14. Amaranthus tuberculatus (Moq.) J.D. Sauer (water hemp)

A. rudis J.D. Sauer

A. tuberculatus var. rudis (J.D. Sauer) Costea & Tardif

A. tamariscinus Nutt.

Acnida tamariscina (Nutt.) A.W. Wood var. tuberculata (Moq.) Uline & W.L. Bray

Acnida tamariscina var. prostrata Uline & W.L. Bray

Pl. 197 a, b, i–m; Map 823

Plants dioecious. Stems (20–)50–200(–350) cm long, usually ascending or erect, glabrous or nearly so, unarmed. Leaves long-petiolate basally, with progressively shorter petioles toward the stem tip. Leaf blades 2–20 cm long, lanceolate to elliptic or ovate, narrowed or tapered to a sharply pointed, bluntly pointed or minutely notched tip, narrowed or tapered at the base, glabrous. Inflorescences dull or grayish green, occasionally dull reddish-tinged, axillary and terminal, the axillary inflorescences mostly elongate spikes or small panicles of elongate spikes at maturity, the terminal inflorescence a panicle with usually numerous ascending branches, the flowers often grouped into discontinuous clusters or regions along the spikes, the tip straight or somewhat curved at maturity, the main axis and branches glabrous to densely but inconspicuously pubescent with mostly crinkled multicellular hairs. Bracts 1.0–2.5 mm long, shorter than to about as long as the sepals, lanceolate to narrowly ovate, with a somewhat thickened green midrib and relatively broad, thin, papery margins, the midrib usually extending beyond the main body as a short awn, usually somewhat spinelike at maturity. Staminate flowers with 5 unequal (the outer 2 more strongly pointed, slightly longer than and strongly overlapping the inner 3) to more or less similar sepals, these 2.2–3.0 mm long, erect or very slightly outward-curved, lanceolate to oblong-elliptic, all tapered and with a short, awnlike extension of the midrib at the tip, or the inner ones sometimes bluntly pointed to shallowly and minutely notched. Stamens 5. Pistillate flowers with the sepals absent or irregularly 1 or 2 and 0.5–2.0 mm long, linear to lanceolate, tapered to a short, awnlike extension of the midrib, this often somewhat spiny at maturity. Stigmas 3 or 4, spreading. Fruits 1–2 mm long, indehiscent or circumscissilely dehiscent at about the midpoint, the surface smooth or more commonly noticeably wrinkled when dry. Seeds 0.8–1.2 mm in diameter, rounded along the rim, the surface reddish brown to black. 2n=32. July–October.

Scattered nearly throughout the state (Wisconsin to Indiana and Alabama west to North Dakota and Texas; Canada; introduced farther east and west to the Atlantic and Pacific seaboards). Banks of streams and rivers, sloughs, and margins of ponds and lakes; also crop fields, fallow fields, banks of ditches, roadsides, railroads, and moist, open, disturbed areas.

Traditionally, A. rudis has been treated as a separate species, based on a number of subtle flower and fruit differences, with intermediate plants written off as hybrids between A. rudis and A. tuberculatus. Pratt and Clark (2001) studied both morphological and allozymic variation in the complex and concluded that there was no morphological or genetic basis for maintaining more than a single biological species of water hemp. Plants traditionally referred to as A. tuberculatus apparently are merely a form in which the sepals of pistillate flowers are reduced or absent and the fruits tend not to dehisce. Costea and Tardif (2003) treated the two entities as varieties of A. tuberculatus, noting that var. tuberculatus and var. rudis are more or less distinguishable in the eastern and western portions respectively of the overall range. However, in the midwestern states, the two taxa are neither morphologically nor ecologically distinguishable. Steyermark (1963) and most earlier authors called plants having pistillate flowers with 1 or 2 well-developed sepals A. tamariscinus Nutt., but Sauer (1972) showed that this name actually refers to a sterile hybrid between what he named A. rudis and one of the members of the monoecious A. hybridus complex.

Amaranthus tuberculatus is extremely variable morphologically. Rare plants with spreading to loosely ascending stems and poorly developed terminal spikes were once called var. prostrata. Sauer (1955) concluded that these represented stunted ecological variants that grew from seeds germinating too late in the season for proper development to have occurred.