Bidens tripartita L., Trifid Bur-marigold

Account Summary

Growth form and preferred habitats

B. tripartita is of rather similar size (perhaps slightly larger, up to 75 cm in height) and appearance to B. cernua (Nodding Bur-Marigold), both species richly branching and mainly growing in bare muddy habitats around waterbodies, especially in summer when the water-table is lower than normal. The habitats of the two species overlap and they regularly occur together, and while they very often colonise bare mud, they can also grow in wet meadows and pasture hollows, usually on mildly acidic to neutral or calcareous soils (Garrard & Streeter 1983). The established strategy of both these Bidens species has been categorised as CR, meaning Competitive Ruderal by Grime et al. (1988, 2007).

It has to be recognised that populations of both these Bur-marigold species are subject to enormous fluctuations in abundance from year to year, plant numbers being greatly affected by amounts, frequency and timing of spring and summer rainfall, factors which determine the extent and duration of bare mud habitat around waterbodies, and its availability for successful colonisation and fruiting by these semi-ruderal species (Salisbury 1942).

Identification of B. tripartita and comparison with B. cernua

Trifid Bur-marigold is distinguished from B. cernua at an early stage of growth and development before it flowers and fruits by its coarsely toothed, opposite leaves being borne on petioles (rather than sessile) and by each leaf having two narrow, leafy green lobes near the base, ie it is deeply divided into three leaf-lobes. Upper leaves are often alternate rather than opposite. Unfortunately, from the point of view of the identifier, there is considerable variation in leaf appearance and B. tripartita plants can sometimes have a variable number of simple, undivided leaves, and occasional individuals producing no trifoliate leaves at all. All leaves and leaflets of B. trifoliata are lanceolate in shape and have deep, coarse teeth on their margins.

When in flower and fruit, the flowerheads nod to a much lesser extent than those of B. cernua and the fruit achenes usually have just two barbed bristles crowning them in place of a pappus of hairs (although there can be up to four of these bristles) and the bristles are also shorter, measuring only 2 mm long, whereas B. cernua typically has three or four bristles on top of the achene, and they are 3 mm long (Garrard & Streeter 1983; Perring & Walters 1989).

Achene variation, structure and dispersal

The protruding bristles and the angles of the achene body are lined along their length by downward directed (ie reflexed) barbs that enable the achene to serve as burrs, readily attaching the fruits to the coats of passing animals, including man. The burr barbs also trap air bubbles when the fruits encounter water, allowing them to float for days or even weeks. The bristles and barbs can also act as an anchoring mechanism, behaving like a miniature grapnel, allowing the achene to attach to substrates, thus facilitating burial and long-term (at least five years) or short-term (one to five years) seed survival (Salisbury 1942; Thompson et al. 1997).

The achenes of B. tripartita are polymorphic, those from marginal florets are short and broad, while those from the centre of the fruiting flowerhead are longer and narrower. Both marginal and central fruits occur in two forms, one with three bristles and the other with two. The three bristle type is ± triangular in cross-section, while the two bristle fruit is more dorsi-ventrally flattened. The latter form occasionally bears a rudimentary third spine, which is half to one third the length of the other two bristles (Salisbury 1942).

Flowering reproduction

Sample statistics for seed production by B. tripartita are given in the current author's B. cernua species account on this website.

Fermanagh occurrence

Slightly less frequent than B. cernua (Nodding Bur-marigold) in Fermanagh, Trifid Bur-marigold occurs in just 43 of our tetrads, ie 8.1% of them as opposed to 62 (11.7%) for the afore-mentioned species. B. tripartita occurs in seemingly identical semi-ruderal, nitrogen-rich, wet, open, marshy and muddy situations to the more common species. These two annual Bur-marigolds often occur together and thus their distributions overlap very considerably. In Fermanagh, this is particularly the case around Upper Lough Erne, where both species are quite frequent. Elsewhere in the VC, apart from two outliers, B. tripartita is really only found around Lower Lough Erne.

The two more remote sites shown on the distribution tetrad map were found by the current author (RSF) and RHN at Drumkeenagh on the shore of Upper Lough Macnean on 16 August 1993 and four years later at Drumswords Lough, 5 km WSW of Rosslea, on 6 September 1997.

The Revised Typescript Flora authors also discovered B. tripartita in a more terrestrial and more closed vegetation setting than usual, growing with other low herbs in damp peaty fields at Cornaleck, Upper Lough Erne; of the two species, this is the one which seems better able to tolerate slightly drier, but still damp soil situations (Hill et al. 1999). Nevertheless, the very substantial overlap in the habitat requirements and tolerances of the two Bur-marigold species, measured by the latter authors, means that the two species must sometimes be involved in intense interspecific competition.

British and Irish occurrence

The more similar plant species requirements are, the more likely it is that their distributions will overlap, and thus there is unavoidable direct competition for limited habitat resources, including the physical space to exist. This situation exists for the two Bidens species and the greater frequency of B. cernua in Fermanagh, which as shown in the BSBI New Atlas and Atlas 2020 is reflected elsewhere in Ireland (Preston et al. 2002; Stroh et al. 2023), suggests it may have the edge on B. tripartita in terms of dispersal, seed survival and competitive ability.

The impression given of the British distribution of the two species by comparison of their hectad maps suggests the opposite of their Irish situation exists on the larger island. B. tripartita appears more frequently in the southern half of England, especially in the Midlands, where both of the Bur-marigold species principally occur. Elsewhere in Britain, there is little to choose between the two Bur-marigolds in terms of frequency, since they both appear rather scarce and widely scattered. Locally in Britain, however, B. cernua does seem to predominate, as detailed in some county Floras, for instance in the Shropshire region (Sinker et al. 1985), in Cumbria (Halliday 1997) and in Norfolk (Beckett et al. 1999).

The BSBI New Atlas and Atlas 2020 distribution maps also suggest that both these Bur-marigold species have been steadily declining since before the 1930s, especially in SE England. The contraction is more obvious in B. cernua than in B. tripartita. Undoubtedly the decline is due to pressures common to all aquatic species, namely, habitat loss through drainage, eutrophication associated with intensive farming methods and pollution including sewage spills into streams, rivers and other larger water bodies (Preston & Croft 1997). However, since bur-marigolds are annuals and essentially behave as pioneer colonists of bare mud, disappearance of these two species will occur naturally if plant communities in sites become mature and a more closed vegetation develops, or if prevailing weather maintains a high water table that restricts the area of exposed muddy ground on which Bidens species depend (Salisbury 1942).

It seems that disturbance is an essential habitat feature, providing bare or relatively open, intermittently wet, waterlogged or muddy, nitrogen-enriched soil for colonisation by B. tripartita and B. cernua in late spring or early summer. Trampling by grazing stock and occasional drainage operations removing silt from streams and ditches, together with more general annual effects, particularly seasonal flooding and water retreat, are clearly extremely important factors providing bare areas for these semi-ruderal annuals to colonise (Rodwell et al. 2000).

European and world occurrence

B. tripartita is a variable species and belongs to the Eurasian temperate phytogeographical element. It is a widespread native in most of Europe, including some of the Mediterranean islands, N Africa and throughout W, C & E Asia. In northern Europe, it reaches 63^oN in Scandinavia, but is absent from both Iceland and Greenland. It has been introduced to N America (where it is much less frequent and more widely scattered than B. cernua) and also to Australia and New Zealand (Hultén & Fries 1986, Map 1799; Sell & Murrell 2006). The current author (RSF) is mystified as to how such a clearly non-agricultural plant species could be introduced across the globe to places like New Zealand. Webb et al. (1988) gives the date of B. tripartita discovery on the W coast of South Island as 1975. Earlier records, eg 1940, were misidentifications of B. frondosa L.

Threats

Agricultural drainage is encroaching upon suitable habitats but, in the short term and locally, disturbance provides bare soil for colonisation.

References

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