MANKATO, Minn. – University of Minnesota Extension faculty took the opportunity at the 2019 Minnesota Ag Expo to quiz farmers. At one of their booths, on the main trade show floor, they displayed four pigweed species and asked farmers to identify them.
“At the request of the Minnesota Soybean Growers, they wanted to increase the educational level of identification of some critical weeds,” said David Nicolai, Extension Educator, while manning the booth. “We grew some weeds in the greenhouse and brought them here to the Ag Expo, both more of an intermediate maturity, not full maturity and then also some seedlings.”
The goal was to help farmers and ag professionals see the differences in the Palmer amaranth pigweed versus other pigweed species that look very similar. On display, there was Palmer amaranth, water hemp, redroot pigweed, and Powell amaranth.
The redroot pigweed and the Powell amaranth are two that are very similar.
“Those two are similar in terms of structure, there is some subtle differences in the amount of hair they have, their inflorescence, which comes out later, but they are both plants that have the male and female flowers on the same plant,” said Nicolai.
Water hemp and Palmer amaranth have separate male and female plants. They also do not have the hair on the stems and leaves that redroot pigweed and Powell amaranth have.
The hairs on the Powell amaranth are finer than the hairs of the redroot pigweed.
“A lot of attendees seem to have a good idea obviously on water hemp and a fair amount on Palmer but are glad to kind of see it here again, in terms of some of the structures,” he said. “It's a learning activity.”
As opposed to the other pigweed species, Palmer amaranth has leaf petioles that are longer than the leaf blade. The female plants have prickly bracts that surround the seed on the seed head. The Palmer species will also grow faster than the other species.
“Here today, Palmer is the tallest seedling compared to everything else and they were seeded on the same day,” he said.
It is very important that farmers and crop consultants are able to identify and distinguish Palmer amaranth from the other pigweed species. Water hemp, redroot pigweed, and Powell amaranth are common in Minnesota, but Palmer amaranth is a new introduction that has been found in limited populations in the state.
“The Minnesota Department of Ag has it on their noxious and eradicate weed list, so they come out, map the area, pull the weed and try to do everything they can to eradicate it,” he said. “They monitor it for a couple of years after that, in terms of making sure they do not have any seed that grows up into plants and reseeds itself.”
If a grower were to find Palmer amaranth in their field, it is important that they contact either the Minnesota Department of Agriculture or their local extension educator. That way samples can be taken of the weed and it can be confirmed as Palmer via testing. The MDA wants to track the weed’s movement into the state and take the necessary steps to eradicating it.
“We do have Palmer amaranth, but it is in just a few particular areas,” said Nicolai. “It came in a couple of years ago in some contaminated seed lots, but recently, this last year, we have been finding it in a couple other fields, including a soybean field in Redwood County.”
Management of the redroot pigweed and Powell amaranth is still fairly easy. A lot of the conventional herbicides, including glyphosate-based herbicides, continue to have an effect on these two species.
“Where on the other two species, water hemp and Palmer amaranth, they really don't (respond to herbicides long-term), due to herbicide resistance,” he said. “You have to do more extensive management plans for them.”