Parthenium hysterophorus |
Parthenium argentatum |
|
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ragweed parthenium, Santa Maria, Santa Maria feverfew, whitetop weed |
guayule |
|
Habit | Annuals, (10–)30–120+ cm. | Shrubs, 30–100+ 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 lanceolate to oblanceolate, 15–25(–40+) × 6–15(–25+) mm, margins mostly entire, some with 1–2(–5) sharp teeth, faces densely strigillose (gray to white) and obscurely or not at all gland-dotted. |
Peduncles | 1–8(–15+) mm. |
1–2(–6+) mm. |
Pistillate florets | 5(–6); corolla laminae reniform or orbiculate to oblong, 0.3–1 mm. |
5; corolla laminae ovate, 1.2–1.5 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 oval-elliptic, 2.5–3+ mm, inner 5 ± orbiculate, 3.5–4 mm. |
Heads | obscurely radiate, borne in open, paniculiform arrays. |
radiate, in glomerules of 3–5+ at ends of ± ebracteate stalks 8–15(–20+) cm. |
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, 2.5–3 mm; pappus-like enations 2(–4), erect to spreading, ± subulate, 0.3–0.8 mm. |
2n | = 34. |
= 36. |
Parthenium hysterophorus |
Parthenium argentatum |
|
Phenology | Flowering Mar–Nov. | Flowering Apr–Jun. |
Habitat | Disturbed sites | Calcareous soils |
Elevation | 0–300 m (0–1000 ft) | 1000–1500 m (3300–4900 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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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.) |
Guayule has sometimes been used for commercial production of natural rubber. Hybrids between Parthenium argentatum and P. incanum have been noted. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Source | FNA vol. 21, p. 22. | FNA vol. 21, p. 21. |
Parent taxa | Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Parthenium | Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Parthenium |
Sibling taxa | ||
Name authority | Linnaeus: Sp. Pl. 2: 988. (1753) | A. Gray: in W. H. Emory, Rep. U.S. Mex. Bound. 2(1): 86. (1859) |
Web links |
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