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This just serves to illustrate the taxonomic confusion that surrounds Conyza! Even Eric Clement is reluctant to do them! I hope someone sorts them out soon.
Might as well add another Conyza species to the discussion ...
Cabbageleek makes significant points that need addressing. This is going to turn into a bit of an essay, though I don't know how many will read it. However it helps clarify my own mind.
In naming this C. floribunda I wasn't too bothered about the hairy phyllaries. This is not a strong taxonomic character in many composites, and I still think it might be a hairy variant of C. floribunda. What I would definitely withdraw is the common name of "Bilbao Fleabane". This is the name Stace (New Flora of the British Isles, ed. 3) uses and I followed it without thinking. However, "Bilbao Fleabane" really applies to Conyza bilbaoana, which Stace regards as a synonym, but which Sell (in Sell & Murrel, Flora of Great Britain & Ireland, vol. 4) regards as a separate species. It would be better to follow Sell and call C. floribunda "Many-flowered Fleabane".
But Cabbageleek is right that this does not fit C. floribunda too well. On the other hand, I do not think it is C. sumatrensis. This is a more distinctly greyish species, with densely pubescent phyllaries and usually less numerous capitula (flowerheads). It is notable that Stace (in his key) comments that the pubescence commonly hides the pale margins of the phyllaries, certainly not nearly the case here.
Living in Scotland I don't often see these species but I spent some time in North London this summer, walking canal sides, exploring an industrial estate, the Thames Embankment, etc, improving my health and seeing some of the species that have colonised or become more widespread in London since I was a teenage botanist more years ago than I am prepared to admit. Solanum chenopodioides, Bidens connata, Bidens frondosa, Polypogon viridis, etc, and paying attention to the large Conyza population near where I was staying (Wembley). I already knew C. sumatrensis, but it was in company with another, greener species with abundant, smaller capitula and resembling C. canadensis in general appearance. The population was enormously variable, especially in habit and branching, and these are characters that clearly are of relatively limited value in identification. Having a copy of Stace with me, I named the "not-sumatrensis" plants as C. floribunda, and noted that while the phyllaries were hairy, the hairs were pretty much confined to the midribs and I didn't think it was an important character. However, prompted by this Observation, I have gone though my photographs and the full descriptions in Sell & Murrel, and the only species that they really seem to fit is Conyza daveauiana. Sell notes that it is common around Cambridge and elsewhere in East Anglia and probably mistaken elsewhere for C. sumatrensis but with smaller capitula.
I have compared Graeme's photos above with my own, and while they are not identical, I think they likely are the same species.
Cicuta has rightly referred to the taxonomic confusion with these Conyza introductions and no doubt concepts will change. We may be trying to recognise too many species, or too few. These species are evidently self fertilising, which can potentially lead to a large number of constantly reproducing variants (as in many weed species such as Shepherd's Purse), or, as speculated by Sell, they may be apomictic. In either case we would get the sort of taxonomic confusion, with populations failing to fit 'book' concepts, that we seem in fact to have.
Incidently C. canadensis is problematic, with Stace stating it has disk flowers with 4-lobed corollas, and making this a key character, while Sell says it has disk flowers with 5-lobed corollas. All plants I have looked at, from south of England to Scotland, with ligule characters of C. canadensis, have 5-lobed disk corollas.
So it may well be best to name Graeme's photo as "Conyza sp.", but I think Conyza daveauiana is a distinct possibility.
Thanks for such a detailed account! I will read Sell's account and keep my eyes open for some of the extra characters you noted. I too have had problems with the 4 and 5-lobed corolla character.
If I were a taxonomist I'd be a lumper.
Yes, there could well be a case for treating the South American introductions as one species, of which several strains have been introduced. It would be interesting to know if Sell's speculation they might be apomictic has been investigated by anyone.
To add yet more to my own note, I forgot to mention flowering times. In fact I stayed in Wembley twice this year. At the end of July, the plant I am now calling daveauiana was in full flower, while the plant I have seen a lot round London in the past few years and believe to be sumatrensis was still in bud and looking distinct. At the end of August, both were in flower and not so easy to distinguish. I took a lot of photographs including close-ups of the capitula, cursing the wind all the while, and looking at these again they do support two different taxa being there. The later-flowering plant does have much more densely pubescent capitula and a greater tendency to branch higher on the plant, but as I said above, the branching varied from plant to plant.
My intention was to put the photographs on web pages allowing comparison, which I could still do. Unfortunately I didn't have a plant press with me, and while I took fresh material back home with me, the specimens never made it to the press. This is now a pity. But next year ... ?
Not allways easy to tell apart from C. sumatriensis on a picture. However, in C. sumatriensis the phyllaries do not quite make it to the top of the flower head. C. floribuna typically has few tubular flowers. Best way to tell them apart is to measure the flower heads!!
C. floribunda has smaller flowers (equal to C. canadensis) !!
Here is a good source http://alienplantsbelgium.be/content/erigeron
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