What's Palmer amaranth?
News that a noxious weed, Palmer amaranth, has invaded Douglas County raises questions.
Many people haven't heard of this weed, which has now been confirmed in Douglas, Todd, Lyon and Yellow Medicine counties. What does it look like? Why is it bad? What's being done to stop it?
Here are some questions on the topic, with answers provided by the Minnesota Department of Agriculture.
What does Palmer amaranth look like?
It is a summer annual that commonly reaches heights of 6 to 8 feet but can reach 10 feet or more. The green leaves are smooth and arranged in an alternate pattern that grows symmetrically around the stem. The leaves are oval to diamond-shaped. There is a small, sharp spine at the leaf tip.
The leaves of some Palmer amaranth plants have a whitish V-shaped mark on them. Not all Palmer amaranth plants display this characteristic.
There are separate male and female plants.
Palmer amaranth looks similar to native pigweeds such as tall waterhemp, redroot and smooth pigweeds.
Why is the weed considered noxious?
Palmer amaranth competes aggressively with crops. It has a fast growth rate of 2 to 3 inches per day and greatly inhibits crop growth. Yield losses have been up to 91 percent in corn and 79 percent in soybean.
Palmer amaranth can be also be toxic to livestock if the concentration of nitrates in the leaves is high.
Where was it found in Douglas County?
The Douglas County infestation was found through the Minnesota Department of Agriculture's investigation into a Palmer amaranth find in Todd County.
The location of the field isn't being disclosed by the ag department because the source of the weed seed is still being investigated.
The field had been mowed before the ag department visited the site, so there were no standing plants on it. Ag workers found parts of the plant and there were enough identifiable pieces to send to a lab for confirmation.
Are there any laws to prevent the spread of Palmer amaranth?
Yes; all above and below ground parts of the plant must be destroyed. Additionally, no transportation, propagation, or sale of these plants is allowed. Failure to comply may result in enforcement action by the county or local municipality.
How did the weed get to Minnesota?
Palmer amaranth is an annual plant native to the arid southwestern United States and northwestern Mexico. It is a traditional food of Native Americans including the Navajo, Pima, Yuma and Mohave.
Experts believe the seed was accidentally moved to the southeastern U.S., perhaps with cotton meal, and it continues to spread.
Here's a timeline of its history:
1915 — First reported in Virginia but was not considered problematic for some time.
1989 — Documented in a weed survey in South Carolina.
1995 — Ranked the most troublesome weed in cotton in the Carolinas.
2009 — Ranked the most troublesome weed in cotton in the southern U.S.
How does it spread?
Seed is the means of spread and female plants are prolific seed producers. A study in Missouri documented more than 250,000 seeds produced per plant. Seed can be spread in water movement, by wildlife and via agricultural practices such as plowing, harvesting and spreading manure.
Palmer amaranth is documented in 28 states including Minnesota and neighboring South Dakota, Iowa and Wisconsin.
What can be done to prevent the spread?
Palmer amaranth developed resistance to multiple classes of herbicides and their different modes of action making it challenging to control. Agricultural experts recommend the following:
• Be proactive and prevent Palmer amaranth establishment. Familiarize yourself with Palmer amaranth identification and actively look for it in crop fields, borders, ditches and around dairies.
• Mowing alone is not as effective as cultivation, as Palmer amaranth plants are usually not killed by mowing. They can regrow from cut stalks and set seed close to the ground. Mowing must therefore be done in conjunction with other methods of control, like herbicide application, to be effective.
• Prevent all Palmer amaranth plants from producing seed if possible. Plants can be hand weeded and removed for disposal. If hand weeding is not feasible, contact your local University of Minnesota Extension agent, co-op, or certified landscape care expert for a specific herbicide recommendation.
• Always clean vehicles and equipment after exiting infested areas. If seed was produced, deep tillage will reduce the quantity of seeds that can readily germinate.
• A cereal rye cover crop can reduce Palmer amaranth germination and growth.