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Key to families | Table of families and genera

Specimen numbers are hyperlinked to records in the Consortium of California Herbaria data view where possible. Taxa are hyperlinked to entries in the Jepson Interchange via the "[Online Interchange]" link.


Annual to small tree; monoecious. Leaf: simple, opposite or alternate, generally petioled, entire to 1–4-pinnately lobed or divided, distal-most often reduced, bract-like. Inflorescence: staminate heads and pistillate heads together in distal leaf axils or in terminal, spike-like clusters, or staminate heads in spike- or raceme-like clusters distal to pistillate heads; staminate involucre ± cup-shaped, phyllaries in 1 series, ± fused, receptacle flat or rounded, paleate; pistillate phyllaries 0 or few, in 1 series, free, ± thin, receptacle paleate, palea bases fused, becoming a hard bur with each pistillate flower in a separate chamber, tips (occasionally not evident in flower heads) becoming hard knobs, spines, or scarious wings. Pistillate flower: 1–5; corolla 0; style branches elongated. Staminate flower: 3–many; corolla translucent to yellow or red-purple; filaments fused, anthers free; style unbranched, ovary vestigial, pappus 0. Fruit: inside bur with beak(s) surrounding style(s); pappus 0.
45–50 species: native to America, especially southwestern United States, northern Mexico. (Greek: early name for aromatic plants; mythic food of the gods) [Strother 2006 FNANM 21:10–18] Wind-blown pollen often allergenic.
Unabridged note: Spines of bur derived from paleas, not phyllaries.

Key to Ambrosia

1. Leaf blade linear to thread-like or lobes thread-like; free palea tips of bur becoming flat, membranous, wings

2. Wings of bur 0.5–2 mm wide

3. Wings of bur in 1 whorl, ± acute to widely obtuse; leaves lobed or generally entire, abaxially generally glabrous; flowers fall ..... A. monogyra

3' Wings of bur spiraled, spine-tipped; leaves generally lobed, abaxially strigose; flowers spring ..... [A. ×platyspina]

2' Wings of bur 2.5–8 mm wide — flowers in spring ..... A. salsola

4. Wings generally 5–9, generally 2.5–4 mm, 2.5–5 mm wide, spreading from bur body in ± 1 central whorl-like spiral; pistillate head phyllary generally 1, obovate, transparent ..... var. pentalepis

4' Wings generally 10–13, generally 3.5–5.5 mm, 3.5–6.5(8) mm wide, spiraled around bur body, spreading or ascending and enwrapping bur; pistillate head phyllaries generally several, ovate, green or transparent, ± hairy ..... var. salsola

1' Leaf blade linear to round, variously toothed or lobed; free palea tips of burs knob-like or spiny, or not evident

5. Shrub — stems persisting 2+ seasons

6. Bur spines straight, flat or round in ×-section

7. Leaf blade 1–3 × pinnately lobed or divided; bur puberulent ..... A. dumosa

7' Leaf blade coarsely toothed or pinnately lobed; bur densely long-soft-hairy ..... A. eriocentra

6' Bur spines hooked, round in ×-section

8. Cauline leaves sessile, ± clasping, spine-tipped and spiny-dentate ..... A. ilicifolia

8' Cauline leaves petioled, generally toothed or lobed but not spiny

9. Leaf blades lance-triangular, (3)5–20 cm; bur ellipsoid, body finely glandular ..... A. ambrosioides

9' Leaf blades ± ovate, 1–3 cm; bur ± spheric, body densely woolly ..... A. chenopodiifolia

5' Annual or perennial herb

10. Annual from slender taproot

11. Leaf blade unlobed or palmately 3–5-lobed ..... [A. trifida]

11' Leaf blade 1–4 × pinnately lobed or divided

12. Bur spines 0–30, spiraled, sharply pointed; distal staminate phyllary tips ± black-lined ..... A. acanthicarpa

12' Bur spines 4–12, in ± 1 whorl below beak, generally blunt; staminate phyllary tips uniformly green ..... A. artemisiifolia

10' Perennial herb from rhizome-like roots or taprooted caudex

13. Bur spines many, spiraled

14. Stems forming mat on coastal dunes; bur 5–10 mm; spines straight ..... A. chamissonis

14' Stems erect; bur 2–5 mm; spines hooked ..... A. confertiflora

13' Bur spines 0–few, generally on distal 1/2

15. Leaf coarsely toothed or generally 1–2 × pinnately lobed or divided; plant generally 3–20 dm ..... A. psilostachya

15' Leaf 2–4-pinnately divided into many minute segments; plant generally 1–3.5 dm ..... A. pumila


Citation for the whole project: Jepson Flora Project (eds.) [year] Jepson eFlora, [accessed on month, day, year]
Citation for an individual treatment: [Author of taxon treatment] [year]. [Taxon name] in Jepson Flora Project (eds.) Jepson eFlora, [URL for treatment]. Accessed on [month, day, year].
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