Amaranthus retroflexus

redroot pigweed

Amaranthaceae

Tropical and subtropical Americas; naturalized worldwide on every continent except Antarctica

Drought Tolerant

Native to North America

SunFull Sun

WaterLow · drought-tolerant

Overview

Amaranthus retroflexus is a summer annual in the family Amaranthaceae spp. reaching 12–72 inches (30–180 cm) tall. Stems are erect, stout, and rough-hairy, often branching from the base, with a reddish to pinkish taproot that gives the species its common name redroot. Leaves are alternate, ovate to rhombic, 1.5–6 inches (4–15 cm) long, dull green with prominent veins on the underside, borne on long petioles. Flowers are small and greenish, densely packed into stiff bristly terminal and axillary panicles 2–8 inches (5–20 cm) long. A single mature plant produces 100,000 to over 200,000 seeds, and the seeds are small, round, glossy black, approximately 0.04 inch (1 mm) in diameter, remaining viable in soil for 10–40 years. Foliage accumulates nitrates and oxalates, particularly when the plant grows in nitrogen-rich soil, producing toxicity in livestock that consume the plant in quantity. The species is classified as a major agricultural weed worldwide and competes with row crops for water, nutrients, and light. Some populations have developed herbicide resistance to ALS-inhibiting and triazine herbicide groups.

Native Range

Amaranthus retroflexus is native to the tropical and subtropical regions of the Americas, with the precise center of origin uncertain. The species has naturalized on every continent except Antarctica, occurring from sea level to approximately 7,500 feet (2,300 m) elevation in disturbed agricultural land, roadsides, gardens, and waste areas.

Suggested Uses

Used in weed-science research, particularly in studies of herbicide-resistance evolution. Young leaves are edible when cooked and have been consumed historically as potherbs. Seeds are consumed by ground-feeding birds including sparrows and finches. The species is included in agricultural weed-identification curricula. Intentional garden planting is not a standard horticultural use.

How to Identify

Identified by a stout rough-hairy stem with a reddish to pinkish taproot visible when the plant is uprooted. Leaves are alternate, ovate to rhombic, 1.5–6 inches (4–15 cm) long with a notched tip on a long petiole. Flower panicles are dense, bristly, and terminal. Separated from Amaranthus hybridus (smooth pigweed) by the more slender less-bristly inflorescences of that species and by its whitish (versus reddish) taproot. Separated from Amaranthus albus (tumbleweed pigweed) by the upright single-stemmed habit of A. retroflexus versus the many-branched decumbent habit of A. albus.

Appearance

Size & Dimensions

Height1' - 6'

Width/Spread1' - 2'

Colors

Flower Colors

green

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~10 weeks

Summer Fall

Flowers from June through October, triggered as daylength shortens below approximately 14 hours. Individual plants bloom over 4–6 weeks. In the Pacific Northwest, peak flowering occurs in August and September. Seeds begin maturing 3 weeks after pollination; the species is wind-pollinated.

Detailed Descriptions

Flower Description

Greenish; small in dense stiff bristly terminal and axillary panicles 2–8 inches (5–20 cm) long

Foliage Description

Dull green; ovate to rhombic with prominent veins on the underside; 1.5–6 inches (4–15 cm) long on long petioles

Growing Conditions

Sun Requirements

SunFull Sun

Requires 6-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 8.0(Neutral)

357912

Soil Types

loam sand clay silt

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

tender

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

The species is classified as a major agricultural weed; management rather than cultivation is the standard practice. Shallow cultivation at the 2–4 leaf seedling stage removes plants reliably because taproots become established by the 6-leaf stage and hand-pulling becomes harder beyond that point. A single plant can produce more than 200,000 seeds, so removal before seed set is critical to limiting the soil seed bank. Seeds germinate when soil temperatures reach 60–65°F (15–18°C) at 0.5–1 inch (1–2.5 cm) depth. Repeated cultivation through the growing season depletes germinating seedlings but does not reduce the persistent seed bank, which remains viable in soil for 10–40 years. Mulching with 3–4 inches (8–10 cm) of organic material suppresses germination in garden settings.

Pruning

Pruning is not applicable. Management is by pulling or shallow cultivation before seed maturity. Cutting at ground level prevents seed set on the severed stem but does not prevent regrowth from basal buds if cut early in the season. Cut plants are removed from the site because developing seed heads can ripen viable seed after the plant has been cut.