Feds warn of possible giant pigweed outbreak

An invasive species is on the move and may be headed for Montana. Palmer amaranth, a giant pigweed, is known to have spread to at least 28 states, including Minnesota and South Dakota, but has not yet been reported in Montana. To prevent its spread into Montana, landowners are encouraged to check their fields to ensure the invasive weed is not present.

Palmer amaranth is spreading through contaminated seed, hay and feed purchases, and custom combining or other mobile farm equipment. It is native to the desert Southwest and northern Mexico and has spread throughout the southern, eastern and midwestern parts of United States. It was recently found as a contaminant in conservation plantings in Illinois, Indiana, Iowa, Minnesota and Ohio but is listed as a noxious weed only in Delaware, Minnesota and Ohio.

It was a known contaminant in Conservation Reserve Program (CRP) seed mixes but pollinator, wildlife habitat and cover crop plantings may also been contaminated. Producers with recent conservation plantings should check their fields to ensure this invasive weed is not present.

Identification

Palmer amaranth (Amaranthus palmeri S. Watson) is one of number of pigweeds that are problem weeds in crops throughout the United States. Pigweeds are warm-season annuals, grow quickly and aggressively, compete well with crops, reproduce by seed, are frost sensitive and have a high percentage of hard seed. It has caused severe yield losses up to 91 percent in corn and 79 percent in soybeans.

Palmer amaranth can grow to a height of 1 to 8 feet and has one reddish central stem that is smooth, relatively hairless, with many lateral branches. In comparison, redroot pigweed seedlings have pubescent or hairy stems and leaves. Palmer amaranth leaves are alternate and grow symmetrically around the stem, giving it a poinsettia appearance when viewed from above. Leaves are hairless, lance to diamond-shaped, 2 to 8 inches long and one half to 2.5 inches wide, with a prominent whitish vein on the leaf underside. The leaf petiole (stalk) is longer than the leaf itself.

Seed heads on female Palmer amaranth plants can reach 3 feet long and have stiff, sharp bracts, giving them a prickly feel. It is a prolific seed producer, producing 100,000 to 500,000 small brown-black seeds that remain viable for up to five years.

Impacts

Palmer amaranth is aggressive, growing 2 to 3 inches per day, and has the potential to become a major agronomic problem in western states. In the Midwest, it emerges from May through September, forcing producers to manage it throughout the year. It can hybridize with other pigweeds, and its reproductive habits allows it to readily adapt to new environments and develop resistance to herbicides. It is already resistant to glyphosate and other commonly used crop herbicides.

Anyone who used seed that may have originated from the South or Midwest should inspect and monitor plantings and destroy Palmer amaranth before seed set. This is especially true if the planting will be grown to maturity for fall grazing, pollinator or wildlife habitat enhancement.

Montana Considerations

Federal Seed Act law requires that all agricultural and vegetable seed sold in the United States has a label that includes: name and address of the seed labeler, lot number, germination rate and date, origin, percent of each component, percent weed seed and percent noxious weeds.

However, seed labels list only those species that are considered noxious in the state where the seed is shipped from or sold. Palmer amaranth is not on Montana’s noxious weed list, so it would not be listed on seed labels. Seed that contains species on the Montana noxious weed list or seed that contains more than 2 percent weed seed cannot legally be sold in Montana. The “Laboratory Report of Analysis” from a seed lab lists noxious weed seeds, other crops seeds and weed seeds by species and number of seeds per pound. It should be available from vendors and gives information on all contaminant species in a mix.

 Since Palmer amaranth seed is visually indistinguishable from other Amaranthus species, if any pigweed seed is found, it will be listed only as Amaranth sp. A new DNA test recognized by the Minnesota Department of Agriculture differentiates Palmer amaranth from other amaranth species but is currently only available on a limited basis.

Options for Preventing Spread of Palmer Amaranth

• Use seed from reliable and trusted sources; purchase certified seed if possible since it is field inspected for weeds and other contaminates.
• Read seed label carefully prior to purchase; check percent pure seed, inert matter, other crop, weed seed, test date, germination, hard or dormant seed, pure live seed (PLS) and origin.
• Ask for the “Laboratory Report of Analysis” for your mix, or all individual species in the mix. Check species and amount (number of seeds/pound) of other crop seeds, weed seeds and noxious weed seeds.
• If dealer won’t provide label or "Lab Report of Analysis,” consider other vendors or obtain analysis for all individual species and mix seed yourself.
• Sample the purchased seed prior to planting; send in a sample and request a "Noxious Weed Only Seed Analysis" that includes amaranth species. Consult with the seed lab for the appropriate test, since prices vary based on noxious weed species.
• If Amaranth species are listed under weed seeds, consider using a different species or a different vendor.
• If mixes have been or will be planted, walk fields to ensure Palmer amaranth or other noxious weeds are not present. Use seedling and mature plant keys to identify species.
• Since Palmer amaranth is resistant to many common herbicides, pulling the entire plant prior to seed set is effective. Place the plant in a plastic bag while in the field, and then ferment, burn, or dispose of properly.
• Scout fields for several years and use appropriate weed control to ensure Palmer amaranth or other noxious weeds are not introduced into Montana.