Q: What is this weed in my garden?

A: It’s Virginia copperleaf, Acalypha virginica.

 It is an annual weed, which means it can be prevented with the proper pre-emergent chemical. The best pre-emergent for broadleaf weeds is dithiopyr (click for sources). Apply the chemical in late March and again in mid-May. If you have a huge copperleaf population, make a third application in early July.

Broadleaf weed killer (click for sources) and any of the glyphosate (click for sources) products will kill copperleaf.

Imazaquin (click for sources) can be used to kill it when growing in landscaped areas

The key is to pull or kill the weed before it drops seed, which it can do only a few weeks after germination.