Answer: Your description of the weed makes me think it is Chamberbitter (Phyllanthus urinaria), which is a tropical weed that has become a major pest of lawns, landscapes and plant nurseries. Glyphosate (Roundup) will kill it but you have to be careful to keep the chemical off nearby foliage. In a lawn, broadleaf weedkiller (Weed-B-Gon, Weed Stop, etc.) applied twice, seven days apart, is also effective.
The best way to control the weed is to prevent it. Apply isoxaben (Green Light Portrait) pre-emergent herbicide in late April and again in mid-June to areas where you will not be planting seeds for anything else. As with every herbicide, read the label and follow it exactly.
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