By Mark Glady
Palmer amaranth, a summer annual broadleaf weed primarily found in the southern half of the U.S., is unfortunately traveling north and has been confirmed in six Minnesota counties.
With its large stature and the thousands of seeds per plant it produces, Palmer is extremely aggressive and easily chokes out crops, particularly soybeans.
The key to controlling it? Get it when it’s small. Or better yet, keep it from emerging at all.
Here are some tips on how to keep Palmer from becoming a problem in the first place and what to do if you discover it in your fields.
1. Identify it. The challenge with identifying Palmer is that it looks similar to waterhemp and redroot pigweed, which are more common in Minnesota. Here’s how you can tell the difference: In a mature Palmer plant, the petiole is longer than the leaf. If you bend the leaf where it attaches to the petiole, the leaf does not touch the stem. The disadvantage of this identification method is that you need a weed that’s already at least 6 to 12 inches tall to do it.
2. Manage it. Two words: Preemergence herbicide. With late planting in spring 2019, not a lot of preemergence herbicide got applied because farmers were focused on just getting crops planted. This year, we have to rely on in-season residual herbicides with multiple modes of action for control. Farmers and agronomists will need to keep a close eye on fields and eliminate any Palmer that has emerged with contact herbicides when it’s less than 1 inch tall.
3. Get rid of it. If you have relatively tall Palmer growing in your fields, you can dig the plant up or chop it down. However, be careful. If you cut the stalks an inch above the soil, they could still sprout branches that produce seed heads. Don’t leave cut or pulled Palmer in the field. Remove it, contain the seeds and then burn it.
4. Help prevent it from emerging. Starting in spring 2020, use a preemergence herbicide at planting plus a second or even a third layer of residual herbicide to help prevent Palmer from emerging in-season. You can do this for a couple of years in a row, or plant continuous corn (or wheat or a rotation crop), which has more herbicide options. There are many more herbicide options to control Palmer in corn than there are in soybeans. If you can control Palmer for three to five years, many of those seeds will no longer be viable and you might be able to keep it out of your fields.
5. Call on the experts. If you see Palmer in your fields, ask your agronomist to come out and provide a second opinion on identification and to document the situation. He or she can help you find other resources or experts to report the problem to and determine if further consultation is needed with University of Minnesota Extension or the Minnesota Department of Agriculture.
6. Strive for prevention. Preventing Palmer from emerging is our optimal outcome. That happens when we’re diligent about applying a preemergence herbicide at planting and residual herbicides with multiple modes of action in-season. Once Palmer emerges it’s a toss-up as to whether we can manage it or not.
Consult your agronomist about a herbicide plan that includes not only Palmer but other more common Minnesota weeds such as waterhemp. As with any weed-control regimen, diligent scouting and persistence are key.
Mark Glady is a regional agronomist with WinField United in west-central Minnesota. Contact him at firstname.lastname@example.org.