Plant of the Week – 9th November – Canadian Fleabane (Conyza canadensis)

Canadian Fleabane Conyza canadensis is no beauty. Its flowers are small, inconspicuous and lacking in colour¹. Its leaves are not very interesting either, rather a pale shade of green and somewhat shapeless and variable. Unlike many entries in our Plant of the Week, you are unlikely to find versions of it in a garden centre. It is no match for many other fleabanes which have good-looking daisy-flowers, often showing delightful shades of blue or pink. The flowers are groundsel-like, but lacking groundsel’s bright yellow colour and beguiling nodding habit. As an ultimate insult, in North America this fleabane is called Horseweed.

If we follow the latest edition of Stace, we should be calling it Erigeron canadensis not Conyza canadensis. He has combined the fleabanes into one broad genus, Erigeron, going against the 1943 definition (Cronquist 1943). The evidence for this move is unclear (Greuter 2003), perhaps someone can enlighten me. In the meantime, I’m going to stick with Conyza.

Its family, the Asteraceae, contains so many useful plants: lettuce, chicory and chamomile spring to mind; and a good many others are loved as garden flowers:  sunflowers, asters, dahlias, chrysanthemums and marguerites. Even thistles have their uses – as the emblem of Scotland and an important nectar source. Canadian Fleabane has little to admire, and doesn’t have any uses that I can find except as a remedy for nasal congestion in the North American Zuni people.

So, why is it Plant of the Week? Answer: this species and its close relatives are spreading from England to Scotland. Do I mind? I love all plants but this one might become a troublesome weed, as it is in many parts of the world. It is said to be in the top ten of the most invasive species in Europe, and it was one of the first species reported to be resistant to the herbicide glyphosate, and other herbicides besides.

I first became aware of Conyza canadensis when we were surveying the Edinburgh tramline for the Urban Flora project in 2019. We would meet on summer evenings at the Haymarket station to catch the tram. There it was, where the pavement joins the road. Days later, I walked the part of the tramline that runs along Princes Street. Very few species can survive the harsh conditions of this place, just a handful of species and all stunted. Conyza canadensis is there along with Taraxacum officinale agg. and Sonchus oleraceus, growing from cracks and crevices of pavement and stones.

Shortly afterwards I found many more plants at a demolition site on Dalry Road, Edinburgh, not far from the cemetery and just in front of Lidl’s supermarket. The site is well protected by an impenetrable fence with camera surveillance; however, it has been allowed to accumulate ‘weeds’ for at least two years. Conyza canadensis now shares the site with many typical urban species including Sonchus oleraceus, Buddleja davidii, Senecio squalidus and Ranunculus repens. It seems to form a completely urban plant community as in London where Stace & Crawley (2015) describe the Conyza-Buddleja scrub.

I started to see it all over north Edinburgh as I ‘got my eye in’, but rarely south of The Meadows. I also found it in Ayr, in the car park by the Gaiety Theatre.

I realised there are records of three other Conyza species in the city: the three quite recent introductions, C. bonariensis, C. floribunda and C. sumatrensis. In the south of England, Stace & Crawley (2015) note a tendency for floribunda to take over from canadensis, something to watch out for here. However, they are hard to tell apart, and I wondered whether they really are distinct species.  For identification, help is at hand in the form of a set of line drawings in a very useful article by B. Leaney in the 2017 BSBI News. Regarding their status as truly different species, there is an important paper by Thebaud and Abbott (1995). These authors collected five species of Conyza from a site in France, and grew them in a common garden in Montpellier. They recorded the morphology and life histories; they also separated the isozymes by electrophoresis. The conclusion was rather clear: the five species are indeed different, although C. canadensis is markedly different from all the others². I duly visited the Edinburgh sites where Conyza had already been recorded (four species), took specimens and made photographs of the flowers (see below).

I found Conyza many times this year during my Covid-19 walks in Edinburgh: it was in the New Town, Johnston Terrace and Castle Rock, and then in Leith. At the end of one small road in Leith, in somewhat grim conditions outside a lock-up garage, Conyza canadensis shared the site with the Great Lettuce Lactuca virosa (the subject of a previous Plant of the Week). Of course, it is common in the southern half of England as the BSBI map shows, and the other species are there too.

What of its biological traits? It is an annual plant, 0.2 to 1.4 metres tall; it produces over 10,000 seeds per plant, and has a persistent seed bank in the soil. Typical of the Asteraceae, the tiny seeds (0.05 g) have a pappus of hairs to aid wind dispersal. Seeds germinate either in the autumn (overwintering as rosettes) or in the spring- and flowering occurs from July to November. It does not survive after flowering, although Thebaud and Abbott (1995) found that C. floribunda does. It thrives in well-lit conditions, on all well-drained, non-saline, moderately fertile soils.

Canadian Fleabane is thought to originate from North America, but was recorded in London as early as 1690. It began to spread rapidly in Britain and Europe fairly recently. My guess is that it is spreading north in response to climatic warming. I’ve looked at the recent data on summer temperatures for Scotland (Kendon et al 2020). Since 1980 Scotland has warmed by about 1 ^oC. Of course, some years have been much warmer – 2018 was especially warm. I suspect many species are migrating in leaps and bounds. It is one of several species that have increased greatly in Scotland in the last few years. Perhaps we are witnessing the start of an important trend.

Can I tell the difference between the Conyza species? Only by collecting them for close examination at home and working through Stace’s key³ with Leaney’s drawings in front of me; even then I’m not sure because there are phenotypic variations and some individuals are definitely intermediate in some characters. The highly variable characters have been much-discussed in the BSBI’s News (Rand 2008, Leaney 2017), and will continue to be for some while. C. canadensis is the rather easy one, but the others take much time and eye-strain to be sure.

John Grace.

Footnotes

¹ Its outer florets are white, its central florets pale yellow – from a distance the impression is a lack of colour.

²The chromosome count of C. canadensis is 2n=18 whereas the other species are polyploid – they have many more chromosomes. C. bonariensis has a chromosome number of 54 the same as recorded for C. sumatrensis with which it has been previously confused.

³It must be the 4th Edition- earlier editions do not have all the species.

References

Stace CA and Crawley M (2015) Alien Plants. Collins.

Cronquist A (1943) The separation of Erigeron from Conyza. Bulletin of the Torrey Botanical Club 70, 629-632.

Greuter W (2003) The Euro+Med treatment of Astereae (Compositae) – generic concepts and required new names. Willdenowia 33, 45-47.

Kendon M et al (2020) State of the UK Climate 2019. International Journal of Climatology 40, 1-69.

Leaney B (2017) Common Problems with identification in Conyza: Norfolk experience. BSBI News 135, 7-17. https://bsbi.org/wp-content/uploads/dlm_uploads/BSBI-News-135.pdf

Rand M (2008). Difficulties with Conyza (Fleabanes). BSBI News, 108: 40-43.

Thebaud C & Abbott RJ (1995) Characterization of Invasive Conyza Species (Asteraceae) in Europe: Quantitative Trait and Isozyme Analysis. American Journal of Botany 82, 360- 368. https://www.jstor.org/stable/2445581?seq=1#metadata_info_tab_contents