By Bob Hartzler, Department of Agronomy
Several people have submitted images of escaped "pigweeds" wondering whether the plants were Palmer amaranth. 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.
Palmer amaranth is generally characterized by longer and thicker terminal inflorescences (flower stalks) than found on waterhemp. 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.
The most reliable characteristic to differentiate the species is the size of the bracts at the base of flowers. 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 show the difference between bract size of Palmer amaranth and waterhemp on both individual flowers and on the inflorescence (Palmer amaranth is on the left). The bracts are the dark green, pointed structures at the base of the flowers. Bract size and stiffness of the female Palmer amaranth bracts are the most consistent characteristics to differentiate the species.
Bob Hartzler is a professor of agronomy and weed science extension specialists with responsibilities in weed management and herbicide use.