Palmer amaranth is a difficult to control weed that's been causing a lot of problems in recent years for farmers in the southern United States. It's been marching north and has been confirmed in Missouri, Illinois and Nebraska. Last month it was confirmed as being present in Iowa. It was found in a farmer's field in Harrison County in western Iowa and was positively identified by Iowa State University Extension weed management specialist Bob Hartzler and county Extension director Rich Pope.
After that discovery and positive identification, Hartzler spread the word with an article he wrote in the weekly ISU Integrated Crop Management newsletter. He asked that if any farmer or crop consultant in Iowa sees what they think is Palmer amaranth in a field, they should let the local ISU area field agronomist know. Or let Hartzler or his ISU weed science colleague Mike Owen know directly and one of the ISU weed specialists will check it out to make sure the weed in question is or isn't Palmer amaranth.
Palmer amaranth is close relative of common waterhemp, which is all over Iowa
Palmer amaranth is a close relative to Iowa's No. 1 weed problem -- waterhemp. Like waterhemp, Palmer amaranth can develop resistance to herbicides. Glyphosate-resistant Palmer amaranth is widespread in the southern U.S. Knowing how to tell the difference between these types of pigweeds is the key to keeping Palmer amaranth from becoming established and spreading throughout Iowa, say Hartzler and Owen.~~~PAGE_BREAK_HERE~~~
"Several people have submitted images of escaped "pigweeds" wondering whether the plants were Palmer amaranth," says Hartzler. "In all cases, except the earlier reported situation in Harrison County, the plants in question have been waterhemp, although at casual glance they could easily be mistaken for Palmer amaranth. These cases highlight the difficulty in differentiating the two species by the general growth habit and shape of plants."
Watch your fields for the presence of odd-looking pigweeds
Palmer amaranth is generally characterized by longer and thicker terminal inflorescences (flower stalks) than found on waterhemp, explains Hartzler. However, both species are highly variable and there is much overlap in their general appearance. With many waterhemp control failures across the state due to herbicide resistance, the entire phenotypic variation of waterhemp is on display, and there often are plants present that have inflorescences typical of Palmer amaranth. "I have seen numerous waterhemp plants with inflorescences as thick as those of Palmer amaranth," he says.
The most reliable characteristic to differentiate the species is the size of the bracts at the base of the flowers, says Hartzler. Bracts are modified leaves, and on Amaranthus species they are narrow, triangular and ending in a sharp point. The bracts of Palmer amaranth are up to ¼ inch in length (3 to 7 mm) and extend far beyond the tepals (modified petals and sepals of Amaranthus flowers). On female plants, the bracts become stiff as they mature and are painful to the touch. The bracts on waterhemp are less than 3 mm in length and rarely longer than the tepals.
The photos accompanying this article show the difference between bract size of Palmer amaranth and waterhemp on both individual flowers (Photo 2) and on the inflorescence (Photo 1). Palmer amaranth is on the left in both photos. The bracts are the dark green, pointed structures at the base of the flowers (see Photo 2). Bract size and stiffness of the female Palmer amaranth bracts are the most consistent characteristics to differentiate the species, says Hartzler.