Flora of North America species comparison

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Parthenium hysterophorus

Parthenium

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

feverfew
Habit Annuals, (10–)30–120+ cm. Annuals, biennials, perennials, subshrubs, or shrubs [treelets], 1–120[–400] cm.
Stems

± erect, usually branched.
Leaves

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).

usually cauline, sometimes in rosettes; alternate; petiolate or sessile;

blades elliptic, lanceolate, linear, lyrate, oblanceolate, obovate, ovate, rounded-deltate, spatulate, sometimes (1–)2-pinnately lobed, ultimate margins entire or toothed, faces usually hairy and gland-dotted (at least the abaxial).
Peduncles

1–8(–15+) mm.

Involucres

± hemispheric [rotate], 3–8(–12+) mm diam.
Receptacles

flat to conic;

paleae cuneate to flabelliform, scarious or membranous, distally papillate and/or fimbrillate, all or the peripheral each ± enfolding a disc floret.
Ray (pistillate) florets

5(–8), fertile;

corollas ochroleucous, tubes stout, glandular, laminae oblong to reniform or orbiculate, or ± coroniform (none in P. alpinum).
Pistillate florets

5(–6);

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

Disc florets

12–30[–60].

12–60+, functionally staminate;

corollas ochroleucous, funnelform, lobes 5 (filaments distinct, anthers connate).
Phyllaries

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

falling, 10(–16) in 2 series, usually distinct (partially connate in P. alpinum), outer 5(–8) herbaceous to scarious, inner 5–8 ± scarious to membranous.
Heads

obscurely radiate, borne in open, paniculiform arrays.

usually radiate, sometimes ± disciform (P. alpinum), usually borne in corymbiform or paniculiform arrays (in glomerules in P. argentatum, borne singly in P. alpinum and P. ligulatum).
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).

(black) oblanceoloid, obovoid, or pyriform, often ± obcompressed (shed together with subtending phyllary and 2 contiguous disc florets and their investing paleae);

pappi 0 (shoulders of cypselae may bear 1–3 pappus-like, triangular to ovate, or ± subulate enations).
x

= 9.
2n

= 34.

Parthenium hysterophorus

Parthenium
Phenology Flowering Mar–Nov.
Habitat Disturbed sites
Elevation 0–300 m (0–1000 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 USDA
New World; mostly warm-temperate and tropical; some temperate [Introduced in Old World]
[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.)

Species ca. 16 (7 in the flora).

(Discussion copyrighted by Flora of North America; reprinted with permission.)
Key
1. Plants 1–2 cm (plants cespitose or forming mats); heads borne singly
→ 2
1. Plants (5–)15–120 cm (plants not cespitose or forming mats); heads in glomerules, or in simple or compound, corymbiform or paniculiform arrays
→ 3
2. Corollas of pistillate florets bearing laminae 1–2 mm
P. ligulatum
2. Corollas of pistillate florets lacking laminae
P. alpinum
3. Shrubs
→ 4
3. Annuals, biennials, or perennials
→ 5
4. Leaf blades oval-elliptic to obovate (sometimes pinnately 3–7-lobed or round-toothed, ultimate margins entire)
P. incanum
4. Leaf blades lanceolate to oblanceolate (margins mostly entire, some with 1–5 sharp teeth)
P. argentatum
5. Perennials; leaf blades usually crenate to serrate, sometimes coarsely toothed or somewhat lobed (then mostly toward bases); pappus-like enations from shoulders of cypselae 0 or 2(–4), erect to spreading, ± subulate or threadlike
P. integrifolium
5. Annuals or biennials (sometimes persisting); leaf blades mostly 1–2-pinnately lobed; pappus-like enations from shoulders of cypselae 2, erect, ± deltate to ovate.
→ 6
6. Biennials (sometimes flowering first year or persisting); leaf blades ± pinnately lobed, abaxial faces strigillose (usually with erect hairs 1–2 mm as well)
P. confertum
6. Annuals (rarely persisting); leaf blades (1–)2-pinnately lobed, abaxial faces scabrellous (seldom with erect hairs 1–2 mm)
P. hysterophorus
Source FNA vol. 21, p. 22. FNA vol. 21, p. 20. Author: John L. Strother.
Parent taxa Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Parthenium Asteraceae > tribe Heliantheae > subtribe Ambrosiinae
Sibling taxa
P. alpinum, P. argentatum, P. confertum, P. incanum, P. integrifolium, P. ligulatum
Subordinate taxa
P. alpinum, P. argentatum, P. confertum, P. hysterophorus, P. incanum, P. integrifolium, P. ligulatum
Name authority Linnaeus: Sp. Pl. 2: 988. (1753) Linnaeus: Sp. Pl. 2: 988. (1753): Gen. Pl. ed. 5, 426. (1754)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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