Keep and Eye Out for New Pigweed Species
Posted: June 26, 2012
Keep in mind that herbicide rate, coverage, weed size, adjuvant, environmental conditions, etc. all impact the success of the herbicide application; evolution of resistance in a weed species not previously documented in our area is quite rare. However, if more than one failure to control the weed occurs with the same herbicide, the situation should be quickly assessed to determine if resistance is the potential reason. The other problem brought to our attention was a sample of a waterhemp (Amaranthus rudis and tuberculatus) brought in for identification. The waterhemps are somewhat rare species of pigweed in our region, but this is the second year in a row that someone has brought us a sample at this late June field day. Although The Plants of Pennsylvania lists waterhemp as occurring in PA, up until last year, we had never encountered it in the region. The key features of this plant are longer more slender (lanceolate) leaves borne on long petioles. In addition, male and female flowers tend to be long and narrow and are on separate plants (dioecious). The waterhemps have been troublesome for a number of years in the Midwest with documented resistance to the ALS-inhibitors, glyphosate, the triazines, the PPO inhibitors, and most recently the HPPD-inhibiting herbicides.
Most of you have been hearing about a weed problem to our South and West called Palmer amaranth or pigweed. Palmer amaranth (Amaranthus palmeri) is an annual pigweed that infests much of the cotton belt and some areas to the west. Its recent claim to fame is wide spread glyphosate resistance, but biotypes have been found that are resistant to the dinitroanilines such as Treflan and Prowl in South Carolina, the ALS inhibitors (Pursuit, Raptor, Harmony, etc.) in numerous states, the photosynthesis inhibitors including atrazine in Texas, and most recently glyphosate in Alabama, Arkansas, Georgia, Illinois, Louisiana, Michigan, Mississippi, Missouri, North Carolina, Tennessee, and Virginia (www.weedscience.org). Last summer, Dr. Mark VanGessel at the University of Delaware reported some infestations in Delaware and Maryland and Michigan reported a Palmer pigweed infestation back in 2010.
The leaves and stem of Palmer pigweed are smooth and like waterhemp, the petioles are longer than smooth or redroot pigweed (long as or longer than the leaf blades). The plants often have a poinsettia-like appearance with symmetrical leaf arrangement. Palmer amaranth is also dioecious, so cross pollination is necessary for successful seed production. Wind dispersed pollen is believed to be a primary avenue for the spread of the resistant populations. Seeds are known to be spread on equipment, in flowing water, by animals, and likely contaminated crop seed or hay. Palmer pigweed females can produce almost half a million seeds per plant and under optimal conditions; a single plant can produce up to a million seeds. As we move through the summer, keep an eye out for any unusual pigweeds that crop up.
Some useful links to learn more about both waterhemp and Palmer amaranth as well as some of the other pigweeds includes:
- Penn State Weed Science