Redroot Pigweed: Amaranthus
Weed Description: An erect
summer annual that may reach 6 1/2 feet in height. Redroot pigweed is an abundant
seed producer that may be found throughout the United States in horticultural, nursery,
and agronomic crops, landscapes, roadsides, and also in pastures and forages.
|Seedling: Stems below
the cotyledons (hypocotyls) are without hairs (glabrous) but may sometimes be slightly
hairy, and are often red in color, especially near the base. Cotyledons are
narrow (10-12 mm long) and green to reddish in color on the upper surface. Lower
surfaces of cotyledons have a reddish tint. First true leaves are alternate, ovate
in shape, and are slightly notched at the tip of the leaf blade (apex). Hairs may
occur on the leaf margins and along veins, especially along the lower leaf surfaces.
||Leaves: Alternate, ovate in outline,
with petioles that reach 1/2 inch in length. Leaves have wavy margins and hairs that
occur along the veins of the lower leaf surfaces.
|Stems: Stout, erect, branched, and
reaching 6 1/2 feet in height. Stems usually have short hairs, especially near the
upper portions of the plant.
Roots: A shallow
taproot that is often, but not always, reddish in color.
Fruit: A single seeded utricle that reach 2 mm in
length and are wrinkled when dry. Each utricle splits open in the middle to expose a
single glossy black to dark brown seed that is 1 to 1.2 mm long and ovate in outline.
||Flowers: Small, green, inconspicuous
flowers are produced in dense, compact, terminal panicles that are approximately 3/4 inch
wide and from 2 to 8 inches in length. Smaller inflorescences also occur between the
stem and the leaf petioles (leaf axils). Male and female flowers occur on the same
plant (monoecious). Each terminal panicle contains many densely packed branched
spikes that have bracts that are 4 to 8 mm long and 2-3 times longer than the sepals.
|Identifying Characteristics: Dense,
compact terminal panicles and relatively tall plants with alternately arranged
leaves. Redroot pigweed is often confused with other similar pigweed species.
For example, smooth pigweed is very similar, however this species has terminal panicles
that appear less dense, compact, and bristly than those of redroot pigweed.
Additionally, the bracts of smooth pigweed are only slightly longer than the sepals,
unlike those of redroot pigweed. Palmer Amaranth
(Amaranthus palmeri) also resembles redroot and smooth pigweed, however the
terminal panicles of this species are much longer and narrower than the other pigweed
species. These species may also resemble Common
Lambsquarters (Chenopodium album) in the cotyledon stage, however common
lambsquarter's cotyledons often have a mealy gray cast, and the first true leaves are
alternate, unlike any of the pigweed species.