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

alpine feverfew, Arkansas River feverfew

Habit Annuals, (10–)30–120+ cm. Perennials, 1–2 cm (underground caudices 2–5+ cm, branched; plants cespitose or forming mats).
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 oblanceolate to spatulate, 6–18(–35) × 1–3(–4+) mm, margins entire, faces strigilloso-sericeous (gray) and obscurely gland-dotted.

Peduncles

1–8(–15+) mm.

0–5(–30) mm.

Pistillate florets

5(–6);

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

5–8;

corolla laminae 0 (tubes ± compressed, obscurely 2–4-lobed).

Disc florets

12–30[–60].

18–28+.

Phyllaries

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

outer 5–8 ± linear, 4 mm, inner 5–8 ± orbiculate, 4–5 mm.

Heads

obscurely radiate, borne in open, paniculiform arrays.

± disciform, borne singly.

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).

oblanceoloid, 4 mm (narrowly winged); pappus-like enations 2, erect to spreading, ± subulate, 0.5–1 mm (a third, ± subulate element sometimes at apex of adaxial face).

2n

= 34.

= 36.

Parthenium hysterophorus

Parthenium alpinum

Phenology Flowering Mar–Nov. Flowering May–Jul.
Habitat Disturbed sites Shale and calcareous outcrops, red clays
Elevation 0–300 m (0–1000 ft) 1300–1700 m (4300–5600 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
CO; NM; WY
[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.)

Parthenium alpinum is in the Center for Plant Conservation’s National Collection of Endangered Plants.

(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. argentatum, P. confertum, P. hysterophorus, P. incanum, P. integrifolium, P. ligulatum
Synonyms Bolophyta alpina, P. tetraneuris, P. alpinum var. tetraneuris
Name authority Linnaeus: Sp. Pl. 2: 988. (1753) (Nuttall) Torrey & A. Gray: Fl. N. Amer. 2: 285. (1842)
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