Virginia Tech Weed Identification Guide

Redroot Pigweed: Amaranthus retroflexus


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. redpigweed4-6.jpg (161299 bytes)
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.