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Ontario Weeds: Redroot pigweed

Excerpt from Publication 505, Ontario Weeds, Order this publication

Table of Contents
  1. Name
  2. Other Names
  3. Family
  4. General Description
    • Photos and Pictures
  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. 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.

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

OMAFRA Crops

Redroot Pigweed

This weed can be difficult to control in non-competitive crops like flax. It can also act as a host for tarnished plant bug which, in turn, can infest many field crops.

click to enlarge

Biology

Redroot pigweed grows in cultivated fields, pastures, roadside ditches, and undeveloped areas. It is a common annual weed, producing many seeds that remain viable for up to 5 years. It has a long, fleshy, reddish to pink taproot. The stems are erect, light green, stout, branched, 60-90 cm (1-3 ft.) high, rough, and angular. The leaves are alternate, stalked, dull green, ovate, rough, and 8-10 cm (3-4 in.) long. The flowers are numerous, small, green, crowded into dense finger-like spikes in axils of the leaves, and in a large terminal spike or panicle.

Redroot pigweed requires high temperatures for germination (optimally, in the 20-30 degrees C or 70-85 degrees F range) and will continue to germinate throughout the summer if there is adequate soil moisture. It will grow almost anywhere and in any crop but is most abundant on rich soils, thriving at higher temperatures.

Scouting Techniques

Take a minimum of 20 weed counts across the field. Scout frequently because under hot conditions, redroot pigweed can advance through it’s growth stages quickly, potentially escaping the window for herbicide application.

Effects On Crop Quality

This weed, because it is an alternate host for many crop insect pests, can indirectly affect the development of field crops.

Threshold/Yield Loss

Redroot pigweed causes yield losses in most crops, particularly under hot conditions.

Control Tips

Crops established before soils warm and hot weather sets in are very competitive with redroot pigweed.

Herbicides are available in most crops for control of redroot pigweed. Watch growth stages carefully under hot conditions because the weed can quickly advance beyond the recommended stages for herbicide application.

Redroot Pigweed This weed can be difficult to control in non-competitive crops like flax. It can also act as a host for tarnished plant bug which, in turn, can infest many field crops.