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green amaranth, green pigweed, hybrid amaranth, slender pigweed, slim amaranth, smooth amaranth, smooth pigweed

amaranth, pigweed

Habit Plants glabrous or glabrescent, or distal parts of stem and branches slightly pubescent when young. Herbs, usually annual, rarely perennial, monoecious (subg. Amaranthus and Albersia) or dioecious (subg. Acnida), glabrous or pubescent.
Stems

erect, green or sometimes reddish purple, rarely under-developed plants ascending, branched to nearly simple, 0.3–2(–2.5) m.

erect, ascending, decumbent, or prostrate, usually branched, occasionally simple or nearly so;

without nodal spines (except in A. spinosus).

Leaves

petiole 1/2 as long as to equaling blade;

blade ovate, rhombic-ovate, or lanceolate, (2–)4–15 × (1–)2–6 cm, base cuneate to broadly cuneate, margins entire, apex acute to obtuse, with mucro.

alternate, petiolate;

blade rhombic-ovate, ovate, obovate, spatulate, lanceolate, oblanceolate, or orbiculate to linear, base rounded to narrowly cuneate, margins usually entire, usually plane, slightly undulate, or crispate, rarely undulate-erose, apex acute, obtuse, or emarginate, usually mucronulate.

Bracts

lanceolate-linear to subulate, 2–3.5(–4) mm, subequal to or 2 times as long as tepals, apex spinescent.

ovate, lanceolate, linear, subulate, deltate, or broadly triangular (in A. acanthochiton), or proximal bracts modified into spines (in A. spinosus);

bracts of pistillate flowers not keeled (keeled in A. scleropoides and A. crassipes);

bracteoles absent or 1–2.

Inflorescences

terminal and axillary, erect or reflexed, occasionally nodding, green or olive green, occasionally with silvery or reddish purple tint, leafless at least distally, terrminal inflorescence often slightly nodding with numerous shorter branches at base.

terminal and/or axillary or exclusively terminal, compound dichasia arranged in spikes, thyrses, panicles, or glomerules;

components of terminal inflorescences often subtended by reduced leaves (pseudobracts), each dichasium unit subtended by persistent bracts.

Flowers

unisexual.

Staminate flowers

at tips of inflorescences;

tepals 5;

stamens (4–)5.

tepals 3–5, equal or subequal;

stamens 3–5, filaments distinct, anthers 4-locular, pseudostaminodes absent;

pistils absent or rudimentary.

Pistillate flowers

tepals 5, lanceolate to lanceolate-linear, subequal or unequal, 1.5–3 mm, membranaceous, apex acute or acuminate, gradually narrowing into aristate tip;

style branches erect, shorter than body of fruit;

stigmas 3.

tepals absent or (1–)3–5, distinct (connate in proximal 1/3 in A. polygonoides), equal or outer tepals larger than inner ones, usually membranaceous, sometimes scarious at maturity;

stamens absent [rudimentary];

pistil 1;

ovule 1;

style 0.1–1 mm, or absent;

stigmas 2–3(–5), slender.

Seeds

black to dark reddish brown, lenticular to lenticular-globose, 1–1.3 mm, smooth, shiny.

1, subglobose or lenticular, usually smooth, shiny, sometimes indistinctly puncticulate or reticulate;

embryo annular.

Utricles

obovoid or elongate-ovoid, 1.5–2.5 mm, shorter than tepals, smooth proximally, lid verrucose or rugose, dehiscence regularly circumscissile, or rarely in some presumably hybrid forms, irregularly dehiscent or indehiscent.

loosely enclosed by inner tepals, occasionally conspicuously 3(–5)-veined, usually globose, ovoid, or elongate-ovoid, thin walled, membranaceous, rugose or tuberculate, glabrous, dehiscence regularly circumscissile, irregularly dehiscent, or indehiscent.

x

= 16, 17.

Amaranthus hybridus

Amaranthus

Phenology Flowering summer–fall.
Habitat Waste places, agricultural and fallow fields, railroads, roadsides, riverbanks, other disturbed habitats
Elevation 0-2500 m (0-8200 ft)
Distribution
from FNA
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; PA; RI; SC; SD; TN; TX; VA; VT; WA; WI; WV; BC; MB; NS; ON; QC; Mexico; Central America; South America; West Indies [Widely introduced or naturalized in tropical, subtropical, and warm-temperate regions worldwide]
[WildflowerSearch map]
[BONAP county map]
from USDA
Mostly tropical; subtropical; and warm-temperate zones; some species in temperate zones [Some taxa are at present almost worldwide as introduced and naturalized weeds]
[BONAP county map]
Discussion

Originally a riverside pioneer in eastern North America, now Amaranthus hybridus is extremely abundant in agricultural fields and other disturbed habitats. Related cultivated species have been reported from the flora area, including A. caudatus, A. hypochondriacus, and A. cruentus; there is no evidence that they are established; specimens identified as these species are often variants of A. hybridus.

Distribution of Amaranthus hybridus in North America needs clarification because the name was misapplied to other species, notably A. powellii, and specimens of A. retroflexus, A. powellii, and A. hybridus are frequently interchangeably misidentified. Forms of A. hybridus and A. powellii with reddish inflorescences are often misidentified as escaped and hence presumably naturalized, cultivated species A. caudatus Linnaeus, A. hypochondriacus Linnaeus, and A. cruentus Linnaeus.

Amaranthus hybridus is extremely variable. In particular, there are numerous North American specimens with subobtuse tepals and thick inflorescences, suggesting hybridization with A. retroflexus. In Europe such presumably hybrid forms are known as A. ×ozanonii Thellung (A. Thellung 1914–1919).

A new, presumably hybridogenous taxon, Amaranthus ×tucsonensis Henrickson, was recently described from Arizona (J. Henrickson 1999). It was suggested that one of its parents is A. hybridus; the other parental species (probably a species with obtuse or spatulate tepals) remains unknown. The problem of proper taxonomic position and origin of A. ×tucsonensis needs further study.

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

Species ca. 70 (38 in the flora, including cultivated species).

Some segregate genera of Amaranthus, in the broad sense, have been proposed and sometimes recognized (see synonymy). In the present treatment, Amaranthus is accepted in its broad sense. Three subgenera are currently recognized (S. L. Mosyakin and K. R. Robertson 1996): subg. Acnida, subg. Amaranthus, and subg. Albersia.

Morphologic terminology in Amaranthus, as used in different floristic and taxonomic treatments, is rather confusing, especially regarding the terms applied to inflorescences and flowers. In the present treatment, we follow the traditional inflorescence terminology only for brevity and convenience; see T. A. Fedorova (1997) for a more complex scheme. A flower is subtended by a bract, often termed a “bracteole,” and 0–2 lateral bracts, the true bracteoles. Structures that are clearly reduced green leaves subtending portions of the inflorescence are sometimes incorrectly called bracts.

Specimens of Amaranthus are often difficult to identify by someone not familiar with the group. When using the key, look closely at the tips of pistillate inflorescence branches for staminate flowers to determine whether the plant is monoecious or dioecious; this is especially important for some monoecious species that produce few staminate flowers. Also, pistillate plants of dioecious species are usually required for positive identification. Descriptions and measurements of floral parts are given in more detail for pistillate flowers, unless noted otherwise.

Determining the exact distribution of some species of Amaranthus in North America requires additional floristic and taxonomic studies. Because of the weedy life strategies of some Amaranthus species, they may occasionally occur as naturalized weeds or waifs very far from their original areas of distribution. Some of such isolated populations exist only as long as conditions are favorable and may eventually disappear or, vice versa, become expansive and invasive. These factors, together with frequent misidentifications in herbaria and the literature, obscure the distribution patterns of some Amaranthus species in North America. Weedy and introduced species of Amaranthus are often neglected or misidentified by collectors. Consequently, some taxa are known only from scattered localities in various regions of the flora, and their actual distribution may be much wider than present data indicate. Some species have been reported for the flora only as rare, casual, non-naturalized aliens, e.g., on ballast, or as grain immigrants or wool contaminants, and may not now be present in North America. Because of all these factors, the maps and distribution statements in the treatment show the generalized distribution and may not properly reflect the actual changing distribution patterns of some species, especially those that have expanded their ranges over the decades due to various anthropic factors. In addition to the taxa discussed below, some other South American or Old World species may be found in North America in the future as introduced weeds.

Species of Amaranthus occasionally form interspecific hybrids. Such hybridization seems to be especially important and widespread in cultivated grain-amaranths, in wild representatives of the A. hybridus aggregate, between species of sect. Amaranthus, and between A. tuberculatus and species of sect. Amaranthus. The degree and scope of hybridization in Amaranthus are often overestimated, especially by European authors, and some taxa described as putative hybrids are in fact nonhybrid infraspecific forms of morphologically variable species. Hybrids between more distantly related species, if they occur at all, are usually highly sterile, such as hybrids between taxa of the subgenera Amaranthus and Acnida, or at least show much decreased fertility. There are no verified records of hybrids between representatives of the subgenera Amaranthus and Albersia.

Some species of Amaranthus are cultivated as pseudocereal and leaf-vegetable crops, or as ornamental or fodder plants (J. D. Sauer 1967; D. M. Brenner 1990; J. T. Williams and D. M. Brenner 1995; S. Cheatham et al. 1995). The most commonly cultivated taxa are A. caudatus Linnaeus, A. hypochondriacus Linnaeus, and A. cruentus Linnaeus of American origin, and south Asian A. tricolor Linnaeus. The cultivated species may occur occasionally as escapes near places of cultivation; they cannot be regarded as truly naturalized.

Species of Amaranthus were widely used by prehistoric and modern Native Americans as food, forage for livestock, medicinal plants, and, occasionally, for some other uses, such as face and body paint, ceremonial items, and fuel (S. Cheatham et al. 1995; D. E. Moerman 1998).

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

Key
1. Stems with paired nodal spines, rarely some forms spineless or with short and weak spines; utricles irregularly dehiscent or indehiscent [3b. subg. Amaranthus]
A. spinosus
1. Stems without nodal spines; utricles dehiscent or indehiscent
→ 2
2. Plants dioecious; inflorescences terminal spikes, thyrses, or panicles [3a. subg. Acnida]
→ 3
2. Plants monoecious; inflorescences of terminal spikes and panicles or axillary glomerules or clusters
→ 21
3. Plants in hand with pistillate flowers
→ 4
3. Plants in hand staminate (pistillate flowers are usually required for positive identification)
→ 12
4. Bracts deltate or rhombic-deltate, leaflike, margins crenate or denticulate, completely enfolding flower; leaf blades linear or narrowly linear-lanceolate, margins crispate or erose, or irregularly undulate [3a.3. sect. Acanthochiton]
A. acanthochiton
4. Bracts ovate to narrowly lanceolate, not leaflike, margins entire, not enfolding flower; leaf blades variable in shape, margins entire to slightly undulate
→ 5
5. Pistillate flowers with tepals absent or 1-2(-3), usually less than 2 mm; utricles usually indehiscent (dehiscent in A. tuberculatus) [3a.1. sect. Acnida]
→ 6
5. Pistillate flowers usually with 5 tepals, at least outer tepals longer than 2 mm; utricle dehiscence usually circumscissile (indehiscent in A. greggii) [3a.2 sect. Saueranthus]
→ 9
6. Utricles 2.5-4 mm; seeds 2-3 mm diam
A. cannabinus
6. Utricles 1-2.5 mm; seeds 0.7-1.2 mm diam
→ 7
7. Utricles with 3(-5) prominent longitudinal ridges, smooth or only slightly rugose, usually stramineous to brown
A. australis
7. Utricles with longitudinal ridges faint or absent, usually ± rugose, reddish or brown
→ 8
8. Leaf blades consistently linear to narrowly oblong, 1 cm or less wide
A. floridanus
8. Leaf blades variable, narrowly to broadly ovate, obovate, elliptic, usually more than 1 cm wide
A. tuberculatus
9. Bracts longer than tepals, 4-6 mm
→ 10
9. Bracts shorter than or equaling tepals, 1.5-3(-4) mm
→ 11
10. Outer tepals acuminate or acute-acuminate at apex
A. palmeri
10. Outer tepals obtuse and short-mucronulate at apex
A. watsonii
11. Utricles 3 mm, indehiscent; outer tepals acuminate at apex
A. greggii
11. Utricles 1.5-2 mm, dehiscent; outer tepals rounded at apex
A. arenicola
12. Leaf blades linear or linear-lanceolate, margins crispate or erose [3a.3 sect. Acanthochiton]
A. acanthochiton
12. Leaf blades variable in shape, margins entire, or sometimes slightly undulate
→ 13
13. Outer tepals without prominent midribs (sometimes moderately prominent), not appreciably longer than inner tepals; bracts 2 mm or shorter, midribs mostly not prominent (moderately heavy in floridanus and australis) [sect. Acnida]
→ 14
13. Outer tepals with prominent midribs, usually longer than inner tepals; bracts longer than 2 mm (1-2 mm in tuberculatus), mostly with prominent midribs
→ 17
14. Bracts less than 1 mm, midribs scarcely excurrent
A. cannabinus
14. Bracts 1 mm or longer, and/or midribs conspicuously excurrent
→ 15
15. Cauline leaf blades consistently linear or narrowly elliptic, 1 cm or less wide
A. floridanus
15. Cauline leaf blades variable, narrowly to broadly ovate, obovate, elliptic, or spatulate, mostly more than 1 cm wide
→ 16
16. Bracts with moderately prominent midribs; midribs of outer tepals excurrent
A. australis
16. Bracts with slender midribs; midribs of outer tepals not excur- rent
A. tuberculatus
17. Outer tepals with apex acuminate, midribs excurrent as rigid spines
→ 18
17. Outer tepals with apex acute or obtuse, apiculate, dark midribs not excurrent [3a.2 sect. Saueranthus]
→ 19
18. Bracts 2 mm, shorter than outer tepals, apex acuminate to short-subu- late [3a.1 sect. Acnida]
A. tuberculatus
18. Bracts 4 mm, equaling or exceeding outer tepals, apex usually long- subulate [3a.2 sect. Saueranthus]
A. palmeri
19. Bracts equaling outer tepals, apex long-acuminate or mucronulate
A. watsonii
19. Bracts shorter than outer tepals, apex acute
→ 20
20. Leaves usually thin, soft
A. arenicola
20. Leaves usually thick, coarse
A. greggii
21. Inflorescences axillary clusters or glomerules, distal nodes sometimes condensed into leafy spikes
→ 22
21. Inflorescences terminal spikes and/or panicles, leafless or almost leafless at least in the distal part, axillary spikes or clusters usually also present
→ 36
22. Pistillate flowers usually with only 1 well-developed tepal, sometimes with 1-3 distinctly unequal tepals [3c. subg. Albersia]
A. californicus
22. Pistillate flowers with 3-5 equal or subequal tepals, at least 2 tepals well developed
→ 23
23. Tepals of pistillate flowers fan-shaped, margins fimbriate or denticulate; utricles dehiscent [3b. subg. Amaranthus]
A. fimbriatus
23. Tepals of pistillate flowers spatulate or narrowly ovate to oblanceolate, lanceolate to linear, margins entire to minutely erose; utricles indehiscent or dehiscent
→ 24
24. Pistillate flowers usually with 3 tepals; fruits usually regularly dehiscent (indehiscent in A. blitum) [3c. subg. Albersia]
→ 25
24. Pistillate flowers usually with (4-)5 tepals; fruits usually indehiscent or tardily dehiscent (regularly dehiscent in A. blitoides, A. scleropoides, A. tamaulipensis, and A. torreyi)
→ 29
25. Utricles indehiscent; leaf blades usually deeply and broadly emarginate at apex
A. blitum
25. Utricles dehiscent; leaf blades obtuse or acuminate to short-mucronate or shallowly emarginate at apex
→ 26
26. Leaf blades brightly colored, showy; plants cultivated
A. tricolor
26. Leaf blades green; plants wild, usually weedy
→ 27
27. Tepals of pistillate flowers long-aristate apically, usually re- flexed outward; seeds 1-1.4 mm diam
A. thunbergii
27. Tepals of pistillate flowers acute to short-acuminate apically, not reflexed; seeds 0.6-1.6 mm diam
→ 28
28. Bracts shorter to slightly longer than tepals of pistillate flow- ers, subspinescent; seeds 1-1.6 mm diam
A. graecizans
28. Bracts 2 times as long as tepals of pistillate flowers, not spinescent; seeds 0.6-1 mm diam
A. albus
29. Inflorescence axes thickened, becoming indurate at maturity [3c. subg. Albersia]
→ 30
29. Inflorescence axes not thickened, not indurate at maturity
→ 31
30. Utricles indehiscent
A. crassipes
30. Utricles with dehiscence regularly circumscissile
A. scleropoides
31. Utricles indehiscent or tardily dehiscent
→ 32
31. Utricles with dehiscence regularly circumscissile
→ 34
32. Leaf blade margins crisped-erose, conspicuously undulate
A. crispus
32. Leaf blade margins entire or erose, plane or slightly undulate
→ 33
33. Leaf blades ovate, obovate-rhombic, to narrowly ovate, sometimes lanceolate; plants not fleshy
A. polygonoides
33. Leaf blades orbiculate, broadly ovate, or obovate; plants fleshy
A. pumilus
34. Tepals narrowly ovate to broadly linear; leaf blades usually obovate to elliptic-spatulate [3c. subg. Albersia]
A. blitoides
34. Tepals spatulate; leaf blades various in shape [3b. subg. Amaranthus]
→ 35
35. Leaf blades lanceolate, oblanceolate, or ovate-lanceolate
A. torreyi
35. Leaf blades ovate or rhombic-ovate
A. tamaulipensis
36. Tepals of pistillate flowers fan-shaped to spatulate, base contracted into claw; terminal spikes unbranched or nearly so, usually interrupted, narrow and slender; leaf blades linear to ovate-lanceolate [3b. subg. Amaranthus]
→ 37
36. Tepals of pistillate flowers spatulate-obovate, oblanceolate, ovate-elliptic or elliptic to lanceolate-linear, base never contracted into claw; terminal inflorescences variable, usually branched and ± dense; leaf blades usually rhombic-ovate to elliptic (linear to narrowly lanceolate in A. muricatus)
→ 39
37. Utricles indehiscent
A. obcordatus
37. Utricles with dehiscence regularly circumscissile
→ 38
38. Tepals of pistillate flowers fan-shaped, margins fimbriate or denticulate
A. fimbriatus
38. Tepals of pistillate flowers spatulate, margins entire, rarely minutely erose
A. torreyi
39. Utricles indehiscent; tepals of pistillate flowers usually 2-3 (5 in A. muricatus); inflorescence bracts shorter than tepals [3c. subg. Albersia]
→ 40
39. Utricles dehiscent; tepals of pistillate flowers usually 5 (or 3-5 on the same plant in A. powellii); inflorescence bracts exceeding tepals (shorter than tepals in some cultivated species) [3b. subg. Amaranthus]
→ 43
40. Tepals of pistillate flowers 5; leaf blades linear to narrowly lanceolate
A. muricatus
40. Tepals of pistillate flowers usually 2-3; leaf blades ovate, rhombic, or elliptic
→ 41
41. Utricles distinctly rugose, equaling or slightly exceeding tepals; terminal inflorescences usually thin and interrupted
A. viridis
41. Utricles smooth to faintly rugose, occasionally wrinkled or rugose in dry plants, distinctly exceeding tepals; terminal inflorescences usually thick and dense (occasionally thin and interrupted in some forms of A. blitum)
→ 42
42. Utricles subglobose to obovate, compressed; seeds filling fruit almost completely; leaf blades usually deeply emarginate at apex; plants an- nual
A. blitum
42. Utricles ellipsoid, slightly to distinctly inflated; seeds filling only proximal portions of fruit; leaf blades retuse or shallowly emarginate at apex; plants short-lived perennial or annual
A. deflexus
43. Fully developed inflorescences large and robust, usually brightly colored, red, purple, deep beet-red, occasionally white or yellowish, rarely green in some forms; bracts usually not exceeding style branches at maturity, occasionally longer than style branches in A. hypochondriacus; seeds white, ivory, reddish, brown, or black; plants cultivated and rarely escaped
→ 44
43. Inflorescences moderately large, usually green, occasionally silvery green, sometimes with reddish tint; bracts in most species exceeding style branches and tepals, distinctly shorter than tepals in A. dubius, almost equal to tepals in some rare forms of A. retroflexus; seeds brown to black; plants wild, often weedy
→ 46
44. Inflorescences stiff, erect
A. hypochondriacus
44. Inflorescences lax, erect to drooping
→ 45
45. Tepals of pistillate flowers oblong to lanceolate, apex acute; style branches erect or slightly reflexed
A. cruentus
45. At least inner tepals of pistillate flowers spathulate-obovate or lanceolate-obovate, apex obtuse to emarginate; style branches spreading or reflexed
A. caudatus
46. Plants densely viscid-pubescent; inflorescences usually unbranched
A. viscidulus
46. Plants not viscid (occasionally slightly viscid in some forms of A. retroflexus); inflorescences branched
→ 47
47. Tepals of pistillate flowers obtuse, rounded, or emarginate at apex
→ 48
47. Tepals of pistillate flowers acute or acuminate to aristate at apex
→ 49
48. Plants glabrous or nearly so; tepals of pistillate flowers 1.5-2 mm
A. wrightii
48. Plants densely to moderately pubescent; tepals of pistillate flowers (2-)2.5- 3.5(-4) mm
A. retroflexus
49. Bracts shorter than 2 mm, shorter than tepals; style branches strongly spread- ing
A. dubius
49. Bracts 2-7 mm, longer than or equaling tepals; style branches erect or slightly reflexed
→ 50
50. Bracts 2-4 mm; inflorescences variable, usually soft and lax, with spread- ing branches
A. hybridus
50. Bracts 4-7 mm; inflorescences usually stiff, with erect branches
A. powellii
Source FNA vol. 4. FNA vol. 4, p. 410. Authors: Sergei L. Mosyakin, Kenneth R. Robertson.
Parent taxa Amaranthaceae > Amaranthus > subg. Amaranthus Amaranthaceae
Sibling taxa
A. acanthochiton, A. albus, A. arenicola, A. australis, A. blitoides, A. blitum, A. californicus, A. cannabinus, A. caudatus, A. crassipes, A. crispus, A. cruentus, A. deflexus, A. dubius, A. fimbriatus, A. floridanus, A. graecizans, A. greggii, A. hypochondriacus, A. muricatus, A. obcordatus, A. palmeri, A. polygonoides, A. powellii, A. pumilus, A. retroflexus, A. scleropoides, A. spinosus, A. tamaulipensis, A. thunbergii, A. torreyi, A. tricolor, A. tuberculatus, A. viridis, A. viscidulus, A. watsonii, A. wrightii
Subordinate taxa
A. acanthochiton, A. albus, A. arenicola, A. australis, A. blitoides, A. blitum, A. californicus, A. cannabinus, A. caudatus, A. crassipes, A. crispus, A. cruentus, A. deflexus, A. dubius, A. fimbriatus, A. floridanus, A. graecizans, A. greggii, A. hybridus, A. hypochondriacus, A. muricatus, A. obcordatus, A. palmeri, A. polygonoides, A. powellii, A. pumilus, A. retroflexus, A. scleropoides, A. spinosus, A. tamaulipensis, A. thunbergii, A. torreyi, A. tricolor, A. tuberculatus, A. viridis, A. viscidulus, A. watsonii, A. wrightii
Synonyms Acanthochiton, Acnida, Albersia, Amblogyna, Euxolus, Mengea, Sarratia, Scleropus
Name authority Linnaeus: Sp. Pl. 2: 990. (1753) Linnaeus: Sp. Pl. 2: 989. (1753): Gen. Pl., ed. 5, 427. (1754)
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