Farm Progress

Cropping Systems: If you can identify Palmer amaranth, you’re one step closer to controlling it in your fields.

July 11, 2017

5 Min Read
GROWING TALL: Female Palmer amaranth plants with tall terminal inflorescences or seed heads often reach 6 to 7 foot heights and may grow up to 10 feet tall.

By Meaghan Anderson

It’s that time of year again. Time to scout for Palmer amaranth, the troublesome Southern weed that has invaded Iowa and spread the past few years through 49 out of the state’s 99 counties. 

While Palmer amaranth made its name last year for moving into many Iowa counties with native seed that was used for conservation plantings, it also moves with traditional agriculture practices. The first detection of Palmer amaranth in 12 counties in 2013 was related to traditional agriculture practices. Equipment movement, and feed and bedding for livestock are two of the most threatening sources. Other documented sources include bird seed and bird movement from the Southern states heading north.

Get out and scout
Scouting is the only way to find and remove this weed in a timely manner, and this time of year is perfect to be out looking. While checking corn and soybean fields for weed control, foliar diseases and insect issues, be sure to keep an eye out for Palmer amaranth.

Other high-risk areas should be prioritized for scouting as well, including 2016 conservation plantings, farms in Iowa that use feed and bedding from Southern states, fields that use manure from those farms, areas with out-of-state equipment movement or high traffic, and fields near any of the above. In reality, all crop fields in Iowa should be considered “high risk” for Palmer amaranth infestations now.

Identification still tricky
Palmer amaranth is extremely difficult to differentiate from waterhemp, but Palmer does have several characteristics that would be unusual to find on waterhemp plants. Prior to flowering, look for leaves with long petioles near the base of the plant. Petioles are small stalks that attach leaves to the plant stem.

PALMER LEAF: A Palmer amaranth leaf can have a petiole longer than the leaf. Folding the leaf over at the base is the fastest way to check for this trait.

Upper leaves on Palmer amaranth are not as likely to have long petioles, but Palmer is more likely to have ovate or egg-shaped leaves than waterhemp, and leaves on Palmer amaranth are often clustered at the top of the plant. People often observe Palmer amaranth as a more leafy and dense canopied weed as well.

Once it flowers, female Palmer amaranth plants are easy to distinguish from waterhemp due to their long, sharp bracts surrounding the flowers on tall terminal inflorescences. If you discover this weed in your field, steps should be taken to remove all plants to prevent seed from “raining” on the soil surface. Eradication is possible and is the best opportunity to manage this weed if infestations are discovered early.

COMPARISON: Waterhemp’s open canopy (left) compared to Palmer amaranth’s denser, leafy canopy (right) is another way to distinguish these two weeds.

The good news
We may have a small grace period to try and eradicate Palmer amaranth before it becomes fully adapted to conditions in Iowa. If farmers take advantage of this adaption period to eradicate Palmer amaranth, we may save ourselves from the disaster Southern states have already experienced.

Most significant populations of Palmer amaranth are on atypical soils in Iowa, like extremely sandy soils, and Palmer amaranth continues to germinate in these areas long into the fall. This assists in reducing population sizes as September-emerging Palmer amaranth are unlikely to produce any viable seed. Palmer amaranth will not be any easier to manage during the growing season, but these traits may allow you the opportunity to get ahead of this troublesome weed.

If you scout fields and find Palmer, take action to remove the weeds before they go to seed. If you find Palmer on Conservation Reserve Program acres or other conservation acres, get permission from your local Farm Service Agency office before applying herbicide.

Palmer amaranth should not persist in conservation plantings, as perennial plants should outcompete this annual weed once the conservation seeding is well-established. This may take several years, so management during the initial years is important to allow for good establishment of the conservation planting. Most well-established conservation plantings from last year appear to be effectively suppressing Palmer this year. However, plantings with open gaps in the canopy will allow more opportunity for Palmer amaranth to survive and reproduce. 

To mow or not to mow?
Working closely with your local USDA Service Center is important to determine appropriate management strategies during this second year. Mowing during the second year of a prairie planting is a technique to prevent heavy weed pressure from suppressing prairie forbs and grasses that are establishing. I’ve scouted several well-established conservation plantings with known Palmer amaranth infestations and have suggested avoiding mowing unless absolutely necessary for establishment of the prairie plants, as mowing will open up the canopy and increase the likelihood of Palmer amaranth establishment.


FLOWERS: A female Palmer amaranth flower has long, sharp bracts on the tall inflorescences, differing from a waterhemp flower, which has small bracts.

Iowa State University has publications and online resources available to help manage Palmer. “Herbicide options for Palmer amaranth in CRP,” CROP 3137, is one such publication; “Palmer amaranth identification,” CROP 3105, is another. Also, ISU Extension weed scientist Bob Hartzler has a 10-minute video online discussing Palmer, explaining ways to detect it early and steps to take to eradicate the weed.

Palmer workshop Aug. 3
For those who advise farmers and others on establishing conservation plantings and managing weeds in conservation plantings, Iowa State University will host a “Palmer amaranth management in conservation plantings” workshop at the ISU Field Extension Education Laboratory from 8:50 a.m. to noon on Aug. 3. The FEEL lab is located near Boone, just west of Ames.

This workshop will provide attendees with best management practices for seeding and establishing natives, weed management in conservation plantings, and a crash course in Palmer amaranth identification. The $25 registration fee includes refreshments and class materials. Go online for more information or to register for the event.

Meaghan Anderson is an ISU Extension field agronomist based in Iowa City, covering east central Iowa. Contact her at [email protected].


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