Containment, diligence needed in Palmer amaranth control

AFP Correspondent

PAINTER, Va. (Aug. 25, 2015) — There’s a new weed in town and she’s ba-a-a-ad.
A single female Palmer amaranth (Amaranthus palmeri) plant can produce a million seeds in one season says Dr. Charlie Cahoon, assistant professor and weed Extension specialist at Virginia Tech’s Eastern Shore Agricultural Research and Extension Center.
The weed, which has been honored more than once with the dubious title “weed of the year,” is an aggressive, invasive pigweed common in the South that is now on Delmarva. Some research in the South has found the weed can cause a 78 percent yield loss in soybeans and 91 percent loss in corn.
“This is the time to do something before it gets out of hand,” Dr. Cahoon urges. “We have two to three years before it becomes a huge problem. There has to be zero tolerance for seed production.”
The weed, which has separate male and female plants, grows extremely rapidly and aggressively in hot climates.  With adequate moisture, it can grow two inches a day and reach 6 to 7 feet in height. It is not deterred by temperatures up to 108 degrees.
Identifying the plant is not difficult. Dr. Cahoon explains, “It is similar to redroot pigweed, but has long petioles – the area between the leaf and the stem – that are at least half the length of the leaf.
“The leaves have a sheen like a clear coat was applied to it,” he adds. Redroot pigweed leaves are not shiny.
“It is widely adapted and can grow anywhere,” Dr. Cahoon says, adding that control can be difficult because the weed is glysophate- and ALS-resistant. He says PPO inhibitors and Liberty can be used, but cautions that the plants will develop resistance to these herbicides over time as well.
Liberty can be used on beans, some corn and, in Virginia, on cotton. PPO inhibitors can be used in soybeans.
Dr. Cahoon says “corn is good to use in rotation because you can use Atrazine, which still has activity against Palmer amaranth.” If the weed emerges following corn harvest, he recommends Gramoxone to kill it. 
Weeds that survive chemical treatment should be pulled or cut before flowering to prevent seed formation. If flower heads have formed, the plants should be severed or uprooted and removed from the field. 
Many seeds will stay in the head until winter so removing the weeds in early fall can reduce weed populations.
Dr. Cahoon stresses the need to pay serious attention to this weed.  “It is very competitive with our crops and can cause very high yield losses, even in low density.”
Good cultural practices are a must. “Farmers must practice containment in fields that have it so they don’t transport the seed in their equipment,” Dr. Cahoon says.