control options for both oxalis and wild violet in our weed gallery: http://extension.umd.edu/hgic/lawns/lawn-weed-identification They are both "broad-leaved perennial" weeds.
In the wild violet info, follow the Link to the "Broad-leaved Weed Control in Established Lawns" publication for specific chemical recommendations. Fall is a great time (and sometimes the only effective time) to kill hard-to-manage weed. Apply twice, a few weeks apart, starting in late August to September.
Fall is the best time to seed a lawn. Thickening turf prevents many weed problems from ever occurring. Turf shade makes weed seed germination impossible or weed seedlings are out-competed.
Fall is also the best time to fertilize a lawn.