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San Diego bur ragweed, San Diego bur-sage, San Diego burr ragweed

annual ragweed, common ragweed, petite herbe à poux

Habit Shrubs, 30–80+ cm. Annuals, 10–60(–150+) cm.
Stems

erect.

erect.

Leaves

mostly alternate;

petioles 5–20 mm;

blades ovate to rounded-deltate, 20–35(–50) × 18–30(–45) mm, bases broadly cuneate to ± truncate, margins toothed, abaxial faces ± densely tomentulose (between veins), adaxial faces sparsely tomentulose, glabrescent.

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.

Staminate heads

peduncles 0–3 mm;

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

florets 12–20+.

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+.

Pistillate heads

clustered, proximal to staminates;

florets 2(–3).

clustered, proximal to staminates;

florets 1.

Burs

bodies obconic to fusiform, 4–6 mm, tomentulose (little, if at all, stipitate-glandular), spines 15–20+, scattered or on distal 1/2, ± subulate (the proximal basally flattened), 2–3 mm, tips usually uncinate, sometimes straight.

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.

2n

= 72.

= 34, 36.

Ambrosia chenopodiifolia

Ambrosia artemisiifolia

Phenology Flowering Mar–Apr. Flowering Jul–Oct.
Habitat Clays or sandy soils, among lava rocks Wet to dry soils, disturbed sites
Elevation 50–200 m (200–700 ft) 0–1000+ m (0–3300+ ft)
Distribution
from FNA
CA; Mexico (Baja California, Baja California Sur)
[WildflowerSearch map]
[BONAP county map]
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
[WildflowerSearch map]
[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. 14. FNA vol. 21, p. 15.
Parent taxa Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Ambrosia Asteraceae > tribe Heliantheae > subtribe Ambrosiinae > Ambrosia
Sibling taxa
A. acanthicarpa, A. ambrosioides, A. artemisiifolia, A. bidentata, A. chamissonis, A. cheiranthifolia, 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. 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
Synonyms Franseria chenopodiifolia A. artemisiifolia var. elatior, A. artemisiifolia var. paniculata, A. elatior, A. glandulosa, A. monophylla
Name authority (Bentham) W. W. Payne: J. Arnold Arbor. 45: 419. (1964) Linnaeus: Sp. Pl. 2: 988. (1753)
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