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ragweed parthenium, Santa Maria, Santa Maria feverfew, whitetop weed

Habit Annuals, (10–)30–120+ 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).

Peduncles

1–8(–15+) mm.

Heads

obscurely radiate, borne in open, paniculiform arrays.

Disc florets

12–30[–60].

Phyllaries

outer 5(–6), lance-elliptic, 2–4 mm, inner 5(–6) ovate to ± orbiculate, 2.5–4 mm.

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).

Pistillate

florets 5(–6);

corolla laminae reniform or orbiculate to oblong, 0.3–1 mm.

2n

= 34.

Parthenium hysterophorus

Phenology Flowering Mar–Nov.
Habitat Disturbed sites
Elevation 0–300 m [0–1000 ft]
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.)

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]
Parent taxa Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Parthenium
Sibling taxa
P. alpinum, P. argentatum, P. confertum, P. incanum, P. integrifolium, P. ligulatum
Name authority Linnaeus: Sp. Pl. 2: 988. (1753)
Source Flora of North America vol. 21, p. 22.
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