Flower:

Small clusters of thistle-like flower heads at branch tips and arising from leaf axils, usually arranged in flat-topped clusters. The 20 to 40 disk flowers in a flower head are pinkish-purple, rarely white, with dark purple-tipped stamens surrounding a long white style. Surrounding the base of the flowers is a dense, broadly egg-shaped to round array of softly spiny bracts (phyllaries), the entire set (involucre) up to about 1 inch (1.5 to 2.5 cm) diameter, usually densely covered in cobwebby hairs, though the hairs can thin out later in the season. Each phyllary is lance-linear with a tiny hook at the tip, the edges usually minutely fringed with glandular hairs. Flower stalks are ½ to 4+ inches (1.5 to 12 cm) long.

Leaves and stems:

Leaves are basal and alternate. Basal leaves are 12 to 16 inches (30 to 40 cm) long, about half as wide, heart-shaped at the base, rounded to pointed at the tip, on hollow or solid stalks up to 6 inches (15 cm) long. Edges are flat to wavy and toothless or minutely toothed; surfaces are gland-dotted and variably hairy, the lower surface usually densely covered in white, woolly hairs.

Leaves become smaller and shorter stalked as they ascend the stem, becoming stalkless or nearly so up into the flower clusters. Stems are single, usually much branched, stout, green or reddish purple, minutely hairy.

Fruit:

At maturity, the persistent bracts dry to brown and become stiff, the hooks attaching the seed head to clothing, animal fur, and anything that passes by, spreading seed far and wide.

Seeds are 5 to 8 mm long, light brown and may be mottled or streaked with darker spots. At the top is a tuft of light brown bristles 1 to 3 mm long that fall off as seed dries.

Notes:

Woolly Burdock is less common in Minnesota than its cousins Common Burdock (Arctium minus) and Great Burdock (Arctium lappa) but is likely more common than current records indicate. The most distinctive characteristic is its cobwebby flower heads. These are prominent when first blooming and, even though the hairs can thin out over time, are still a pretty good indicator. It is rare for either of the other two Burdocks to also have this trait, though not unheard of, but in any case the others lack the glandular hairs on the phyllaries that should be present on Woolly Burdock. Common Burdock is further distinguished by the flowers being mostly very short-stalked, arranged in tight clusters, and Great Burdock by its large flower heads, well over 1 inch diameter.

The size and arrangement of flowers plus phyllary characteristics can lead you towards one of these 3, but without flowers or fruiting heads it is less straight forward. One characteristic to check is whether basal and lower leaf stalks are solid or hollow. A. minus has hollow stalks, A. lappa has solid stalks, and A. tomentosum might be either. A. tomentosum leaves are also said to be generally more densely white hairy on the lower surface where the other two are less densely gray-hairy, but I've found any of these can have white or gray hairy leaves at some point so it is not a very reliable characteristic. All three species are conveniently located in a city park near me so I had the opportunity to examine and compare them at various stages.