Bidens cernua L., Nodding Bur-marigold
Account Summary
Native, occasional, but locally frequent. Circumpolar temperate.
1806; Scott, Prof R.; Co Fermanagh.
July to November.
Growth form and preferred habitats
The two bur-marigold species (Bidens cernua and B. tripartita (Trifid Bur-marigold) occurring in Fermanagh are spring germinating annuals and pioneer colonists of lowland, nitrogen enriched, sheltered, muddy ground with a fluctuating water-table. They have very similar habitat requirements and regularly occur together on mud exposed when summer water levels are low (Sinker et al. 1985; Stace 1997; Hill et al. 1999).
Bur-marigold plants are tap-rooted and may grow up to 60 cm or more in height, especially in disturbed, nutrient-enriched ground trampled and manured by cattle or other stock, especially around the area where the animals habitually drink or shelter (Ellenberg 1988). Plants of B. cernua have stems ± branched, glabrous or sparsely hairy with opposite linear-lanceolate leaves, 4-15 cm long, simple and undivided, stalkless, the margins coarsely toothed and the apex acute or acuminate (Sell & Murrell 2006).
Flowering reproduction
Both B. cernua and B. tripartita flower from July to September and the former can be distinguished by its stalkless, linear-lanceolate leaves, as opposed to the stalked, three- or five-lobed leaves (rarely simple and unlobed) of B. tripartita.
The flowerheads are of two types, with or without ray-florets. The more normal form contains rather drab brown and greenish-yellow coloured, bisexual, tubular florets and lacks any of the very more showy, ligulate, marginal ray-florets (see Variation section below). Nodding Bur-marigold also has larger flowers than Trifid Bur-marigold and it has flowerheads 15-25 mm in diameter, which more decidedly droop when both flowering and fruiting, and hence the English common name, 'Nodding Bur-marigold' (Grieve 1931). The drooping head is also present in the Latin specific name 'cernua', which is from 'cernuus', meaning 'nodding' or 'bowing' (Gilbert-Carter 1964).
The flowers are insect pollinated and attract flies, bees and hoverflies rather sparingly (Garrard & Streeter 1983; Sell & Murrell 2006). The chromosome numbers of these two Bur-marigolds species differ and thus hybrids between them are unknown (Stace 1975).
Measurements of the reproductive capacity of B. cernua indicated that plants developed a mean of 14 flowerheads, producing 1,847 ± 216 fruits per plant (Salisbury 1942). Equivalent mean figures from the same author for B. tripartita were 32 flowerheads, producing a total of 1,669 ± 160 fruits per plant. The fruits are compressed achenes and they lack a hairy pappus. However, the achenes are quite large, 5-6 mm long, barbed on the angles and crowned with two-to-four rigid, tough, barbed bristle projections, which are large enough to serve as burrs. These barbs may attach the fruits to the fur of passing animals, and hence give rise to the common name 'Bur-marigold' (Melderis & Bangerter 1955). The achenes are also clothed with air tissue which enables them to float for between six and twelve months, so that they are regularly water-dispersed (Ridley 1930, p. 216).
Germination rates are high and seed can persist in mud for many years, enabling sudden population explosions when conditions are favourable, and surviving equally rapid collapse after the first frost of the year (Salisbury 1942; Grime et al. 1981).
Fermanagh occurrence

The Cen Cat Fl Ir 2 indicates that the two species have been found in every VC on the island of Ireland. In Fermanagh, however, B. cernua is more common than B. tripartita. B. cernua has been recorded in 62 Fermanagh tetrads, 11.7% of those in the VC. As the tetrad map clearly illustrates, B. cernua is frequent around Upper Lough Erne where it grows along marshy, muddy lakeshores, stream-sides and ditches, but it is very widely scattered and only very occasional elsewhere in Fermanagh.
The Lough Erne basin (and the Upper Lough in particular) has been receiving considerable runoff of slurry and other fertiliser over recent years, so that a slow but steady nutrient enrichment (eutrophication) has been taking place, favouring species like both Bur-marigolds. They are not always seen in abundance, but along with other nitrogen-loving annuals, such as Polygonum spp. and other members of the family Asteraceae, they may be luxuriant in a dry summer when the water-table drops and more bare mud is exposed around lakes and ponds, but the Bur-marigolds appear much more scarce in wetter seasons (Salisbury 1942).
Variation
The rayless form of B. cernua flowerhead, composed entirely of brownish- or greenish-yellow tubular florets, is the one normally found, there being only two records in the Fermanagh Flora database of the form with spreading strap-shaped ray-florets, var. radiata, compared to 190 of the rayless form. The only site for var. radiata in the VC is on the shores of Lough Head and in bog drains nearby, where MCM & D discovered it back in 1950. It was still there in August 1991 when RHN re-found it.
British and Irish occurrence
Locally common in C & S England, Wales and Ireland, becoming scarce or rare and scattered further north in England and Scotland.
European and world occurrence
A member of the Circumpolar temperate phytogeographical element, B. cernua is widespread and fairly abundant across most of Europe from N Spain to the N Balkans and eastwards across Asia, where it becomes increasingly and more decidedly disjunct. It is absent from all of the Mediterranean islands and in the north it becomes scarce and confined to coasts beyond S Scandinavia (Hultén & Fries 1986, Map 1797). It is absent from Iceland and Greenland (Schou et al. 2023). It is also widespread across N America, where it is generally considered native, making it circumpolar, although Hultén (1968) regards it as an alien introduction in Alaska and neighbouring territories.
Uses
Bidens spp., most probably both B. tripartita and B. cernua, were formerly valued by herbalists for their diuretic and astringent properties, being employed in cases of fever, bladder and kidney troubles (for example, stones). Bidens spp. were also considered a good stypic (ie a drug used to contract blood vessels or tissues) in cases of bleeding of every description, including for consumptive patients (Grieve 1931).
Names
The generic name 'Bidens' is derived from the Latin for 'two teeth', referring to the bristles on top of the achene, which are often paired (Gilbert-Carter 1964). Additional English common names for Bidens species include 'Tickseed', 'Beggar's tick' and 'Stick-tight', all obviously referring to the burr-like property of the fruit (Stearn 1992; Le Strange 1977).
Threats
Drainage causing habitat loss.
References
Sinker, C.A., Packham, J.R., Trueman, I.C., and Oswald, P.H., Perring, F.H. and Prestwood, W.V. (1985); Grieve, M. (1931); Stace, C. (1997); Hill, M.O., Mountfield, J.O., Roy, D.B. and Bunce, R.G.H. (1999); Ellenberg, H. (1988); Stearn, W.T. (1992); Gilbert-Carter, H. (1964); Le Strange, R. (1977); Garrard, I. and Streeter, D. (1983); Ridley, H.N. (1930); Melderis, A. and Bangerter, E.B.(Eds.) (1955); Salisbury, E.J. (1942); Stace, C.A.(Ed.) 1975;, Grime, J.P., Mason, G., Curtis, A.V., Rodman, J., Band, S.R., Mowforth, M.A.G., Neal, A.M. and Shaw, and S. (1981); Scannell, M.J.P. and Synnott,D.M. (1987); Hultén (1968); Hultén & Fries 1986; Schou et al. 2023; Sell & Murrell 2006;