Palmer Pigweed and Waterhemp Becoming a Wide Spread Problem

Palmer Pigweed and Waterhemp are becoming a huge problem on farms across the Commonwealth.  Until recently, we did not have a wide infestation of these two weeds in Kentucky.  However, both are showing up more and more, and both of these invasive weeds are showing a strong resistance to glyphosate and ALS chemistries, creating a huge problem when trying to control the two.

Waterhemp and Palmer Pigweed are both members of the Amaranthus family.  Waterhemp has an egg shaped cotyledon with a slightly longer petiole than the other Amaranthus species. As waterhemp matures, the leaves average 2 to 12 centimeters in length while maintaining a slender 1 to 8 centimeter width giving a lanceolate appearance. Waterhemp has glabrous stems and leaves giving it a smooth surface and a shiny appearance. Stem and branch colors range from green to a deep purple. Plant height and width varies from 0.5 to 3 meters tall with erect to prostrate branching. Flowering structures are mainly located at the ends of the branches, but often appear from the node to the tip of the branch.  Young Palmer Pigweed plants can look similar to other pigweed species. This species has smooth stems and longer leaf petioles than other types of pigweeds.  Unlike other common pigweeds, Palmer does not have any tiny hairs.  Also, its petioles, especially on older leaves, will be as long, if not longer than the leaf blade itself. 

How these weed specifically arrived on all these farms is yet to be determined. However, it is known that the seed of Palmer can be introduced when cotton hulls are fed to livestock and the subsequent manure spread on crop fields. Other possible routes for introduction can be from transport of farm equipment and trucks hauling supplies from the southern regions of the US where these weeds are widespread. 

After these weeds become established it can be difficult to control. Many populations of this plant are known to be resistant to glyphosate and ALS chemistries. Crop producers across Kentucky need to be on the lookout for this weed in both soybean and corn fields.  UK Weed Specialist Jim Martin suggests these tips for helping to manage these weeds: start clean, plant liberty link soybeans, use a soil-applied herbicide, be timely with postemergence herbicide, include residual products with post herbicides, follow up with a post herbicide if needed and rotate crops.

If you suspect these weeds are on your farm or if you would like more information about weed control recommendations, please contact the McLean County Cooperative Extension Service at (270)273-3690.  Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.