Spiny Amaranth

Amaranthus spinosus

Alternate names
Spiny Pigweed

Weed Description
A summer annual that is very similar in appearance to other pigweeds but has spines along the stems. Spiny amaranth is primarily a weed of pastures and hay fields, and occurs less often in agronomic crops and turfgrass. Spiny amaranth is found throughout the eastern half of the United States.

Seedlings
Stems below the cotyledons (hypocotyls) are usually reddish in color but sometimes green, without hairs. Cotyledons are without hairs, long and narrow.

Leaves
Alternately arranged along the stem, ovate in outline. Leaves are approximately 1 1/4 to 2 1/2 inches long, without hairs, and occur on long petioles.

Stems
Erect, branching, without hairs, reaching 5 1/2 feet in height. A pair of spines that are from 5 to 10 mm long occurs at the base of most of the leaf petioles.

Flowers
Seedheads occur at the ends of stems and also in small clusters in the area where the leaf petioles meet the stem (leaf axils).

Fruit
An utricle that is 1 1/2 to 2 mm long.

Identifying Characteristics
Plants that resemble most other pigweed species but with pairs of spines at the base of the leaf petiole and the central stem. The spines of spiny amaranth help to distinguish it from all other closely related pigweed species, like Redroot Pigweed (Amaranthus retroflexus), Palmer Amaranth (Amaranthus palmeri), and Smooth Pigweed (Amaranthus hybridus). This weed may also be confused with Spiny Cocklebur (Xanthium spinosum) however the spines of this weed are 3-parted unlike those of spiny amaranth.

Office of Programs

University of Missouri

Spiny Amaranth

Amaranthus spinosus