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Time to scout for palmer amaranth

The frequency of Palmer amaranth infestations in Ohio has been holding relatively steady again into this year. We have mostly an isolated field or patch in about 10 counties, with the exception of two small epicenters of Palmer amaranth — far southern Scioto County and an area along the Madison-Fayette County line north of Jeffersonville.

Several new infestations of Palmer amaranth in soybeans have been reported over the past several weeks though. It was also found in a first-year hayfield, where cutting and competition from the alfalfa/grass stand will likely keep it under control in coming years. None of these appear to have produced viable seed yet. This is typical based on our Palmer seed collection over the past several years, and it may be due to the early-season control provided by preemergence soybean herbicides. This is a good thing, since it provides a window to remove plants from fields before viable seed are produced.

Palmer amaranth in Fayette County.
Palmer amaranth in Fayette County.

The number one recommendation for managing Palmer amaranth in Ohio is to prevent it from getting established, because it can take over a field faster than any other annual weed we deal with. Taking the time to remove any Palmer plants from fields now will go a long way toward maintaining the profitability of farm operations (same goes for waterhemp really, which is no picnic to manage either). There is information on Palmer amaranth identification on most university websites, including ours:

http://agcrops.osu.edu/specialists/weeds and http://u.osu.edu/osuweeds/ . Or Google “Palmer amaranth” and then use the “images” link. The dead giveaway at this time of the year is the long seedheads, and those on female seed-bearing plants are extremely rough to the touch. We recommend the following as we progress from now through crop harvest:

▪   Take some time now to scout fields, even if it’s a from the road or field edge with a pair of binoculars. This would be a good time to have a friend with a drone that provides real-time video, or a satellite. Scouting from the road is applicable mostly to soybean fields, since corn will often hide weed infestations.

▪   Walk into the field to check out any weeds that could be Palmer amaranth or are otherwise mysterious. If you need help with identification, send photos to us or pull plants and take them to someone who can identify them.

▪   Where the presence of Palmer amaranth is confirmed, check to see whether plants have mature seed (the plants with the rough seedheads), by shaking/crushing parts of the seedhead into your hand or other surface that will provide contrast. Mature seed will be small and very dark.

▪   Plants without mature seed should be cut off just below the soil surface, and ideally removed from the field and burned or composted. Plants with mature seed should be cut off and bagged and removed from the field, or removed via any other method that prevents seed dispersal through the field.

▪   If the Palmer amaranth population is too dense to remove from the field, some decisions need to be made about whether or how to harvest. Harvesting through patches or infested fields will result in further spread throughout the field and also contamination of the combine with Palmer amaranth seed that can then be dispersed in other fields. So consider: 1) not harvesting areas of the field infested with Palmer amaranth, and 2) harvesting the infested field(s) after all other fields have been harvested, and cleaning the combine thoroughly before further use. This also applies to any Palmer amaranth infestations that are discovered while harvesting.

▪   Scout field borders and adjacent roadsides, and also CREP/wildlife area seedings, which can be infested due to contaminated seed produced in states where Palmer amaranth is endemic and not considered noxious. Reminder – ODA will test any seed used for these purposes at no charge.

Feel free to contact OSU weed science for help with identification or management of Palmer amaranth. Mark Loux – 614-292-9081, loux.1@osu.edu.

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