1	Shrubs		(2)
+	Annuals, perennials, or subshrubs (usually rhizomatous)		(10)
2 (1)	Leaf blades and/or lobes filiform (0.5–1.5 mm wide); burs winged, wings 5–20+, oblanceolate to cuneiform or flabellate to orbiculate		(3)
+	Leaf blades and/or lobes mostly deltate, elliptic, lanceolate, ovate, rhombic, or triangular (broader than filiform); burs spiny, spines 8–30(–80+), ± lanceolate (sometimes navicular at bases) to subulate or acerose (tips sometimes uncinate)		(4)
3 (2)	Wings on burs mostly around middles, 2–3 × 1–2 mm; flowering (May–)Aug–Nov		1 Ambrosia monogyra
+	Wings on burs ± scattered, 3–4(–6) × 2–4(–8) mm; flowering Mar–May(–Jun)		2 Ambrosia salsola
4 (2)	Leaf blades (green) hirsutulous to hirtellous abaxially		(5)
+	Leaf blades (often gray, silvery, or white) ± densely pilosulous, puberulent, scabrellous, strigillose, or tomentulose abaxially		(6)
5 (4)	Leaves: petioles 0–2 mm, blades elliptic to ovate, margins spiny-toothed		3 Ambrosia ilicifolia
+	Leaves: petioles 10–35 mm, blades lanceolate to narrowly triangular, margins coarsely toothed (not spiny)		4 Ambrosia ambrosioides
6 (4)	Leaf blades (1–)2–3-pinnately lobed; pistillate heads often intermixed with staminates		5 Ambrosia dumosa
+	Leaf blades not pinnately lobed (irregularly toothed to ± laciniate in A. eriocentra); pistillate heads proximal to staminates		(7)
7 (6)	Leaf blades elliptic, lance-linear, or rhombic (margins irregularly toothed to ± laciniate); pistillate florets 1; burs densely villous and stipitate-glandular		6 Ambrosia eriocentra
+	Leaf blades deltate, lance-deltate, ovate, or rounded-deltate; pistillate florets (1–)2(–3); burs puberulent to tomentulose and/or ± stipitate-glandular		(8)
8 (7)	Leaf blades: bases cordate to truncate, abaxial faces densely puberulent (including veins)		7 Ambrosia cordifolia
+	Leaf blades: bases cuneate to truncate, abaxial faces tomentulose (mostly between veins)		(9)
9 (8)	Leaf blades ovate to rounded-deltate; burs usually tomentulose (little, if at all, stipitate-glandular)		8 Ambrosia chenopodiifolia
+	Leaf blades deltate to lance-deltate; burs usually stipitate-glandular (little, if at all, tomentulose)		9 Ambrosia deltoidea
10 (1)	Annuals		(11)
+	Perennials or subshrubs		(14)
11 (10)	Leaves mostly opposite (usually some blades palmately 3–5-lobed)		10 Ambrosia trifida
+	Leaves mostly opposite (in A. bidentata, opposite and alternate, or mostly alternate, usually some or all blades 1–2-pinnately lobed, except in A. bidentata with 0–4 basal lobes)		(12)
12 (11)	Leaf blades with (1–)2(–4) basal lobes or not lobed; peduncles of staminate heads 0–0.5 mm; burs 5–8 mm		11 Ambrosia bidentata
+	Leaf blades mostly 1–2-pinnately lobed; peduncles of staminate heads 0.5–2 mm; burs 2–5 mm		(13)
13 (12)	Involucres of staminate heads (2–)3–5(–7) mm diam. (usually each with 1–5+ black nerves); burs ± fusiform to obpyramidal, 3–5 mm, spines 8–18+, 2–4(–5) mm		12 Ambrosia acanthicarpa
+	Involucres of staminate heads 2–3+ mm diam. (usually without black nerves); burs ± globose to pyriform, 2–3 mm, spines or tubercles 3–5+, 0.1–0.5+ mm		13 Ambrosia artemisiifolia
14 (10)	Stems ± prostrate or decumbent (usually beaches or strand)		(15)
+	Stems ± erect (rarely beaches or strand)		(16)
15 (14)	Staminate involucres 2–3+ mm diam.; burs ± pyriform, 1–2 mm, spines or tubercles 0–5+, 0.1–0.5+ mm		14 Ambrosia hispida
+	Staminate involucres 4–6+ mm diam.; burs fusiform to ± pyriform, 4–7+ mm, spines 8–16+, 0.5–1.5+ mm		15 Ambrosia chamissonis
16 (14)	Leaf blades sometimes 1–2(–3)-pinnately lobed (if lobed, lobes mostly deltate to lanceolate); involucres of staminate heads usually ± saucer-shaped (cup-shaped in A. cheiranthifolia from s Texas)		(17)
+	Leaf blades usually 1–4-pinnately lobed (lobes or non-lobed blades mostly lanceolate to linear); involucres of staminate heads cup-shaped		(19)
17 (16)	Leaf blades elliptic to lance-elliptic, 50–80(–180) × 12–20(–50+) mm, 1–2(–3)-pinnately lobed (lobes ± deltate), abaxial faces densely finely scabrellous		16 Ambrosia tomentosa
+	Leaf blades elliptic, lanceolate, oblanceolate, oblong, or ovate, 20–60(–100+) × 8–45(–75+) mm, 1(–2)-pinnately lobed or not lobed, abaxial faces densely sericeous, strigillose, or strigose		(18)
18 (17)	Leaves: petioles none, blades lanceolate or lance-elliptic to lance-oblong or oblanceolate, 20–50(–70+) mm, rarely ± pinnately lobed (lobes ± deltate)		17 Ambrosia cheiranthifolia
+	Leaves: petioles 10–45+ mm, blades elliptic to ovate, 45–60(–100+) mm, usually 1–2-pinnately lobed (lobes ± lanceolate)		18 Ambrosia grayi
19 (16)	Burs: spines (1–)8–13+, subulate, tips usually uncinate		(20)
+	Burs: spines or tubercles 0 or 1–6, stoutly conic to ± acerose, tips straight		(21)
20 (19)	Leaf blades mostly linear (some 1-pinnate, lobes linear); involucres of staminate heads 4–6+ mm diam		19 Ambrosia linearis
+	Leaf blades lanceolate to ovate, laciniately 2–4-pinnately lobed (lobes ± lanceolate); involucres of staminate heads 1.5–3+ mm diam		20 Ambrosia confertiflora
21 (19)	Leaf blades deltate to elliptic, 15–35(–75) × 12–25(–45) mm, laciniately (1–)2(–3)-pinnately lobed; burs ± fusiform, 2–2.5 mm, strigillose		21 Ambrosia pumila
+	Leaf blades deltate to lanceolate, 20–60(–140) × 8–35(–50+) mm, pinnately toothed or 1(–2)-pinnately lobed; burs ± obpyramidal to globose, 2–3 mm, hirsutulous		22 Ambrosia psilostachya