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Argentine fleabane, asthmaweed, flax-leaf fleabane, flax-leaf horseweed, hairy horseweed, South American conyza, wavy-leaf fleabane

conyza, horseweed

Habit Plants erect, 10–100(–150+) cm, branched mostly distally. Annuals [perennials], 10–120(–350+) cm.

usually erect, branched mostly distally (spreading and branched throughout in C. ramosissima), glabrous or hispid, hispidulous, strigillose, or strigose.


faces ± densely strigose or hispidulous;

proximal blades oblanceolate, 30–80(–120+) × 10–25+ mm, obscurely lobed to coarsely toothed or entire;

distal narrowly oblanceolate to linear, 10–50 × 2–10 mm, obscurely toothed or entire.

basal and cauline (mostly cauline at flowering); alternate; petiolate or sessile;

blades mostly lanceolate to oblanceolate or linear, margins rarely lobed, ultimate toothed or entire, faces usually hispid, hispidulous, strigillose, or strigose (eglandular).


3.5–5 mm.

± turbinate, 2–5[–7+] mm diam.


3–5 mm diam. in fruit.

± flat, pitted or smooth, epaleate.

Pistillate florets


corollas ± equaling or surpassing styles, laminae 0 or to 0.3 mm.

Disc florets


3–30+, bisexual, fertile;

corollas yellowish (nerves sometimes prominently resinous), tubes shorter than narrowly funnelform throats, lobes 5, erect or spreading, deltate;

style-branch appendages deltate.


usually strigose or hispidulous;

outer greenish to purplish, lanceolate, shorter;

inner stramineous to purplish, linear-attenuate (more chartaceous to scarious, less hairy).

20–40+ in 2–4 series, appressed (usually reflexed in fruit), the larger usually 3-nerved (midnerves orange to brownish; not notably keeled), lanceolate to linear, unequal, ± herbaceous medially, margins membranous, abaxial faces glabrous or hirsutulous, hispidulous, or strigose.


usually in paniculiform to racemiform, rarely corymbiform arrays.

radiate or disciform, usually in spreading to strict, paniculiform or corymbiform arrays (borne ± singly in C. ramosissima).


pale tan, 1–1.5 mm, faces glabrous or sparsely strigillose;

pappi of 15–25+, pinkish, sordid, or tawny bristles 3–4+ mm.

compressed, oblong to elliptic, 1-nerved on each edge, faces glabrous or strigillose (hairs 0.05–0.1+ mm), eglandular;

pappi persistent, of 15–25+ pinkish, sordid, tawny, or white, ± equal, barbellulate, apically attenuate bristles in 1 series.


(“ray”) florets pistillate, fertile: either 20–45+ in 1–2+ series, corollas white to purplish (filiform with laminae filiform to elliptic, 0.1–1[–1.5+] mm), or 20–150+ in 2–5+ series, corollas ochroleucous (filiform, laminae lacking, distally truncate or 2–5-toothed).


= 9.


= 54.

Conyza bonariensis


Phenology Flowering year round, mostly late summer–fall.
Habitat Disturbed sites, along roads and streets
Elevation 0–500 m (0–1600 ft)
from FNA
AL; AZ; CA; FL; GA; LA; MS; NC; NM; OR; SC; TX; UT; VA; South America [Introduced in North America]
[WildflowerSearch map]
from USDA
Widespread; mostly in subtropical and warm-temperate zones

Conyza bonariensis is widespread in tropical and warm-temperate regions of the world. It is thought to be native to South America.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

1. Plants spreading, 5–25 cm, branched throughout (stems usually ± strigose); receptacles 0.7–1 mm diam. in fruit
C. 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
C. 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)
C. 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
C. canadensis
Source FNA vol. 20, p. 349. FNA vol. 20, p. 348.
Parent taxa Asteraceae > tribe Astereae > Conyza Asteraceae > tribe Astereae
Sibling taxa
C. canadensis, C. floribunda, C. ramosissima
Subordinate taxa
C. bonariensis, C. canadensis, C. floribunda, C. ramosissima
Synonyms Erigeron bonariensis
Name authority (Linnaeus) Cronquist: Bull. Torrey Bot. Club 70: 632. (1943) Lessing: Syn. Gen. Compos., 203. (1832)
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