Jepson eFlora
Key to families | Table of families and genera

Key to Ambrosia

View taxon page for 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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