Palmer amaranth, sometimes referred to as carelessweed, is a summer annual broadleaf weed species closely related to other pigweed species such as waterhemp, smooth and redroot. It evolved in deserts of the southwestern United States, including areas of the Sonoran Desert, but because of its adaptability, has now colonized very different agricultural landscapes across the United States. As an example, it’s been found in Illinois, and Aaron Hager, University of Illinois Extension weed specialist says that confirmed reports indicate it will cut soybean yields by almost 80%, and corn yields by over 90%. Adam Davis, USDA plant ecologist says there are few barriers to the establishment of Palmer amaranth and they’re competitive with crop species. While his observation follows Illinois research, his conclusion could be applied to nearly every state:
…it’s not a question of if Palmer amaranth will become established in Illinois, but rather when and where it will become established…
Palmer amaranth plants are either male or female, and male plants produce only pollen while female plants produce only seed. Aaron Hager’s colleagues at Purdue, Bill Johnson and Travis Legleiter, have produced an 11-page guide to this weed which they say will produce 100,000 seeds per plant when competing with a crop, and a half million seeds per plant when not in competition with other plants. The seeds are spread by birds, furry-footed wild animals, dirt clods on tillage equipment, and particularly combines. Farmers who operate across wide expansive areas are more likely to quickly spread the seed than those who work close to home.
Iowa State University weed specialists Bob Hartzler and Mike Owen says Palmer amaranth is just moving into Iowa. They stress the importance of properly identifying this weed. If you think you may have discovered Palmer amaranth, you can verify its identity by testing a leaf tissue sample. Hartzler and Meaghan Bryan also offer a guide which helps identify Palmer from other varieties of amaranth.
So how should this weed be controlled? Experts say that management programs that rely on a single mode of action will typically be ineffective. Hager and other specialists recommend some specific precautions to take where the plant exists:
- Fields with Palmer amaranth populations should be the last fields harvested in the fall and the last fields planted in the spring.
- Mark or flag areas where Palmer amaranth plants have produced seed. These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production.
- Do not mechanically harvest mature Palmer amaranth plants with crop harvesting equipment. Physically remove the plants immediately prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants in the field. Bury or burn the bags in a burn barrel as soon as possible.
- Fields in which Palmer amaranth seeds were produced should NOT be tilled during the fall or following spring. Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.
The University of Illinois weed science program has developed recommendations for management of Palmer amaranth which hold three general principles:
- Prevention is preferable to eradication.
- It is not uncommon for annual herbicide costs to at least double once Palmer amaranth becomes established.
- Control of Palmer amaranth should not be less than 100%
Hager sums things up by saying the threshold for this species is zero. Because of the amount of seeds produced by the female Palmer amaranth, in less than five years a few surviving plants could completely shift the weed spectrum in any specific field.
Have you spotted any carelessweed in your fields? What steps are you taking to control this invasive and extremely competitive species?
-Terry Olson, Titan Outlet Store Team
Midwest Ag Journal (an edition of High Plains Journal) 2014 Spring Planting Issue