What's Cropping Up? Blog

Articles from the bi-monthly Cornell Field Crops newsletter

New problem weeds in NY – waterhemp and Palmer amaranth

Bryan Brown, New York State Integrated Pest Management, Cornell University
Mike Hunter, Regional Field Crops Specialist, Cornell Cooperative Extension

Several populations of waterhemp have been found on farms in the central and western parts of our state. These populations have established over the past couple years by plants that have escaped control, likely due to resistance to certain herbicide sites of action. One waterhemp population survived several herbicides and reduced soybean yields by around 50 percent.

Additionally, Palmer amaranth was found growing near one farmer’s barn. He believes the seed arrived on some used equipment from the Midwest. The plants were collected and burned, but we’ll continue to monitor the site for future emergence.

Both of these weeds are likely resistant to glyphosate and PPO inhibitors, among others. We’re hoping to run some spray chamber trials this winter to determine their resistance to certain sites of action.

Fig. 1. Palmer amaranth leaf and leaf-stem. ID tip: if the leaf-stem is longer than the leaf, as seen here, it is Palmer amaranth, not waterhemp. Photo: W. Curran and D. Lingenfelter, Penn State

While other pigweed species have short hairs on their stems, waterhemp and Palmer amaranth have smooth stems. The best way to distinguish waterhemp and Palmer amaranth is to rip off one of the lower leaves. As shown in Figure 1, if the leaf-stem (petiole) is longer than the leaf, it’s Palmer amaranth.

Unlike other pigweeds, waterhemp and Palmer amaranth have separate male (pollen producing) and female (seed producing) plants. Herbicide resistance traits can transfer by pollen, which has allowed these weeds to develop resistance faster.

To prevent these weeds from taking hold, growers are also recommended to start weed-free with tillage, followed by a 2-pass program of residual and post-emergence herbicides that utilizes several effective sites of action. Foliar applied herbicides should be used when these weeds are less than four inches tall. Since these weeds emerge over a broader timeframe than most weeds, mid-season residual herbicide applications should be considered, along with increased planting density or tighter row spacing to help close the canopy earlier.

If you do find yourself with escapes of these weeds, it makes economic sense to go hand-rogue those weeds out of your fields rather than deal with 200,000 to one million seeds in your soil from each weed. If there are too many to bag up by hand, consider sacrificing that patch of your crop by mowing and tilling the area before the weeds produce seed. Avoid harvesting these areas. Combines are especially good at spreading weed seeds. If you must harvest these areas, know that combines can carry 150 pounds of plant material even you think it’s empty, so check out some of the great online videos on how to clean them out after going through weedy fields.

The weakness of waterhemp and Palmer amaranth is the short lifespan of their seeds in the soil. Of those that don’t germinate, very few will survive in the soil for more than four years. So, if you can keep it under control for four years, you won’t have much of it after that. But as one Pennsylvania grower put it, “the cheapest way to control Palmer amaranth is to never get it in the first place.” So, it’s important to make sure that your seed, feed, bedding, and equipment are clean from the start.

Resources:

Ohio State University has been dealing with these weeds for a while, and has a helpful listing of resources: https://u.osu.edu/osuweeds/super-weeds/palmer-amaranth/

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