Syn. Gen. Compos.,203. 1832
Widespread, mostly in subtropical and warm-temperate zones.
Species 25–40+ (4 in the flora).
Distinctions between Conyza and Erigeron, as usually circumscribed in the past 50+ years, are not always clear. Usually, conyzas have unequal (graduated) phyllaries and 2–20+ times as many pistillate florets as bisexual florets in each head (rarely more bisexual than pistillate), and corollas of pistillate florets either lack laminae or have laminae usually less than 1(–1.5) mm. Erigerons usually have subequal phyllaries and more bisexual than pistillate florets (rarely more pistillate than bisexual) and corollas of pistillate florets (if any) usually have laminae 2–10+ mm.
In studies by R. D. Noyes (2000) and by Noyes and L. H. Rieseberg (1999), Conyza, as traditionally circumscribed, was found to be nested within Erigeron and to be para- and/or polyphyletic. Here, absent an alternate taxonomy, four species traditionally treated in Conyza are retained in Conyza and a separate suite of six species that have usually been included in Conyza are treated in Laënnecia.
|1||Plants spreading, 5–25 cm, branched throughout (stems usually ± strigose); receptacles 0.7–1 mm diam. in fruit||Conyza ramosissima|
|1||Plants erect, (10–)30–350+ cm, branched mostly distally; receptacles 1–5 mm diam. in fruit||> 2|
|2||Phyllaries usually hispidulous or strigose; receptacles 3–5 mm diam. in fruit; pistillate florets 60–150+; pappi 3–4+ mm||Conyza bonariensis|
|2||Phyllaries glabrous or sparsely strigose; receptacles 1–3 mm diam. in fruit; pistillate florets 20–45+; pappi 2–3 mm||> 3|
|3||Phyllaries usually sparsely strigose; corollas of pistillate florets with laminae 0 or to 0.3 mm; cypselae pale tan (usually some in each head with reddish nerves)||Conyza floribunda|
|3||Phyllaries usually glabrous, sometimes sparsely strigose; corollas of pistillate florets with laminae 0.3–1+ mm; cypselae uniformly pale tan to light gray-brown||Conyza canadensis|
|Author||John L. Strother +|
|Common name||Horseweed +|
|Etymology||Ancient name for fleabane + and perhaps from Greek konops, flea, or konis, dust, alluding to powdered dry plant being used to repel insects +|
|Illustrator||Yevonn Wilson-Ramsey +|
|Reference||cronquist1943a + and nesom1990i +|
|Taxon name||Conyza +|
|Taxon parent||Asteraceae tribe Astereae +|
|Taxon rank||genus +|
|Volume||Volume 20 +|