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annual ragweed, common ragweed, petite herbe à poux

dwarf bur ragweed, dwarf burr ragweed, San Diego ambrosia, San Diego ragweed

Habit Annuals, 10–60(–150+) cm. Perennials, 10–20(–40+) cm.
Stems

erect.

erect.

Leaves

opposite (proximal) and alternate;

petioles 25–35(–60+) mm;

blades deltate to lanceolate or elliptic, 25–55(–90+) × 20–30(–50+) mm, 1–2-pinnately lobed, bases cuneate, ultimate margins entire or toothed, abaxial faces sparsely pilosulous to strigillose, adaxial faces strigillose, both gland-dotted.

mostly alternate (sometimes crowded at bases of stems);

petioles 5–35(–55) mm;

blades deltate to elliptic, 15–35(–75) × 12–25(–45) mm overall, laciniately (1–)2(–3)-pinnately lobed, bases cuneate to truncate, ultimate margins entire, abaxial and adaxial faces ± strigillose to sericeous (often grayish) and gland-dotted.

Staminate heads

peduncles 0.5–1.5 mm;

involucres shallowly cup-shaped (usually without black nerves), 2–3+ mm diam., glabrous or hispid to pilosulous;

florets 12–20+.

peduncles 0.5–6 mm;

involucres obliquely cup-shaped, 3–5 mm diam., ± strigillose;

florets 8–25+.

Pistillate heads

clustered, proximal to staminates;

florets 1.

clustered, proximal to staminates;

florets 1.

Burs

bodies ± globose to pyriform, 2–3 mm, ± pilosulous, spines or tubercles 3–5+, near middles or distal, ± conic to acerose, 0.1–0.5+ mm, tips straight.

bodies ± fusiform, 2–2.5 mm, strigillose, spines or tubercles 0 or 1–5, mostly distal, stoutly conic, 0.1–0.5 mm, tips straight.

2n

= 34, 36.

= 72.

Ambrosia artemisiifolia

Ambrosia pumila

Phenology Flowering Jul–Oct. Flowering May–Jun(–Oct).
Habitat Wet to dry soils, disturbed sites Disturbed sites, damp to wet, alkaline soils
Elevation 0–1000+ m (0–3300+ ft) 100–200 m (300–700 ft)
Distribution
from FNA
AK; AL; AR; AZ; CA; CO; CT; DC; DE; FL; GA; IA; ID; IL; IN; KS; KY; LA; MA; MD; ME; MI; MN; MO; MS; MT; NC; ND; NE; NH; NJ; NM; NV; NY; OH; OK; OR; PA; RI; SC; SD; TN; TX; UT; VA; VT; WA; WI; WV; WY; AB; BC; MB; NB; NF; NS; NT; ON; PE; QC; SK
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from FNA
CA; Mexico (Baja California)
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[BONAP county map]
Discussion

Hybrids between Ambrosia artemisiifolia and A. psilostachya have been called A. ×intergradiens W. H. Wagner. The name Ambrosia ×helenae Rouleau applies to hybrids between A. artemisiifolia and A. trifida.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 21, p. 15. FNA vol. 21, p. 18.
Parent taxa Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Ambrosia Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Ambrosia
Sibling taxa
A. acanthicarpa, A. ambrosioides, A. bidentata, A. chamissonis, A. cheiranthifolia, A. chenopodiifolia, A. confertiflora, A. cordifolia, A. deltoidea, A. dumosa, A. eriocentra, A. grayi, A. hispida, A. ilicifolia, A. linearis, A. monogyra, A. psilostachya, A. pumila, A. salsola, A. tomentosa, A. trifida
A. acanthicarpa, A. ambrosioides, A. artemisiifolia, A. bidentata, A. chamissonis, A. cheiranthifolia, A. chenopodiifolia, A. confertiflora, A. cordifolia, A. deltoidea, A. dumosa, A. eriocentra, A. grayi, A. hispida, A. ilicifolia, A. linearis, A. monogyra, A. psilostachya, A. salsola, A. tomentosa, A. trifida
Synonyms A. artemisiifolia var. elatior, A. artemisiifolia var. paniculata, A. elatior, A. glandulosa, A. monophylla Franseria pumila
Name authority Linnaeus: Sp. Pl. 2: 988. (1753) (Nuttall) A. Gray: Proc. Amer. Acad. Arts 17: 217. (1882)
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