On the Trail of a Monster

- Craig Nelson

Waterhemp is both a monstrous and remarkable weed (Amaranthus tuberculatus (A. rudis is occasionally used to identify waterhemp plants that have developed herbicide resistance.)) that has been a part of the North American landscape for centuries. It is an edible plant that was important for Native Americans who collected its seeds for food. That might be the only good thing that canbe said about this plant, a close relative to redroot pigweed, Palmer amaranth and other common pigweeds. Waterhemp is widely adapted and can produce several hundred thousand seeds per plant. These plants have a profound propensity to develop herbicide resistance. One plant population found in Missouri was confirmed in late 2018 to have developed resistance to six herbicide modes of action.

It is impossible to make a definitive, visual identification of waterhemp seeds. Its seeds along with seeds of other Amaranthus species all have similar phenotypic shape and size. They all can vary in color from dark to light brown with varying amount of seed coat mottling. While one species may tend to be generally darker or lighter than another species, there is a significant amount of coloration overlap that makes it impossible to identify individual seeds. When waterhemp or other pigweed seeds are found in a seed sample, they will be listed as Amaranthus spp. on the seed analysis report. The best solution for handling waterhemp and related seeds, of course, is to keep them out of a seed lot in the first place.

When found in any seed lot but especially in grass or cover crops these seeds present an immediate problem. Like Palmer Amaranth, Waterhemp has been classified as a prohibited noxious weed in the state of Wisconsin, making proper identification of Amaranthus turberculatus imperative. If a seed control official pulls a sample that tests positive for Amaranthus spp. the lot could be subject to an immediate stop sale order. Eurofins BioDiagnostics has recently validated a PCR test that can be used to positively identify individual waterhemp seeds discovered within a physical seed purity examination.

Identifying weed seeds within any seed lot is always important, but when Amaranthus seeds are discovered. A PCR analysis is the only method except for completing a time-consuming growout to determine whether or not noxious weed seeds are present.