NDSU Extension - Griggs County

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November 27

Extension Connection Column by Jeff Stachler

Waterhemp

Good day!  I hope things are going well for you.  Since harvest is completed, I’m no longer driving the county, so I have no field report.

Central Dakota Ag Day will be happening again this year, but in a much different format due to COVID-19.  Prerecorded presentations will be available on November 19, 2020 at the Carrington Research Center website.  There will be 35 presentations available in the following categories:  Crop Pest Management; Soil Management; Precision Agriculture; SARE; Ag Economics and Marketing; and Livestock.  Then on December 3, 2020 there will be a live panel discussion using zoom.  This will give you an opportunity to ask any questions or delve deeper into the presentation with each speaker.  To gain access to the presentations and the Panel Discussion register for the meeting at www.ag.ndsu.edu/carringtonrec .

Waterhemp is becoming a very large weed problem throughout the United States and is present and is increasing in Griggs County.  Last fall I observed waterhemp in 13% of soybean fields at harvest time.  This is not a good situation as weed control will become more difficult and expensive.

Waterhemp (Amaranthus tuberculatus) is now considered to be one species.  At one time there were two species, common and tall waterhemp.  Waterhemp belongs to the Amaranthaceae, Amaranthus, or pigweed family.

Waterhemp can be identified by the lack of hair on the stem and petioles (structure that attaches the leaf blade to the stem) compared to the hairy stem and petiole of redroot pigweed and Powell amaranth.  Another pigweed species, called Palmer amaranth also lacks the hair on the stem and petiole.  The leaves of waterhemp are longer and narrower and shinier or glossier than all other pigweed species.  The leaf shape is considered lanceolate.  In the young seedling stage waterhemp can be identified by its smaller and more egg-shaped cotyledon compared to the other pigweed species.  Another identifying characteristic is that waterhemp is dioecious meaning it has male and female plants.  So only female plants have seeds while our common pigweed species have seeds on all plants because they are considered monoecious, like corn.

Since waterhemp is dioecious it has tremendous genetic diversity making it a good weed.

A single waterhemp plant can produce up to 5 million seeds, but this is not common.  Waterhemp most commonly produces between 100,000 and 500,000 seeds per plant, which is still way too many.  The seeds of waterhemp are smaller than all other pigweed species. 

Waterhemp can reach a height of up to 10 feet, but usually is around five to six feet tall.

Waterhemp begins emerging in early May and continues to emerge into August which presents issues with maintaining waterhemp-free fields at harvest time.

Waterhemp seeds remain viable in the soil for at least 4 years, but likely longer, however the seed longevity is shorter compared to most weed species.

Waterhemp is a moderate competitor, possibly reducing corn and soybean yields by 15 and 44%, respectively.

The biggest problem with waterhemp is that it is very prone to resistance to herbicides making it difficult to manage with herbicides.  In Illinois, waterhemp plants can withstand ALS-inhibiting herbicides like Pursuit (herbicide group 2), growth regulator herbicides like 2,4-D (herbicide group 4), atrazine (herbicide group 5), glyphosate (herbicide group 9), PPO-inhibiting herbicides like Flexstar (herbicide group 14), and very long chain fatty acid herbicides like acetochlor (herbicide group 15) and HPPD-inhibiting herbicides like Callisto (herbicide group 27).  In North Dakota, waterhemp can be resistant to herbicide groups 2 and 9, but may be resistant to other herbicide sites of action.

To manage waterhemp apply preemergence followed by postemergence herbicides that have residual activity.  Only the following herbicides provide excellent waterhemp control:  Acuron; Acuron Flexi; Corvus; Verdict at 10 to 18 fluid ounces per acre; Armezon/Pro plus atrazine; Callisto Xtra; Capreno plus atrazine; Huskie; Huskie Complete; Impact plus atrazine; Laudis plus atrazine; Lumax EZ at 3 pints per acre; Marvel; Paraquat; and Talinor.

In summary, know how to properly identify waterhemp and eliminate it any way possible including removing individual plants by hand!  The only way to stop waterhemp is to stop if from producing viable seed.

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