Forest and Kim Starr, Starr Environmental, Bugwood.org
Annual sowthistle and perennial sowthistle are difficult to tell apart because they both grow tall and produce yellow flowers that resemble dandelions. They also have stems that produce milky sap when they're broken. This annual blooms only from June to August and turns into white puff balls the way dandelions do. The leaves are green with a bluish tint, all of which have rounded and spiny lobes.
If you can't tell annual and perennial sowthistle apart, use an herbicide containing clopyralid because it controls both weed species. If the herbicide is labeled for use on one but not the other, it may not work if you target the wrong weed. Mowing the grass at a low height is one effective way to keep this annual from germinating and spreading on the property.