Common mallow (Malva neglecta)
Also known as cheeseweed (thanks to the fruits, which resemble a round of cheese), common mallow is a widespread annual or
biennial weed with broad, lobed leaves and pinkish white, five-petaled flowers.
Hoe or pull these weeds when they're young. Mature plants have a long, tough taproot that is difficult to extract from the soil, and they are of course more likely to have set seed.
For chemical control, use a preemergence herbicide to prevent seedlings from becoming established in lawns and around ornamentals.