Last fall, American Agriculturist warned you that herbicide-resistant Palmer amaranth was suspected in Pennsylvania and Delaware. The "superweed" had already been confirmed in eastern Ohio, Virginia, most of the South and now much of the Midwest.
This pigweed-on-steroids has now been confirmed in Delaware and on at least seven farms in Pennsylvania, reports Bill Curran, Extension weed specialist at Penn State University. While it can't leap tall buildings in a single bound, it can leap to 10-foot heights almost overnight and toss 500,000 tiny hard-to-contain seeds into the wind from a single head.
That's why it's the "hot topic" at this winter's crop management meetings, according to Curran. Herbicide-resistant or not, containing new infestations and preventing its spread is a critical first step to managing this new threat. And that's an understatement.
Why it's a huge threat
Extremely small seed size, makes the Palmer pigweed nearly impossible to completely eliminate with standard seed-cleaning procedures. Because of their small size, seeds are easily transported from field to field, carried along by wind, wildlife and farm machinery – especially combines.
Unlike redroot, smooth, Powell, and spiny pigweed species, Palmer amaranth grows faster and can be either male or female. Pollen from herbicide-resistant male plants can easily travel on the wind to susceptible females, making a portion of the offspring also resistant.
Herbicide resistance breeds double-trouble with this weed. It can develop resistance to glyphosate and the Group 2 herbicides (ALS-inhibitors). Resistance to Group 3 and 5 herbicide classes has also been documented, notes Curran.
If you spot this 'pig' . . .
Early ID and aggressive management is critical. While seedlings look similar to other pigweeds, survival of glyphosate application might be your first clue.
Palmer leaves, stems and petioles are hairless, and petioles are usually longer than the leaf blade.
For close-up pictures on how to ID this baddy, click on this newly updated Penn State website. It shows shows how Palmer seedlings differ from redroot and other pigs.
Sometimes, its leaves will also have a "V" mark or dark red/purple patch (watermark) on the leaf blade.
- If you suspect Palmer amaranth, report it immediately to your local Extension office and/or to a professional crop advisor.
- Team up soil residual herbicides and postemergents. Plants that are less than 4 inches tall are easier to manage.
- With smaller infestations, physically remove plants from the field before flowering.
- For flowering plants, hand rogue smaller infestations and remove plants from within field. Then burn removed plants at field edge.
- Manage the field with no-till if possible to leave any potential seeds near the soil surface – enhancing seed predation and mortality.
- If the problem is too large to handle by hand, consult with Extension and/or a professional crop consultant on best management strategy.
USDA developing a biocide control
Scientists at USDA's Agricultural Research Service at Stoneville, Miss., have discovered a fungus that attacks Palmer amaranth. They're exploring ways to formulate the Myrothecium verrucaria fungus that causes wilt, lesions and other disease symptoms.
It can kill young plants and weaken older ones. But it's most effective on Palmer pigweeds that are 2-inches tall or less.