Hogweed (Ambrosia artemisiifolia) is a shallow-rooted, erect herb, growing 1-2 m high (rarely higher) with fern-like, slightly rough, grey foliage. The leaves are deeply divided with hairy undersides and have a distinct leaf-stalk; upper leaves become almost entire (undivided) and stalkless. Two types of flowerheads are borne on each plant. Male flower heads are born in cone-shaped aggregations up to 20 cm long, with each head (made up of 10-100 tiny florets) small and greenish, appearing yellow when mature because of the abundant pollen. Female flower-heads (each with a single floret) are born singly or in clusters and hidden by the bases of the upper leaves. The ‘seeds’ (actually 1-seeded fruits or achenes) are small, black, top-shaped, rough and surrounded by a hard ring of bracts (modified leaves) the tips of which form 4-7 spines each about 1 mm long. Flowers late summer and autumn.