Fuller's Teasel, Dipsacus fullonum (formerly D. sylvestris)
Dipsacaceae or Teasel Family

Flowers Tall plant to 6 ft or more tall. Stems prickly. Leaves raggedly toothed, opposite each other, with bases joined around stem. Flowers emerging from a large, egg-shaped, woody ball atop stem with long, narrow, curving bracts, longer than the flower cluster, spreading below. Flowers tiny, lilac, opening in bands around the ball. Blooms in the summer. The ball persists in the fruiting stage, turning brown in the autumn.

A weed of waste places, roadsides and pastures. Considered noxious because of its prickly nature. In Wildwood, occasional along the Bikeway.

Unlikely to be mistaken for anything else in the Park, as the prickly stems, long bracts, and ball-shaped flowerhead are distinctive in flower and in fruit. Cutleaf teasel (D. laciniatus), another weed, is similar, but the leaves are deeply cut, and the bracts are shorter than the flower ball. It has not yet been reported in the Park.

Plant
Flowerhead

Leaves 

Fruiting head