- Name also: Nodding Bur Marigold, Nodding Beggartick (USA)
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Annual herb.
- Height: 20–90 cm (8–35 in.). Stem usually pale, rough.
- Flower: Single flower-like approx. 1.5–2.5 cm (0.6–1 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets usually lacking (sometimes yellow, tongue-like); disc florets yellow, tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucral bracts in 2 rows, outer bracts (5–8) large, leafy, inner bracts small, yellow, black-striped, membranous–with membranous margins. Capitula solitary or borne in a corymbose cluster, nodding.
- Leaves: Opposite, stalkless, amplexicaul. Blade light green, narrowly elliptic, lacking lobes, sparsely large-toothed.
- Fruit: 4-edged, descending bristles along edges, pale, 6–8 mm (0.24–0.32 in.) long achene, tip with (3–)4 barbed bristles.
- Habitat: Shores, waterside meadows which are prone to flooding, reservoirs, ditches, often in shallow water.
- Flowering time: July–September.
Nodding bur-marigold is the rarest, most demanding and most clearly limited to its aquatic environment of the three members of genus Bidens that grow in Finland and are covered in these pages. It thrives best beside ditches and growing out of rotting organic waste. It likes the nitrogenous water that flows out of populated areas and it has probably exploited the enrichment of the water and the drop in water level that is a consequence of regulated water levels in Finnish lakes. Nodding bur-marigold stands are mainly found in southern Finland but it also grows here and there inland and along the coast of the Gulf of Bothnia as far as the Bay of Bothnia.
Nodding bur-marigold is mainly pollinated by flies but also bumblebees and honeybees. Successfully pollinated achenes have hooked tips which enable them to be carried to new habitats by passing animals. It often grows right at the water line where it is not so easy to find suitable animals to attach to. Most of the achenes end up floating off on their own, and as they are light and highly waterproof they can spend over one year on the surface. The plant helps its seeds spread by holding the ripe capitula downwards. The inflorescences do not therefore nod in a weary-looking or otherwise dreamy kind of way – it is rather a deliberate adaptation that has developed over a long time.
Nodding bur-marigold’s close relatives trifid bur-marigold (B. tripartita) and greater bur-marigold (B. radiata) grow in generally drier places and hold their capitula erect. They can however be told apart even when they are not flowering because nodding bur-marigold’s leaves are stalkless, amplexicaul, lobeless and large-toothed, while its two relatives’ leaves are stalked and usually lobed.