Virginia buttonweed is a perennial weed native to the U.S. and is very common in our area. According the LSU Agcenter, "Virginia buttonweed is widely considered the most invasive weed infesting turf grass in the South." If you have Virginia buttonweed in your yard it is pretty easy to identify by it's white flowers and lance shaped leaves. It will also be noticeable because it spreads rather aggressively and squeezes out the desirable turf. This will be quite evident later in the growing season. Unfortunately, control of Virginia buttonweed can be problematic and there is not a one shot solution.
One of the reasons for the difficulty in control is the proficiency at which the Virginia buttonweed can reproduce. Aside from being a prolific seed producer Virginia buttonweed can also reproduce from stem fragments caused by mowing or pulling the plant out of the ground. If any little pieces are left behind, those pieces could root and further spread the problem. Also if the all the roots are not removed (good luck doing that) the plant will come right back, just as strong as ever.
Fortunately there are chemical control solutions available to the DIY homeowner, but timing is the key. Once you notice you have a problem it is probably to late to have effective control. According to the LSU Agcenter, "A program approach works best to control buttonweed." You have to start a control program early in the growing season before the plant has time to harden off. This will give you the best chance to have reasonable control. LSU recommends starting your control program in early April when the seeds start to germinate. The young plants are much more "tender" and easier to kill with chemicals that won't damage the desirable turf grass. For more specific info on control, surf on over to the the LSU Agcenter and read Virginia Buttonweed: No. 1 Weed Problem of Southern Lawns