Trifid Bur-marigold - Bidens tripartita

Very variable plant to about 50 cm. Erect stems. Leaves opposite, usually three lobed occasionally 5 lobed with lanceolate, coarsely toothed divisions. Leaf stalks are short and winged. Flowerheads yellow 10 to 25 mm. Erect in branched clusters. Achenes with 3 or 4 barbed bristles. Detailed examination of the plant is necessary for a safe id.

The commonest Bidens recorded; Bidens cernua and Bidens frondosa are also known in VC55.  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 petiolate (stalked) and lobed, usually deeply and nearly to the midrib, but may be less. The leaflets themselves are not lobed.  The petioles are winged to the base.  The barbs on the edge of the achenes are backwards directed.  (Note that Stace (4th edition) is not entirely certain that the achenes always have the latter characteristic)

A photo of the whole plant and details of leaf shape and achenes.  Note that this plant cannot be verified from a photo just of the flowers

Damp habitats on mildly acid or calcareous soils.

July to September.

Annual.

Locally frequent in England but less so in Scotland and Wales.

Occasional in Leicestershire and Rutland. In the 1979 Flora survey of Leicestershire it was found in 50 of the 617 tetrads.