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Agronomic Crops Network

Ohio State University Extension


Reminders about Palmer amaranth

Palmer amaranth in soybeans.

We spent most of the winter meeting circuit trying to put the fear of (insert deity of your choice here) in everyone about Palmer amaranth.  We appear at this time to have fewer infestations of Palmer amaranth than our neighbors to the north and west, and it is our goal to keep it that way as much as possible.  Some reminders about the Palmer amaranth situation:

1.   Most of the nine infestations we know about are limited to small areas of individual fields at this point.  The exception is an area between Jeffersonville and Midway where Palmer can be found in several fields within the area.  Our assessment of that area following last fall’s preharvest scouting is that several fields will have substantial infestations this year.  If Palmer is not adequately controlled in this area in 2014, the number of infested fields will increase across a larger geographic area, and the severity of the infestations will continue to increase.  

2.  Residual herbicides are an important component of Palmer amaranth management programs, and so by extension are also important for prevention of new infestations.  They will at least control Palmer for the first part of the growing season, reducing the number of plants that have to be controlled with POST herbicides, and ultimately minimizing seed production and the risk of spread.  We have ratings of residual herbicide effectiveness on Palmer in the current “Weed Control Guide for Ohio and Indiana”. 

3.  We have screened only a few Palmer amaranth populations for their sensitivity to herbicides.  One population in far southern Ohio is resistant to glyphosate and ALS-inhibiting (group 2) herbicides.  Several other populations had variable response to herbicides, showing resistance to glyphosate, ALS inhibitors, or Flexstar (PPO inhibitors - group 14) depending upon the population.  So the good news is that POST glyphosate applications can still possibly control some populations of palmer amaranth.  The bad news is that these populations will likely become glyphosate-resistant where management relies on glyphosate as a primary tool.

4.  Take some time now to review the Palmer amaranth identification resources that are available at our website ( Where the presence of Palmer amaranth is suspected this summer, contact us and send photos or plants to get help with identification.

5.  Where Palmer amaranth infestations are discovered in crop fields mid-season, we cannot emphasize enough the importance of removal and destruction of plants prior to the development of mature seed.  Plants should be cut off below ground, removed from the field, and burned or composted.  Remediation of established infestations of Palmer amaranth is difficult and expensive, and these steps are essential to ensure that farm profitability is not reduced by this weed. 

6.  The two sources of Palmer amaranth infestations in Ohio have been: a) contaminated cotton feed products that originate in the south, for use as animal feed here; and b) contaminated seed used for CREP and wildlife seedlings, which originated from points west such as Kansas and Texas.  So extra scouting for Palmer may be merited in fields where manure has been spread from animal operations feeding cotton feed products, and also in recent areas where cover has been established for CREP, wildlife, or similar purposes. 

7.  Seed to be used for any new CREP, wildlife area, or similar seeding can be tested by ODA for the presence of Palmer amaranth and other weed seed.  There is no charge for this testing, but the process requires an ODA representative to come to the site of seed storage and take an official sample (rather than sending seed directly to ODA).  Contact David Simmons at ODA for more information at 614-728-6410 or

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.