Photo: Chris Evans, University of Illinois, Bugwood.org, annual sowthistle, Sonchus oleraceus
Many thistles have spines on the foliage or stem. Annual sowthistle only has very small ones along the leaf margins. Sowthistle reproduces from seeds. Seedlings form a basal rosette low to the ground, so they can often go unnoticed. The leaves at this time have toothed margins and are wider at the tip and taper back towards where they attach to the plant.
As the plant grows, it produces upright stems that can reach approximately 4 feet tall. The leaves on mature plants are alternate from each other. These leaves have a lot of variability in their appearance, though the last portion is triangular in shape. All plant parts (leaves, stems and roots) exude white sap if broken. The yellow flowers are produced in late summer into October. A white, fluffy part (pappus) of the seed enables it to disperse easily by wind. Annual sowthistle does have characteristics similar to other sowthistles and prickly lettuce. Weeds of the Northeast (Uva, Neal and DiTomaso) has a detailed chart for discerning the species from each other.
Annual sowthistle has a shallow taproot. This allows for easy hand removal in landscape settings if practical. There are a number of pre-emergent herbicides that can be used: flumioxazin (BroadStar - nursery; SureGuard - landscape); napropamide (Devrinol); oxadiazon (Ronstar); oxyfluorfen (Goal 2XL - nursery) and others. Post-emergent herbicides include: clopyralid (Lontrel); diquat (Reward); flumioxazin (SureGuard); glyphosate (Roundup); oxyfluorfen (Goal 2XL); and others.
Remember to always read the label for proper application sites, rates and precautions.