Help with identifying Bidens pilosa/alba - what are the key features?

I though that I would try to re-identify some of my old observations of “Bidens” following the post [Bidens alba and Bidens pilosa]

This gave a neat key for separating the two species:
Key to identification
(from Weakley 2020 and Ballard 1986)
Bidens alba: Ray florets 5-8, the ligule 3-18 mm long; cypselas 0-2-awned, the awns 1-2 mm long; outer phyllaries (8-) 12 (-16)
Bidens pilosa: Ray florets absent (or if a few present, the ligule is only 2-3 mm long); cypselas 3 (-5)-awned, the awns 1-3 mm long; outer phyllaries 7-10

However, I ran into conflicts when I tried to id two of my past observations: Murrarie
white ray florets = alba but 2-3 awns = more like pilosa Tinchi Tamba
no ray florets = pilosa but 2 awns = alba

I can’t completely guarantee that the images of the flower and of the cypsela were from the same plant for these past observations (I’ll make sure in future} but they were certainly in very close proximity.

Help please, Pete


This may help, although it’s an older reference:

Norton, R. A. (1991). Bidens alba (Smooth Beggar-Tick) and Bidens pilosa (Hairy Beggar-Tick)

B. pilosa var. minor and B. alba var. radiata are erect annual herbs with opposite pinnate leaves (Fig. 1). Flowers are organized into a capitulum with yellow disc flowers and five or six white (occasionally purple) ray flowers which are 5-7 mm long and have a nonfunctional style in the former species and ray flowers 15-18 mm long with no style in the latter. Both plants, in common with most species of Bidens, are found in moist, disturbed areas. Mature plants are easily recognized by the spherical heads radiating black, barbed seeds in all directions from the receptacle. The two species are cross-fertile, although B. alba var. radiata is a tetraploid (x = 24) and B. pilosa var. minor is a hexaploid (x = 36) (Ballard 1986; Norton unpubl.). Hybrids are fertile pentaploids (x = 30) which are intermediate for flower size, hairiness, time of flowering, and leaf polyacetylene level (unpubl.). The base chromosome number for most species of Bidens appears to be x = 12, although many species are tetra- and hexaploid.

Bidens pilosa is the most widespread; with documented occurrence in 59 countries around the world.

Basically, they’re nearly identical, hybridize easily, and when in doubt it’s most likely Bidens pilosa.

More recent work may have changed that a bit though.

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I had a very similar situation with this observation: where a kind expert provided this resource in the Flora Mesoamericana which may be of some help.
Bidens deserves (and probably needs) to have a complex like Rubus, Veronica anagallis-aquatica and Hypochaeris sessiliflora, I think.

I question this portion:

  • the flower head has disk flowers only, and lacks the strap-shaped flowers

Most of the identification guides I see for it say that ray flowers are often present, and most of the images of it from reputable sources include Bidens pilosa specific images that include ray flowers.

Cabi datasheet

Ray florets absent or (3-)5-8

Kew Plants of the World, which is pretty much the definitive source specifically shows images of Bidens pilosa with ray flowers.

etc, etc.

As for global distribution, Bidens pilosa has a vastly greater introduced range than Bidens alba.

That fact that both species readily hybridize is a complication as well.

If you have information, I’m curious about the use of the two species. I’ve read both about the leaves being poisonous and edible…

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