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Waterhemp Is Here, Is Palmer amaranth Next?

Posted by Albert Adjesiwor, University of Idaho | July 7, 2022

When I took my current job at the University of Idaho about two years ago, one of my fears was that Palmer amaranth, one of the most troublesome weeds in agricultural production systems will be introduced to the PNW. I have looked at the map of Palmer amaranth distribution in the United States (Figure 1), a number of times and still couldn’t believe that we don’t have it here “yet”.

Palmer amaranth in US.


Figure 1. The current distribution of Palmer amaranth in the United States

During the 2022 Snake River Weed Management Tour at the University of Idaho Kimberly Research and Extension Center, we had Palmer amaranth, waterhemp, and redroot pigweed on display to raise awareness among stakeholders (Figure 2). At first glance, these weeds look very similar, especially when they are small. You are right to think so! These weeds do look very similar in a lot of ways. However, there are some subtle differences that can help identify these weeds. Please, see PNW 758 Pigweeds: Current and Emerging Weed Threats in the Pacific Northwest for details.

Palmer amaranth, water hemp, and redroot pigweed

Figure 2. Palmer amaranth, waterhemp, and redroot pigweed demonstration


Figure 3. Waterhemp collected from a farm in Elmore County, Idaho. This population of waterhemp survived two-time application of glyphosate.

Less than a week after the tour, I received a panic call from an Agronomist who suspected he might have waterhemp in a sugar beet field in Elmore County, Idaho. That was not even the scary part! The waterhemp population survived a 2-time application of glyphosate. This is very concerning because glyphosate is the main (and only) herbicide that provides broad-spectrum weed control in sugar beet. Photos and living plants sampled from the field confirmed that the pigweed was indeed waterhemp (Figure 3).

The only recommendation I was able to provide was to get a hand crew out there and do everything possible to prevent these plants from going to seed. These weeds are currently being grown in the greenhouse with the hope that we can get some seeds to conduct a broader screen for herbicide resistance and provide management recommendations (Figure 4).

waterhemp in greenhouse.

Figure 4. Waterhemp being grown in the greenhouse at the University of Idaho Kimberly Research and Extension Center for herbicide resistance screening.

You might be wondering, why are we so concerned about these pigweeds when we have other troublesome weeds like kochia, Italian ryegrass, common lambsquarters, and the like? Well, for one, having one or two additional weeds means one more thing to worry about. The main reason these weeds are so concerning is that they have developed resistance to multiple herbicides labeled for use in crops in our region. For crops like dry bean and sugar beet that have limited herbicides for weed control, this becomes very concerning.  To provide some perspective, from the last time I checked the number of cases of herbicide resistance in Palmer amaranth and waterhemp is about three times that of redroot pigweed, the most common pigweed in our region (Figure 5). These resistant cases in Palmer amaranth and waterhemp encompass multiple herbicide sites of action including group 2 {e.g., metsulfuron-methyl (Osprey)}, group 3 {e.g., pendimethalin (Prowl H2O)}, group 4 {e.g., 2,4-D}, group 5 {e.g., metribuzin (TriCor 4F)}, group 9 {glyphosate (Roundup PowerMax)}, group 10 {glufosinate-ammonium (Liberty 280 SL)}, group 14 {e.g., sulfentrazone (Spartan 4F)}, group 15 {e.g., dimethenamid-P (Outlook)}, group 27 {e.g., mesotrione (Callisto)}. Although it is not likely that any population of Palmer amaranth and waterhemp introduced to our region will be resistant to all these herbicide groups, it safe to assume that it might be resistant to a couple of these herbicide groups.

Figure 5. Cumulative cases of herbicide-resistant redroot pigweed, waterhemp, and Palmer amaranth in the United States. Source: Heap (2021). The International Herbicide-Resistant Weed Database. Accessed on 1 March 2021.

All our efforts in proactive resistance management would mean nothing if we cannot prevent the introduction of herbicide-resistant weeds from other regions. There are a few things we can do to keep these weeds out of our region or keep them contained:

  • Livestock feed. The seeds of Palmer amaranth are very small and difficult to detect or remove from other crop seeds (Figure 2). They can be introduced to the PNW through contaminated hay or cottonseed meal purchased and transported from infested states.
  • Manure. Before purchasing manure to use as fertilizer, note its source. If you do not know or suspect that the manure might be contaminated with Palmer amaranth seed, it is good practice to spread a small amount of the manure at the edge of your field or other areas where it will be used to see if any Palmer amaranth, waterhemp or other weeds not common to that location emerge from the manure. If you detect Palmer amaranth, do not spread the manure. Call your county Extension office for advice on how to proceed.
  • Rights-of-way. If your farm or property abuts a main highway or interstate, scout the area periodically for any signs of Palmer amaranth.
  • Farm equipment. When transporting farm equipment to your field or moving equipment from field to field or purchasing used equipment, inspect it to ensure that it is thoroughly clean and free of soil, debris, and weed seeds.
  • Seed (cover crop, bird feed, and restoration). Seed mixes used as cover crops, restoration seed, pollination habit improvement, or birdfeed can be sources of these pigweeds. Be careful when purchasing these seeds and always be on the lookout.

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