Perennials

Amaranthus retroflexus

redroot pigweed

AmaranthaceaeTropical and subtropical Americas

At a Glance

TypeAnnual
HabitUpright
FoliageDeciduous
Height12-72 inches (30-180 cm)
Width12-24 inches (30-60 cm)

Key Features

Drought Tolerant
Native to North America
Maintenancehigh

Overview

Amaranthus retroflexus is a summer annual in the family Amaranthaceae 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 plant its common name. 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, greenish, and densely packed into stiff, bristly terminal and axillary panicles 2–8 inches (5–20 cm) long. A single plant produces 100,000 to over 200,000 seeds. Seeds are small, round, glossy black, approximately 0.04 inch (1 mm) in diameter, and remain viable in soil for 10–40 years. Foliage accumulates nitrates and oxalates, particularly when growing in nitrogen-rich soils, making it toxic to livestock when consumed in quantity. Classified as a major agricultural weed worldwide, competing with row crops for water, nutrients, and light. Some populations have developed resistance to ALS-inhibiting and triazine herbicides.

Native Range

Native to tropical and subtropical regions of the Americas, with the precise center of origin uncertain. 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. Included in agricultural weed identification curricula.

How to Identify

Identified by the stout, rough-hairy stem with a reddish to pinkish taproot visible when uprooted. Leaves are alternate, ovate to rhombic, 1.5–6 inches (4–15 cm) long with a notched tip and long petiole. Flower spikes are dense, bristly, and terminal, distinguishing it from A. hybridus (smooth pigweed), which has more slender, less bristly inflorescences and a whitish taproot.

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
J
F
M
A
M
J
J
A
S
O
N
D
SummerFall
Flowers from June through October, triggered by day length shortening below approximately 14 hours. Individual plants bloom over 4–6 weeks. In the Pacific Northwest, peak flowering occurs in August through September. Seeds begin maturing 3 weeks after pollination; wind-pollinated.

Detailed Descriptions

Flower Description

Green

Foliage Description

Dull green, ovate to rhombic with prominent veins on the underside

Growing Conditions

Sun Requirements

Full 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
loamsandclaysilt
Drainage
well drained

Water & Climate

Water Needs

Low

Frost Tolerance

tender

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Classified as a major agricultural weed; management focuses on prevention and removal. Shallow cultivation at the 2–4 leaf seedling stage is most effective, as taproots become established by the 6-leaf stage. A single plant can produce over 200,000 seeds, so removal before seed set is critical. Seeds germinate when soil temperatures reach 60–65°F (15–18°C) at 0.5–1 inch (1–2.5 cm) depth. Repeated cultivation throughout the growing season depletes seedlings but does not affect the persistent seed bank. Mulching with 3–4 inches (8–10 cm) of organic material suppresses germination in garden settings.

Pruning

No pruning applicable. Plants are managed by pulling or cultivating before seed maturity. Cutting at ground level prevents seed set on the cut stem but does not prevent regrowth from basal buds if done early in the season. Remove cut plants from the site, as developing seed heads may ripen viable seed after removal.

Maintenance Level

high

⚠️ Toxicity Warning

Toxic to pets