Palmer Amaranth Again in the News
Posted: May 13, 2014
Both Iowa and Ohio State Universities published articles this past week on how early Palmer amaranth was emerging in their respective states.Unlike the other pigweeds which we generally consider later emerging, Palmer amaranth appears to be off and running this time of year and of course this is also a weed that will germinate and emerge over a long period of time.
Remember that we identified Palmer amaranth last fall in seven or eight locations mostly in the southeast part of the state. In a greenhouse assay that we conducted this past winter on some young plants from seeds collected from some PA infestations, all five populations tested were glyphosate resistant and at least four were also ALS-resistant. Of course these seeds were collected from fields where glyphosate failed, so resistance should not be a surprise. Assuming we are often dealing with both glyphosate and ALS-resistance, a number of herbicides can provide some control of Palmer amaranth and particularly in corn. These include atrazine (Group 5), Group 15 (Dual, Harness, Outlook, Zidua, etc.), Group 27 (Balance, Callisto, Impact and Armezon, etc.), and Group 4 herbicides (2,4-D, dicamba, etc.). In soybean, effective herbicides are in shorter supply, but include the Group 14 (Authority, Valor, Reflex, etc.), Group 5 (metribuzin), Group 3 (Prowl), and of course the Group 15 herbicides (Dual, Outlook, Warrant, Zidua, etc.). Liberty (Group 10) is also active on Palmer amaranth in Liberty link crops.
As you scout fields over the next few weeks, keep a special eye out for Palmer amaranth. As young plants, the long leaf petioles may be the best indicator that you are dealing with Palmer amaranth rather than one of the other pigweeds. Here are some key characteristics to watch for:
- Palmer amaranth plants look similar to other pigweeds and especially as seedlings.
- Palmer leaves, stems, and petioles are hairless and petioles are usually longer than the leaf blade.
- Sometimes, Palmer amaranth leaves will also have a “V” mark or dark red/purple patch (watermark) on the leaf blade (spiny as well as the other pigweeds can also sometimes have this mark).
Here are some additional resources to help you identify and manage Palmer amaranth.