These annual weeds are found throughout the U.S. but are most common in the South and along the West Coast. The reddish stems grow upright 1 to 6 feet. In mowed lawns they form tight rosettes. A milky sap oozes from the stems when they are broken. The upper branches may be covered with hairs. From July to September, yellow flowers bloom on branches at the top of the plants. The seeds contained in brownish seedheads germinate that fall or the following spring. In Florida, Texas, and California, the plants may germinate or bloom year-round.
A vigorous, healthy lawn will prevent most sowthistle, so good fertilization and mowing practices are very important. Mow off the flower stalks before they produce seeds and wait for fall frost to kill the plants. Sowthistle can be hand-pulled, but wear gloves because the hairs are irritating.