Updated: September 26, 2022
By Ben Beale , and Kurt Vollmer

Thoughts on the Control of Palmer Amaranth in Organic Systems

Biology of Palmer Amaranth

Factors enabling palmer amaranth to achieve a competitive advantage and become a major weed:

  • Ability to produce a lot of seed- >200,000 per female plant.
  • Seed will germinate throughout the season from mid-April through September.
  • Very aggressive growth habit compared to most crops, especially during hot and dry conditions.
  • Small seed travels well in equipment, contaminated crop seed, animal manure, and wildlife.
  • Resistance to multiple herbicide groups.

Palmer amaranth weaknesses that can be exploited to increase control:

  • Seed is very small, and has a short viability lifespan of 3-4 years.
  • Plants do not tolerate shading, and generally emerge from within the top inch of soil.
  • Plants do not like cooler soils, preferring to germinate in late April into the summer.
  • Seeds are retained on the seed head until harvest, so removing plants from the field before harvest will prevent further spread of this weed.

Management Considerations

Prevention: Palmer amaranth is extremely hard to manage once it reaches a high population in a given field. Therefore, prevention is a key management concept.

  • Do not bring in used equipment from areas with Palmer amaranth present.
  • Be very careful with for-hire custom farm operations such as combining, where seed can be transported from one farm to the other. Be sure the for-hire custom operator is not coming from an infested area.
  • Palmer seed moves readily in dairy and beef manure, as well as contaminated hay. We do not believe it is present in conventional poultry litter from modern integrator houses due to the feed processing mechanisms, or in mushroom compost. There are several examples of Palmer amaranth being spread in manure or hay to new operations. Same is true for cotton hulls used as a feed source. Trucks should be cleaned between loads when hauling potentially contaminated products like grain and then loading with crop inputs such as lime or mushroom compost.
  • Be careful with seed source, especially for specialty pollinator or cover crop mixes from other states. Many Midwestern states do not list Palmer amaranth as a noxious weed, and it can be in the seed mix. Utilize seed suppliers with a good reputation and that offer assurance that neither Palmer amaranth nor waterhemp is not present.

Early Detection and Elimination: Even with your best efforts there is a chance that Palmer amaranth may end up on the farm. This is where early detection and elimination is vital.

  • Learn to quickly and accurately identify Palmer amaranth. If you do not know, ask some who does.
  • Scout fields early and continue scouting throughout the end of the season. Even if plants flower you will still be able to remove them before the seed is dispersed to the ground.
  • If Palmer is found, mark the location and pull, bag and remove any plants from the field. Plants left in the field will re-root!
  • If pulling is not feasible, we advise sacrificing the crop and destroying those plants before they go to seed. It is important to stress again that this year’s seeds make next year’s weeds. Palmer will not be easily put back in the bag once populations explode.
  • Monitor that area for the rest of the summer and the next crop year. It is very likely more plants will germinate in that vicinity.
  • It is very important not to move seed from one isolated field throughout the whole farm. We have seen cases of a dozen plants explode into a total farm infestation in 2-3 years. This means not harvesting, mowing, or conducting other operations and moving to another part of the farm
  • Consider the economics of harvesting areas heavily infested with Palmer amaranth, yields will be low and Palmer will spread more easily.
  • Weed free areas should harvested before areas infested with Palmer. After combining fields with Palmer equipment should cleaned before going to uninfested areas. A tutorial video for cleaning a combine can be found here: https://growiwm.org/the-straw-bale-methodology-for-cleaning-weed-seeds-out-of-a-combine.

Managing populations in Organic systems:

Utilize cover crops to suppress amaranth germination.

  • Cover crops have been proven to significantly reduce germination of Palmer amaranth seedlings, as well as allowing for the crop to grow and canopy.
  • The cover crop needs to reach critical biomass. Plant early, manage like a regular crop, and terminate as late as possible to promote growth.
  • Cover crops must remain on the soil surface throughout the growing season to suppress Palmer amaranth. Incorporating cover crops into the soil will not help with weed suppression.
  • For organic systems, the use of a cereal rye that can be crimper rolled and killed at or prior to planting will work well. There is a lot of research being conducted currently to identify the best cover crops/reduce till cultivation systems to address this issue.
  • Realize that heavy cover crops will reduce population of Palmer, but may also complicate mechanical cultivation later in the season

Consider cultural techniques (growing practices) that are less conducive to Palmer amaranth

  • Include crops like corn in rotation or earlier crops that will canopy before Palmer really gets going.
  • Use narrow rows and heavy plant populations to encourage earlier canopy closure. Once the crop has canopied, germination rate will diminish
  • Set yourself up for success and optimize the desired crop growth with proper fertility, establishment, etc.
  • Harvest seed destruction systems like the Harrington Seed Destructor and the Redekop integrated seed impact mill that greatly reduce viable seeds. Other techniques such as chaff lining, chaff baling, and narrow row burning can also help. These techniques are currently being actively researched in the mid-Atlantic area. https://growiwm.org/how-harvest-weed-seed-control.

Strategic tillage and cultivation

  • For organic growers, multiple timely mechanical cultivation may be needed.
  • There are two major challenges- Palmer will grow incredibly fast and will germinate through the season in any area that allows in light. This means cultivations need to be performed very timely before Palmer plants are too large and also performed often to catch succeeding germinations. This also means that this method will not be compatible with high residue cover crops.
  • Tillage and cultivation are effective on small seedlings (less than 3 inches tall), but as plants increase in size this method is less effective as plants will re-root and continue to grow.
  • Flame weeding or weed zapping are also options for postemergence control. However, these methods provide no residual control and multiple passes will be needed.
  • Research has shown that deep tillage with a moldboard plow at least 4-6 inches will greatly reduce viable seed over time. This method should only be utilized once every 4-5 years or you will just return seeds to the soil surface.

Crop rotation/fallowing:

  • A final option is to take heavily infested fields out of production for a period of time. If keeping the organic certification, fields could be mowed or tilled throughout the season. This will only work if Palmer is never allowed to go to seed. A perennial cover or turf could be established with close mowing as well.
  • If willing to lose the organic certification, fields could be converted to conventional crops with integrated herbicide strategy to eliminate Palmer amaranth. There are many examples of effective IWM programs with herbicides that can be implemented.

This article appears on August 2022, Volume 13, Issue 5 of the Agronomy news.

Agronomy News, August 2022, Vol. 13, Issue 5

Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. This newsletter is published once a month during the growing season and will include topics pertinent to agronomic crop production. Subscribers will receive an email with the latest edition.