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Pigweed or Water hemp?

Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Release Date:  August 11, 2014

Pigweed or Water hemp?
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (08/02/14) — This is a good time of year to take note of weeds that are still lurking around fields, and evaluate whether they point to a need for some change in weed control strategies.

A farm store agronomist brought 4 plants to the office recently, knowing that they were in the pigweed family. He wanted to know whether they were actually pigweed or waterhemp; and to be sure they were NOT Palmer amaranth. The field had been sprayed with glyphosate and these plants showed no sign of injury.

I explained that there are several species of pigweed in Minnesota and waterhemp was also part of the pigweed or amaranth family and they are able to cross pollinate. So there is great variety in the genetic mix of plants you might find in the field. It seems as we go along that Palmer amaranth will show up in Minnesota someday. I’m not sure that anyone over time will be able to be very sure about identifying something specifically as smooth pigweed, redroot pigweed, or tall waterhemp.

I sent pictures and plant samples to U of M Extension weed scientist Jeff Gunsolus. He saw the pictures first and said it was pretty safe to say waterhemp WAS part of the mix and at least some of the waterhemp WAS resistant to glyphosate products. He recommended making future weed control decisions accordingly. If there are a limited number of these plants, it would be good to pull them out and destroy them. Unfortunately by the time most of us notice this kind of a weed problem, they are usually too numerous to generate much interest in pulling them. After Jeff had a chance to see the weed samples, he advised that all 4 plants were likely to be waterhemp. 

A unique characteristic of waterhemp is that it is dioecious meaning that it has male and female flower parts on different plants. This contributes to its ability to maintain a large genetic pool - and within that pool to select for survival of offspring that are resistant to glyphosate AND other herbicides. Glyphosate products were originally known by the trade name Roundup. The glyphosate patent has expired and there other companies producing glyphosate products with other marketing names now.

Gunsolus has talked for many years about the need to make weed control decisions with the goal of reducing the risk of developing resistance to products we are using. Repeated use of products with the same “site of action” is a key factor in developing a resistant population. So using a rotation of products with different sites of action is a key practice for reducing resistance risk. This is true for other herbicides too, as well as for insecticides, fungicides, and other kinds of pesticide products.

Another important resistance management practice is to make treatment decisions based on research based thresholds. Every spray application gives the pest population a chance to select for resistant types. Routine spraying without a documented need, speeds the selection process needlessly. 

More farmers are making a choice between corn and soybeans or other glyphosate crops as the most important place to use glyphosate. They pick other options then for other crops. This can also be important for managing other pests. Volunteer glyphosate tolerant corn in a soybean crop, helps to maintain corn rootworm populations in the field. Volunteer glyphosate tolerant soybeans, as a weed in a soybean crop, helps to maintain soybean cyst nematode populations.  

You don’t have to wait for resistance issues to show up on your farm before you make strategic changes in with weed or other pest control practices. It’s usually better to put a lock on the door before the horse is stolen. That seems a little contrary to our human nature sometimes. How does it go, an ounce of prevention…? 


Daniel Martens
Extension Educator, Ag Production Systems
(320) 968-5077
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