Flower:

Flower heads are numerous in compact clusters at the tips of branching stems and upper leaf axils, often drooping while developing. Flower heads have no petals (ray flowers), just a greenish yellow disk about ¼ inch across that barely exceeds the cylindrical base (involucre) beneath it. There are up to 21 principal bracts (phyllaries), about ¼ inch long with an outer layer of about 10 shorter bracts. Bracts are hairless and green with conspicuous blackish tips. Stalks are hairless or variably covered in matted hairs.

Leaves and stems:

Leaves are simple and alternate, oblong or spatulate in outline, 1 to 4 inches long, up to ¾ inch wide, pinnately lobed, the primary lobes shallowly lobed or coarsely toothed. The lower leaves are largest, stalked and less deeply lobed, becoming smaller, more deeply lobed and stalkless to clasping as they ascend the stem.

Leaf and stem surfaces range from completely smooth to variably covered with matted hairs. Stems are hollow, usually single, heavily branched, and green to purple.

Fruit:

Flowers become a dandelion-like plume of dark brown seeds (achenes), each with a tuft of light brown hairs (pappus) to carry it off in the wind.

Notes:

Unlike it weedy cousin, Sticky Ragwort (Senecio viscosus), Common Groundsel has been around Minnesota for well over a century. It would seem that an annual which is spread by the wind and loves disturbed sites, should be found just about everywhere by now, its presence across the landscape is more cryptic, though it is also likely greatly under-reported. It is unlike just about everything; though Sticky Groundsel leaves are similar in shape, its flowers have yellow rays, it's covered in sticky glandular hairs and has a strong scent, all of which Common Groundsel lacks. And while the flowers of Pilewort (Erechtites hieracifolia) are similar to Common Groundsel, it is a much larger and more robust plant, reaching up to 8 feet tall.