By Mark Loux, Extension Weed Specialist

The frequency of Palmer amaranth infestations in Ohio seems to be holding relatively steady again into this year with the exception of one county, as far as we know.  

In two areas where Palmer amaranth was most prevalent — far southern Scioto County and an area along the Madison-Fayette County line north of Jeffersonville — Palmer has not increased its footprint. Palmer has been found in several new fields in Mahoning County, however, and the discovery occurred too late to implement effective POST herbicide treatments.  

Waterhemp infestations, or at least the discovery of them, have been increasing with no real geographic pattern to their locations. It can be found almost anywhere in the state but the highest concentration is in west central Ohio. Essentially all of the Palmer amaranth in the state is resistant to glyphosate and ALS inhibitors.  

Resistance characteristics of waterhemp populations are more variable. They are all resistant to ALS inhibitors, but resistance to glyphosate is more likely to occur in the western half of the state. We have also characterized some populations in western Ohio as having some level of resistance to PPO inhibitors.

Neither one of these weeds is a picnic to deal with, and the trend across the country is for them to develop resistance to any new herbicide sites of action that are used in POST treatments. So preventing new infestations of them should be of high priority for Ohio growers.  

When not adequately controlled, Palmer amaranth in Ohio can take over a field faster than any other annual weed we deal with. Taking the time to remove any Palmer and waterhemp plants from fields now will go a long way toward maintaining the profitability of farm operations.  

There is information on Palmer amaranth and waterhemp identification on most university websites, including ours — u.osu.edu/osuweeds/ (go to “weeds” and then “Palmer amaranth”). The dead giveaway for Palmer amaranth as we move into late summer is the long seedhead, 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 into late summer to scout fields, even if it’s 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 your own personal 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.