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Greater Bur-marigold

Bidens radiata

  • Written also: Greater Bur Marigold
  • Family: Daisy Family – Compositae, subfamily Asteroideae
    (formerly Aster Family – Asteraceae)
  • Growing form: Annual herb.
  • Height: 10–60 cm (4–25 in.). Stem glabrous–rough-haired, usually yellowish–reddish brown.
  • Flower: Single flower-like approx. 1–2 cm (0.4–0.8 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets lacking; disc florets yellow, tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucral bracts in 2 rows, outer bracts (7–14) large, leafy, inner bracts small, brownish, membranous–with membranous margins. Capitula solitary or borne in a corymbose cluster, erect.
  • Leaves: Opposite, stalked. Blade light green–lime green, with three or sometimes more lobes, lobes lanceolate, with toothed margins, teeth curving towards tip.
  • Fruit: 4-edged, descending bristles along edges, shiny, reddish brownish, 3–5.5 mm (0.12–0.22 in.) long achene, tip with 2 barbed bristles.
  • Habitat: Lake shores, river banks, waterside meadows which are prone to flooding, ditches, puddles.
  • Flowering time: August–September.

Greater bur-marigold usually spreads to new habitats via its floating seeds. The tips of its cypselas also have hooked bristles, however, as anyone who brushes against one in late summer or early autumn will testify to. Originally these tiny harpoons have been aimed at animals. They do not stick well to the oily feathers of water fowl, and suitable four-legged animals are not so common nowadays in wetlands. Things were different long ago however when cattle, wild horses, giant elks and other ancient animals would graze by the waterside. People displaced these beasts, but the species began to exploit human activity in turn: it has found new habitats in shore-side boatyards, and through the creation of lake-side fields and the control of water levels in lakes. Greater bur-marigold has also moved from the wild towards inhabited areas.

Greater bur-marigold is quite common in the south of Finland while elsewhere it only grows here and there. It is capricious everywhere, however: in rainy summers the species can all but disappear, while in warm, dry summers when shore silt is revealed from under the water it becomes abundant. Many annual travellers grow on exposed bits of mud with greater bur-marigold, and it is typical of all of them to vary between being very sparse to highly abundant.

Greater bur-marigold looks quite a lot like its relative trifid bur-marigold (B. tripartita) but it is more of a coastal plant. Its stem and leaves are lime green while trifid bur-marigold’s are clearly dark green. Trifid bur-marigold also has less outer, petaloid involucral bracts, only 5–9. Greater bur-marigold’s achene is shiny and is tipped with a 5–7 mm (0.2–0.28 in.) long bristle while trifid bur-marigold’s is dull and around 1 cm (0.4 in.) long. Greater bur-marigold and trifid-bur marigold can cross-breed with each other.

Other species from the same genus
Other species from the same family

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