Waterhemp

Pigweed (Amaranthaceae family):

Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea & Tardif

EPPO code:

AMARU

Other names:

Common waterhemp

Species information

Lifecycle:

Annual.

Propagation:

Reproduces by seed, dioecious, has separate male and female plants.

Emergence:

Spring, typically after lamb’s-quarters.

Habitat:

Not native to Canada. Now found throughout Ontario, mainly along waterways and beaches. Prefers full or partial sun, moist or wet conditions.

Competitiveness:

In University of Guelph research trials, poor control of waterhemp has resulted in over 85 percent yield loss in both corn and soybeans.

Resistance:

A number of populations are resistant to Group 2 (for example, imazethapyr), Group 9 (for example, glyphosate) and Group 5 (for example, atrazine) herbicides in Ontario and throughout North America.

Identification clues

Seedlings

Cotyledons:

Linear, narrow, purple-red underneath.

Mature leaves:

Lance-shaped (tapered and typically with a pointed tip), with a wavy margin.

Mature Plant

Stem:

Alternate leaf orientation; smooth with no hairs; variable colouring (green to red/ purple); 1.5–2.5 m in height.

Seed heads:

Long, spikey clusters (somewhat similar to ragweed) that can vary in colour from green to red to purple. Only the female plants produce seed.

Roots:

Taproot.

Often mistaken for

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

I know it's not Green pigweed because the upper stem of green pigweed has a cluster of dense hairs whereas the stem of waterhemp is smooth and hairless. Waterhemp will typically have narrower leaves with wavy margins and without a notched tip.

I know it's not Eastern black nightshade because the leaf margins of young eastern black nightshade leaves are hairy and the older leaves have wavy toothed margins whereas waterhemp leaves are hairless with wavy margins.