Dalgial, Wikimedia Commons
Shepherd's purse gets its name from the purse-like seed capsules that grow from the flowering stalks. It thrives in the winter and produces a basal rosette, spreading out as it grows. The leaves are smooth and lobed, and the weed prefers to grow in disturbed soil. Once it produces seed heads, it dies quickly thereafter.
As the stalks grow, they produce seed heads. Routine mowing will prevent the stalks from growing and producing seeds. A well-maintained yard is also beneficial as this weed doesn't compete very well with healthy turf. Shepherd's purse weeds that have established and grown should be dealt with a postemergent herbicide. It's best to use pre-emergent herbicides with isoxaben or dithiopyr in the fall or the spring for greater control.