Nodding Bur-marigold - Bidens cernua

Short to tall plant with erect stems. Leaves opposite, linear lanceolate to lanceolate, toothed and unstalked. Flowerheads yellow, 15 to 25 mm, nodding, with or without rays. Outer flowers bracts 5 to 8, leaf like, the inner with dark streaks.

Bidens tripartita and Bidens frondosa; other Bidens are known to be naturalised in Britain and may turn up.  Identification is tricky and the key in Stace (4th edition) should be referred to; details of leaf and leaflet shape, and the shape of the achenes, is usually needed. 

Leaves tapered to base, but sessile; leaves not lobed, although serrated; achenes flattened with backward directed barbs. 

A photograph showing leaves as well as flowers; this species can't be verified from a photo just showing the flowers (RPR)

Moist habitats. Margins of ponds, lakes, rivers and canals.

Flowering July to September.

Annual.

Scattered throughout much of Britain though scarce in the south west.

Scarce in Leicestershire and Rutland. In the Flora of Leicestershire (Primavesi and Evans 1988) it was found in 12 of the 617 tetrads and in the Flora of Rutland (Messenger 1971) in 3 tetrads.

In the current checklist (Jeeves 2011) it is listed as scarce. 

It is listed on the current VC55 Rare Plant Register (Hall and Woodward 2022) as Locally Scarce (i.e. present in 4-10 sites)