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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
from FNA
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]
[WildflowerSearch map]
[BONAP county map]
from FNA
TX; Mexico
[WildflowerSearch map]
[BONAP county map]
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
P. alpinum, P. argentatum, P. confertum, P. incanum, P. integrifolium, P. ligulatum
P. alpinum, P. confertum, P. hysterophorus, P. incanum, P. integrifolium, P. ligulatum
Name authority Linnaeus: Sp. Pl. 2: 988. (1753) A. Gray: in W. H. Emory, Rep. U.S. Mex. Bound. 2(1): 86. (1859)
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