Ontario Weeds: Redroot pigweed

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Excerpt from Publication 505, Ontario Weeds, Order this publication

Table of Contents

  1. Name
  2. Other Names
  3. Family
  4. General Description
  5. Stems and Roots
  6. Flowers and Fruit
  7. Habitat
  8. Similar Species
  9. Related Links

Name: Redroot pigweed, Amaranthus retroflexus L.,

Other Names: amarante à racine rouge, Green amaranth, Pigweed, Redroot, Rough pigweed, Tall pigweed, amarante réfléchie, armarante pied rouge

Family: Amaranth Family (Amaranthaceae)

General Description: Annual, reproducing only by seed.

Photos and Pictures

Redroot pigweed (A - young flowering plant with dull (non-shiny) leaves; B - thick inflorescence with short lateral branches).

Redroot pigweed (A - young flowering plant with dull (non-shiny) leaves; B - thick inflorescence with short lateral branches).

Redroot pigweed. A. Base of plant. B. Top of flowering plant.

Redroot pigweed. A. Base of plant. B. Top of flowering plant.
Stems & Roots: Stems erect, 10cm-2m (4in.-6½ft) high, but usually 50-90cm (20-36in.), simple or branched, lower part thick and smooth, upper part usually rough with dense short hair, greenish to slightly reddish but usually red near the roots; leaves alternate (1 per node), long-stalked, ovate with a shallow notch at the tip on young plants but on older plants somewhat diamond-shaped, dull green above but lighter green and with prominent whitish veins below, and somewhat hairy.

Flowers & Fruit: Inflorescence a coarse, branching, bristly panicle made up of a short, thick terminal spike and below it several to many short, lateral finger-like spikes, these pointing upward if not crowded or outward if densely crowded, and smaller spikes in some lower leaf axils, each spike made up of many tiny flowers and spiny-tipped bracts up to 8mm (1/3in.) long; each flower unisexual, having either 1 pistil or 5 stamens but never both (similar to Prostrate pigweed); seeds black, shiny, round, flattened with a narrow, thin margin, and about 1mm (1/25in.) in diameter. Flowers from July to August.

Habitat: Redroot pigweed is a common weed in cultivated fields, gardens, pastures, waste places, roadsides and other disturbed areas throughout Ontario.

Similar Species: It is distinguished from Tumble pigweed and Prostrate pigweed by its tall, erect habit of growth, its larger and broader leaves, and its flowers crowded into a thick, terminal panicle as well as in some of the lower leaf axils; from Smooth pigweed by its coarse, harsh inflorescence; and from Green pigweed by the somewhat dull green colour of its leaves, the dense covering of short hair on its upper stem, its thick, coarse, bristly terminal panicle with the uppermost central spike extending only a short distance above the rest of the panicle, and by two features requiring magnification to see: the sepals of each flower are broader above the middle and rounded or somewhat flattened at their tips, and its male flowers usually have 5 stamens each.

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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