Silverleaf Nightshade, maybe?

Here is a link that might be useful: Silverleaf Nightshade

Cara, thanks. That's it.

Looks like it is difficult to kill.

I've never heard of anyone mowing once every three weeks....especially a warm season grass as summer approaches, which is peak growing time. That's very stressful on your grass. Does it look like a hayfield on mowing day? At what height is your mower set?