10.
Ambrosia trifida
Linnaeus, Sp. Pl. 2: 987. 1753.
Giant ragweed, grande herbe à poux
Ambrosia aptera
de Candolle;
A. trifida
var.
integrifolia
(Muhlenberg ex Willdenow) Torrey & A. Gray;
A. trifida
var.
texana
Scheele
Annuals,
30—150(—400+) cm.
Stems
erect.
Leaves
mostly opposite; petioles 10—30(—70+) mm; blades rounded-deltate to ovate or elliptic, 40—150(—250+) × 30—70(—200+) mm, usually some blades palmately 3(—5)-lobed, bases truncate to cuneate (sometimes decurrent onto petioles), margins usually toothed, rarely entire, abaxial and adaxial faces ± scabrellous and gland-dotted.
Pistillate heads
clustered, proximal to staminates; florets 1.
Staminate heads:
peduncles 1—3+ mm; involucres ± saucer-shaped, 2—4 mm diam., scabrellous (often with 1—3 black nerves); florets 3—25+.
Burs:
bodies ± pyramidal, 3—5(—7+) mm, glabrous or glabrate, spines 4—5, ± distal, ± acerose, 0.5—1 mm, tips straight (bases ± decurrent as ribs).
2
n
= 24, 48.
Flowering Jul—Nov. Disturbed sites, waste places, damp soils; 0—1600 m; Alta., B.C., Man., N.B., N.S., Ont., P.E.I., Que., Sask.; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico (Chihuahua).
The name
Ambrosia
×
helenae
Rouleau applies to hybrids between
A. artemisiifolia
and
A. trifida
. Hybrids between
A. bidentata
and
A. trifida
have been recorded.
Ambrosia trifida
may be no longer extant in British Columbia.
