Parthenium hysterophorus |
Parthenium confertum |
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ragweed parthenium, Santa Maria, Santa Maria feverfew, whitetop weed |
gray feverfew, Gray's feverfew |
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Habit | Annuals, (10–)30–120+ cm. | Biennials (sometimes flowering first year or persisting), 10–30(–60+) cm. |
Leaf | blades ovate to elliptic, 30–180+ × 10–50(–90+) mm, (1–)2-pinnately lobed (ultimate lobes lanceolate to linear, 3–50 × 2–15 mm), faces sparsely to densely scabrellous and gland-dotted (seldom with additional erect hairs 1–2 mm). |
blades ovate or rounded-deltate to elliptic, 30–80(–120+) × 10–20(–40+) mm, usually ± pinnately or sub-bipinnately lobed, ultimate margins entire, faces strigillose, usually also with erect hairs 1–2 mm and gland-dotted. |
Peduncles | 1–8(–15+) mm. |
2–8(–12+) mm. |
Pistillate florets | 5(–6); corolla laminae reniform or orbiculate to oblong, 0.3–1 mm. |
5; corolla laminae 0 or ± coroniform, 0.1–0.5 mm. |
Disc florets | 12–30[–60]. |
20–30+. |
Phyllaries | outer 5(–6), lance-elliptic, 2–4 mm, inner 5(–6) ovate to ± orbiculate, 2.5–4 mm. |
outer 5 lance-ovate to elliptic, 2.5–3+ mm, inner 5 ± orbiculate, 3–3.5 mm. |
Heads | obscurely radiate, borne in open, paniculiform arrays. |
disciform or obscurely radiate, borne in open, ± paniculiform arrays. |
Cypselae | obovoid, 1.5–2(–3.5) mm; pappus-like enations erect, deltate to ovate, 0.5–1 mm (sometimes a third, subulate spur near apex adaxially). |
± obovoid, 2–3 mm; pappus-like enations 2, ± erect, deltate to ovate, 0.5–1 mm (sometimes a third, subulate spur near apex adaxially). |
2n | = 34. |
= 36, 68, 72. |
Parthenium hysterophorus |
Parthenium confertum |
|
Phenology | Flowering Mar–Nov. | Flowering Mar–Oct. |
Habitat | Disturbed sites | Sandy plains, openings in mesquite grasslands |
Elevation | 0–300 m (0–1000 ft) | 20–2000 m (100–6600 ft) |
Distribution |
AL; AR; CT; DC; DE; FL; IL; KS; LA; MA; MD; MI; MO; MS; NJ; NM; NY; OH; OK; PA; TX; VA; Mexico [Introduced in subtropical, tropical, and warm-temperate New World and Old World]
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AZ; NM; TX; Mexico
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Discussion | In some places, especially in Old World, Parthenium hysterophorus causes serious contact dermatitis in humans. The plants should be handled with caution. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
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Source | FNA vol. 21, p. 22. | FNA vol. 21, p. 22. |
Parent taxa | Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Parthenium | Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Parthenium |
Sibling taxa | ||
Synonyms | P. confertum var. divaricatum, P. confertum var. lyratum, P. confertum var. microcephalum | |
Name authority | Linnaeus: Sp. Pl. 2: 988. (1753) | A. Gray: Proc. Amer. Acad. Arts 17: 216. (1882) |
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