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.