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

gray feverfew, Gray's feverfew

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
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
AZ; NM; 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.)

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
P. alpinum, P. argentatum, P. confertum, P. incanum, P. integrifolium, P. ligulatum
P. alpinum, P. argentatum, P. hysterophorus, P. incanum, P. integrifolium, P. ligulatum
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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