Pigweed (Amaranthaceae family):
Amaranthus powellii S. Wats
EPPO code:
AMAPO
Other names:
Powell amaranth

Species information

Lifecycle:
Annual.
Propagation:
Reproduces by seed.
Emergence:
Spring, typically after lamb’s-quarters.
Habitat:
Found in most cultivated fields. Less prevalent in winter cereals and established forage crops. Widespread across Ontario.
Competitiveness:
University of Guelph research examined competition in corn with pigweed at densities ranging from 16,000–32,000 plants/ac. When emerging at the 3–4 leaf stage of corn, yield losses have ranged from 22–52%. However when emergence was delayed until the 6–7 leaf stage, the impact on corn yield loss was reduced by 50%.
Resistance:
A number of populations are resistant to Group 2 (for example, imazethapyr) and Group 5 (for example, atrazine) herbicides in Ontario and throughout North America.

Identification clues

Seedlings

Cotyledons:
Linear, narrow. Purple–red underneath.
Young leaves:
Round to ovate with a short petiole, purple-red underneath, the upper surface is smooth and shiny. There is a small notch at the tip.
Mature leaves:
Ovate, shiny green and seemingly hairless. The tip of the leaf has a notch with a small spine (mucron).

Mature plant

Stem:
Alternate leaf orientation; the upper stem will have a cluster of short hairs but otherwise the stem feels hairless. Dark pink to red, contrasting with the dark green leaves.
Seed heads:
Long finger like spikes that are coarse to the touch, but less so then the short compact spikes of redroot pigweed.
Roots:
Taproot.

Often mistaken for

I know it's not Waterhemp because the upper stem of green pigweed will have a cluster of short hairs, whereas waterhemp’s stem is smooth and hairless. The tips of waterhemp leaves are not notched.

I know it's not Redroot pigweed because the upper stem of green pigweed has a cluster of dense hairs whereas the stem of redroot pigweed is extremely hairy all over. The leaves of redroot pigweed are also a dull green compared to the more shiny leaves of green pigweed.

I know it's not Eastern black nightshade because young eastern black nightshade plants have pointed, almost diamond shaped cotyledons compared to the linear cotyledons of pigweed species. The underside of eastern black nightshade leaves are typically a deep purple compared to the violet underside of pigweed species. The older leaves have wavy toothed margins whereas pigweed species lack the wavy toothed margins.

A four-leaf seedling plant with ovate leaves, notched apex and narrow cotyledons
A four-leaf seedling plant with ovate leaves, notched apex and narrow cotyledons.
Three pigweed plants: the top two are redroot pigweed with larger pale green leaves; the bottom green pigweed with darker, shinier leaves
Three pigweed plants: the top two are redroot pigweed with larger pale green leaves; the bottom green pigweed with darker, shinier leaves.
The mature leaf tends to be shinier and darker green than other pigweeds
The mature leaf tends to be shinier and darker green than other pigweeds.
Green pigweed’s stem is almost hairless with the majority of hairs on the top node
Green pigweed’s stem is almost hairless with the majority of hairs on the top node.
The finger-like spikes of green pigweed tend to be longer than the short and compact ones of redroot pigweed
The finger-like spikes of green pigweed tend to be longer than the short and compact ones of redroot pigweed.