Pigweed (Amaranthaceae family):
Amaranthus retroflexus L.
EPPO code:
AMARE
Other names:
Pigweed, redroot, rough pigweed, tall pigweed

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.
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 range between 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.
Mature leaves:
Ovate, dull green and somewhat hairy, has a small notch with a spine (mucron) at the tip.

Mature plant

Stem:
Alternate leaf orientation; the stem is coarse with many short hairs.
Seed heads:
Seed heads have short, thick spikes that are very coarse to the touch.
Roots:
Taproot

Often mistaken for

I know it's not Waterhemp because the stem of redroot pigweed is covered in dense short hairs, whereas waterhemp is smooth and hairless. The tips of waterhemp leaves are not typically notched.

I know it's not Green pigweed because only 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.

The linear/narrow cotyledons and ovate leaves of seedling plants
The linear/narrow cotyledons and ovate leaves of seedling plants.
The dull green and wavy leaf margins of a 10 leaf seedling plant
The dull green and wavy leaf margins of a 10 leaf seedling plant.
Closeup of mature leaf
Closeup of mature leaf.
The short compact finger like spikes of the mature seed head. The finger-like spikes of green pigweed tend to be longer than the short and compact ones on redroot pigweed.
The short compact finger like spikes of the mature seed head. The finger-like spikes of green pigweed tend to be longer than the short and compact ones on redroot pigweed.
The short but dense hairs that cover the stem of redroot pigweed
The short but dense hairs that cover the stem of redroot pigweed.