Blue billygoat weed

Ageratum houstonianum, ageratum mexicanum

A short-lived herbaceous plant with softly hairy stems and leaves. Its toothed leaves are oppositely arranged at the base of the stems, but are often alternately arranged at the top of the stems. Its flower-heads are usually blue (occasionally pink or whitish) and lack any obvious 'petals'. These flower-heads have very hairy bracts and numerous long narrow projections. Its tiny black or brown 'seeds' (about 2 mm long) are topped with five whitish, hair-like, scales.

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