The Purdue Plant and Pest Diagnostic Laboratory

PPDL Picture of the Week for
April 30, 2012

It’s a pigweed, right?

Travis Legleiter, Weed Science Professional Assistant, Botany & Plant Pathology Department

The term pigweed is often used to lump together all species within the Amaranthus genera.  Within the state of Indiana the most prevalent and common Amaranth is smooth or redroot pigweed (Amaranthus retroflexus).  Although two new amaranth species are likely to begin making their appearance in Indiana: Common Waterhemp (Amaranthus Rudis) and Palmer amaranth (Amaranthus palmeri).  Common waterhemp has already become a prevalent species in pockets of Indiana and is anticipated by Purdue weed scientist to continue to spread throughout Indiana.  Currently only a few incidents of palmer identification has been reported in Indiana and the potential spread is less anticipated as compared to waterhemp.

The identification of these individual species can prove to be essential as waterhemp and palmer amaranth are much more aggressive and capable of becoming herbicide resistant as compared to the redroot pigweed populations that currently exist in Indiana.  So when scouting your fields this spring rather than simply glancing at that weed in your field and generalizing it as a  “pigweed”, take a closer look and determine just exactly what species you have.  The following characteristic can help you distinguish the three amaranth species.

Redroot pigweed: Stems will have hairs present especially towards the growing point of younger plants.  Leaves will appear to be rough in texture and are oblong to oval in shape.

Common waterhemp:  hairs will be absent on all parts of the plant.  Leaves appear to have a waxy upper surface and are much more lance to linear in shape than that of either redroot pigweed or palmer amaranth.

Palmer amaranth: Also will have an absence of hairs on all plant parts.  Petioles of palmer are much longer than the other two species and can be as long or longer than the actual leaf blade.  Leaves will be lance to diamond shaped with a much wider apex than waterhemp leaves and less rounded than redroot pigweed.  When viewed from the top, palmer plants will have a rosette-like appearance that is similar to that of a poinsettia plant.

As easy as it is for myself to type this and show you pictures of plants side by side in the greenhouse, it can be very difficult to distinguish these characteristics with isolated plants in the field.  Make sure to become familiar with these characteristics and take along weed id guides and pictures to assist yourself in the field.

If you do identify waterhemp or palmer amaranth in your field, be aware that these two species can quickly become herbicide tolerant when exposed to multiple herbicide applications of a single mode of action.

Click image to enlarge

Photo 1.
The three amaranth species can be identified by their leaf shapes, petiole lengths, and the presence or absence of hairs.  Left to Right: Palmer amaranth, common waterhemp, and redroot pigweed.

Photo 2. 
Redroot pigweed (left) can be distinguished from waterhemp (right) and palmer amaranth by the presence of hairs on the stem as well as it’s oblong to oval shaped leaves.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service