Ambrosia trifida L. var. texana Scheele, giant ragweed. Annual, robust, taprooted, not rosetted, 1–stemmed at base, with ascending branches above midplant, in range < 60—450 cm tall; monoecious; shoots with only cauline leaves widely spaced with long internodes, branching distinctly opposite decussate, foliage conspicuously scabrous, dominant hairs with persistent, hardened, enlarged bases.
Stems initially ± 5–angled aging cylindric, to 15 mm diameter, green with thinner black stripes, internodes to 190 mm long.
Leaves opposite decussate, palmately 3(5)–lobed and unlobed, symmetric mostly with a pair of lateral lobes having deep sinuses, if 5–lobed basal lobes defined by shallow sinuses, petiolate, without stipules; petiole ± hemi–cylindric broadly channeled at base, < 10—75 mm long, tough, often 2–ridged above midpoint, lateral and lower sides with 5 blackish lines, scabrous hairs curved upward; blade mostly ovate in outline, in range 85—240 × (45—)80—230 mm, principal lobes ovate, terminal lobe to 165 mm long, > lateral lobes > basal lobes, broadly tapered at base or with narrow taper along petiole, low–serrate with blunt, short teeth, the teeth aging blackish, acute at tip, when lobed with 3 principal veins near base and pinnately veined above, principal veins raised on upper surface and principal + minor veins raised on lower surface, lower surface with subsessile glandular hairs having colorless hairs.
Inflorescence unisexual heads, in terminal, racemelike arrays of 0—3 alternate, condensed, cymelike shoots at base with up to 3 pistillate heads per shoot and upper portion with 60+ alternate, nodding staminate heads, pistillate portion with sessile heads, staminate portion to 140 × 9.5—10.5 mm with radiating, stalked heads; heads discoid, pistillate head 1–flowered and 2.5 mm across, staminate head 4—1+–flowered and 2—3.5 mm across, bracteate; bract along across opposite decussate, 3–lobed, to 6 mm long, with midvein raised, bract subtending pistillate flower sessile, oblanceolate and weakly 3–lobed, to 15 mm long, lateral lobes diminutive, with 1—3 black lines.
Staminate head bract subtending peduncle absent; peduncle spreading, at pollination 1—3 mm long, pale green, sharply bent at tip making head reflexed ⊥ inflorescence axis, scabrous with short hairs and inconspicuous slender hairs; rachis pale green, hairy; involucre broadly funnel–shaped, 1.6—3.5 × 1.8—4 mm, length < width, of fused phyllaries inconspicuously lobes with undulating margin, green with (1—)2—3 “black” lines, the lines of raised, dark, blackish purple tissue on inner surface, outer surface with hairs.
Staminate flower radial, 1 mm across, ca. 2 mm long, exserted to 0.6 mm from involucre; calyx (pappus) absent; corolla 5–lobed, funnel–shaped, colorless, glabrous; lobes triangular, 0.4 mm long, ascending; stamens 5, free; filaments weakly fused at base of corolla, ca. 0.5 mm long, colorless; anthers not fused, basifixed, dithecal, 0.8—0.9 mm long with threadlike appendages at tip, light yellow, longitudinally dehiscent; pollen light yellow; pistil lacking ovary, club–shaped, 0.8 mm long, colorless, papillate at top.
Pistillate head bracts subtending cymelike array 3, each 3–lobed, the lowest bract alternate, oblanceolate with diminutive lobes near base, to 15 mm long, the upper 2 opposite, like miniature 3–lobed leaves, to 6 mm long, each lateral lobe with blackish vein; involucre obovoid with small hole in conic beak, 4—5 × 2.5 mm, with 8—9 ribs, green and glabrous with blackish purple spots below greenish, short–hairy beak.
Pistillate flower calyx (pappus) absent; corolla absent; stamens absent; pistil 1; ovary “inferior” (no other flower parts present), obovoid, ca. 2.2 × 1.3 mm, white blushed purple–red at top, 1–chambered with 1 ovule; style 2–branched, the branches exserted through short beak, fused at base, stalk to 0.8 mm long, exserted stigmatic branches fingerlike, 1.3—1.5 mm long, whitish, with groove on upper side.
Fruit cypsela within involucre (perigynium), obovoid, < 4—5.5 × 2.5—3.8 mm, brown, blunt at base and pointed at top, having 0—5 short–pointed knobs approaching top above the widest diameter, each knob terminating a poorly defined, decurrent rib, lacking hairs.
A. C. Gibson