Control of Palmer Amaranth

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Palmer amaranth was seen in a number of locations in DE and MD eastern shore last summer. We found fields near Dover and throughout Sussex County with Palmer amaranth infestations. We talked about this plant at most of our winter meetings. It looks much like smooth or redroot pigweed early in its growth. The link below will show you some bulletins on how best to identify Palmer amaranth, but the watermark is very diagnostic. However, many plants never develop these ā€œVā€ markings on the leaves.

http://agdev.anr.udel.edu/weeklycropupdate/?p=3146

This species needs to be taken very seriously; it can overwhelm a field in a few years. It is a species that has developed resistance to glyphosate very quickly and once that happens, it will make control very difficult. (If you think glyphosate-resistant marestail has been a headache; Palmer amaranth is much worse.)

Palmer amaranth grows very rapidly, which means you have only a few days to make postemergence herbicide decisions and get the field treated. Without effective control, Palmer amaranth will grow 5 to 6 feet tall. If you know you have Palmer amaranth, or you suspect you might have it, do not rely on glyphosate alone for postemergence control. Options for corn include HPPD-inhibiting herbicides (Group 27, Callisto, Impact, or Laudis); ALS-inhibiting herbicides (Group 2, Resolve, Steadfast, Permit Plus, Capreno, plus many others); and plant growth regulators (Group 4, such as Status).

Postemergence options in soybeans include PPO-inhibiting herbicide (Group 14, Reflex) and ALS-inhibiting herbicides (Group 2, FirstRate, Pursuit, Classic, etc).

We had a number of reports of poor performance with glyphosate last year for Palmer amaranth control, and so tankmixes will be essential for resistance management. Most of these postemergence options need to be applied to Palmer amaranth before Palmer amaranth plants are 4 inches tall.

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