How to identify pigweeds

Pigweeds (Amaranthus spp.) are difficult to identify from each other. But identification is crucial for developing a successful weed management program, particularly in an agricultural landscape that is heavily reliant on herbicides (and where resistance is common). Species can also differ in their growth rates, which can impact the success of physical management, like cultivation.  When thinking about pigweed identification, focus on a few key characteristics:

  • The shape of the leaf blade and the length of the petiole, as well as the hairiness of the stem.
  • The presence of male and female flowers on the same (monoecious) or separate (dioecious) plants and the structure of the flower heads (inflorescence).
  • The size of the plant.

In the above two pictures, the top row shows the leaves and flower heads  of smooth pigweed, redroot pigweed, and Powell amaranth (left to right). The bottom row shows the leaves and male and female flowers of waterhemp (left) and Palmer amaranth (right). Despite similarities across species, there are traits/combinations of traits that can be used to distinguish among the pigweeds to successfully identify them.

Identifying traits of five pigweed species
Trait Palmer amaranthWaterhemp Powell amaranthRedroot pigweedSmooth pigweedPurple Pigweed
LeavesDiamond-shapedLong and linear Diamond-shapedOval- to egg-shaped with wavy marginsOval- to egg-shaped with wavy marginsWedge-shaped, deeply notched tip with stiff hair.
PetiolesLONGER than leaf bladeShorter than leaf bladeShorter than leaf bladeShorter than leaf bladeShorter than leaf bladeShorter than leaf blade
Stems and plant heightSmooth
Up to 10 feet
Smooth
Up to 10 feet
Sparsely hairy
3 to 6 feet
Sparsely to very hairy
3 to 6 feet
Very hairy
3 to 6 feet
Smooth 
3 to 4 feet
Male and female flowersSEPARATE plantsSEPARATE plantsSame plantSame plantSame plantSame plant
Flower headsThick branches and tightly clustered flowers, female flowers have SHARP bractsBranches are thinner than Palmer amaranth, flowers are less tightly clustered, no bractsSparsely branched, but branches can be long and flowers have bracts resembling PalmerBranches on flowerheads are compact and short/stubbyMany branched flower heads with branches longer and thinner than redrootTerminal and in the axils of leaves and at the base of branches