“It’s the only weed I’ve seen that can drive a farmer out of business.” – Bill Johnson, Purdue University Extension Weed Specialist
Stutsman County Extension agent Alicia Harstad with a Palmer Amaranth plant (NDSU Photo) “Palmer amaranth’s prolonged emergence period, rapid growth rate, prolific seed production, and propensity to evolve herbicide resistance quickly makes this the most pernicious, noxious, and serious weed threat that North Dakota farmers have ever faced.” – Rich Zollinger, Retired NDSU Extension Weed Scientist
Palmer amaranth is a competitive and aggressive pigweed species that poses a major threat to North Dakota crop production. Here’s why.
Another distinguishing feature of Palmer Amaranth is the small spike sometimes found at the tips of the leaves. (NDSU Photo)
Palmer amaranth seedlings have egg-shaped leaves with a hair-like protusion at the leaf tip (Photo source: Christy Sprague, Michigan State University)
Blackshaw, R. and Rode, L. 1991. Effect of ensiling and rumen digestion by cattle on weed seed viability. Weed Sci. 39(1):104-108.
Reducing Palmer amaranth seed in feed
Eckford, R., Newman, J., Li, X., and Watson, P. 2012. Thermophilic anaerobic digestion of cattle manure reduces seed viability for four weed species. Int. J. Agric. & Biol. Eng. 5(1):71-75.
Stubbendieck J, Friisoe GY, Bolick MR (1994) Pigweed family. Pages 32–38 in Weeds of Nebraska and the Great Plains. Lincoln, NE: Bureau of Plant Industry, Nebraska Department of Agriculture.
Occurrence in Nebraska
5–10 tons/ac), applied within a day after cultivating an established crop, can reduce subsequent pigweed emergence by 90%. Alternatively, a synthetic mulch such as black plastic can be laid before crop planting, and alley weeds controlled by cultivation, mowing, organic mulch, or cover crop. Note: If plastic or other synthetic mulch is used to organic crops, it must be removed from the field at the end of the harvest or growing season.
Pigweeds have the C4 photosynthetic pathway, which confers an ability to grow rapidly at high temperatures and high light levels, to tolerate drought, and to compete aggressively with warm-season vegetables for light, moisture, and nutrients. Growth is related to cumulative Growing Degree Days, with a base temperature of 50 °F (Shrestha and Swanton, 2007; Horak and Loughin, 2000); thus, pigweeds grow much faster in hot climates than in northern regions with cooler summers.
Organic farmers manage pigweeds by taking advantage of their points of vulnerability. The small seeds have minimal nutrient reserves; thus seedlings can emerge only from seeds located within an inch of the soil surface, and are immediately dependent on the soil for readily available nutrients. Transplanted and large-seeded crops have substantial nutrient reserves, and can gain a competitive edge over pigweed seedlings if slow-release nutrient sources are used.
Crop Rotation, Planting Schedules, and Stale Seedbed
Pigweed is the common name for several closely related summer annuals that have become major weeds of vegetable and row crops throughout the United States and much of the world. Most pigweeds are tall, erect-to-bushy plants with simple, oval- to diamond-shaped, alternate leaves, and dense inflorescences (flower clusters) comprised of many small, greenish flowers. They emerge, grow, flower, set seed, and die within the frost-free growing season.
Figure 7. In this field trial, a flush of pigweed competes against broccoli planted no-till into killed hairy vetch (foreground), while broccoli planted in killed rye or rye–vetch are relatively free of pigweed (background). Rapid N mineralization from the all-legume cover crop residues apparently stimulated pigweed germination and growth. Photo credit: Mark Schonbeck, Virginia Association for Biological Farming.
Reproductive development is accelerated by shortening daylength after the summer solstice in field populations (Keeley et al., 1987), and proceeds faster in short (
Because the small seeds have minimal nutrient reserves, pigweed seedlings are initially more dependent on readily available nutrients from the soil, especially phosphorus (P) and potassium (K), than larger-seeded plants such as corn, beans, and cucurbits (Hoveland et al., 1976; Mohler, 1996). However, in studies conducted on organic (muck) soils in Florida, smooth pigweed and spiny amaranth were less responsive than lettuce to P levels, and a band application of P fertilizer improved the crop's ability to compete against these weeds (Santos et al., 1997; Shrefler et al., 1994).