On the Lookout for Palmer Amaranth

Written by Jeremiah Vardiman

Wyoming’s agriculture community has a new weed pressure to be concerned about. Palmer amaranth, which is perhaps the most aggressive pigweed species, has been found growing in Goshen County.

Weed pressure can and does have significant impacts to agriculture crop yields. As an example, weed pressure alone can cause up to 40 percent yield loss in corn crops when weeds are not managed.  There are certain situations when crop failure occurs because weeds get out of control. 

Palmer amaranth has gained a lot of attention in the agriculture industry because of the difficulty to manage this weed, especially in vegetable and row crops. Several attributes make this weed difficult to control, such as fast growth, continual emergence and herbicide resistance. Palmer amaranth is native to the southwest United States, an area where plants need to be water efficient to survive. Along with water efficiency, palmer amaranth has dioecious reproduction, meaning the plants are either male or female.  The combination of these two features allow it to adapt and spread quickly. This plant exhibits very aggressive growth in the Midwest where it can be more than six feet tall – as tall as or taller than corn – and cause yield losses up to 91 percent in corn. 

Palmer amaranth is also very challenging to control because of its continual emergence during the growing season. This summer, the annual plant was documented emerging from May until the middle of September. Once crops have emerged and are growing, control options are limited or not available, depending on crop and type of weed, because of injury to the crop itself. This pigweed specie also germinates faster than other pigweed species and favors shallow emergence depths found in no-till and reduced tillage systems.

To make palmer amaranth even more difficult to control, this dioecious plant has adapted a resistance to single modes of action in herbicides and, in some cases, to multiple modes of action. The modes of action that palmer amaranth can be resistant to are ALS inhibitors, triazines, HPPD inhibitors, dinitroanalines, PPO inhibitors, and ESPS inhibitors or glyphosate herbicides. Reduced tillage and no-till systems, along with other agriculture systems that rely mainly on post-emergence herbicides have limited affect against this already-resistant specie. 

The best control for this weed is be identifying it before establishment and infestation levels are achieved.

One key method to identifying palmer amaranth from other pigweeds is that the petiole or leaf stalk is longer than the leaf blade. The other pigweed species have longer leaves than the petiole.

If palmer amaranth infestations are identified, hand pulling of small populations is an option. For large infestations levels, an integrated management plan is the best option for control and decreasing populations. This can be accomplished by cultivating crops with optimal plant spacing and canopy development to enhance crop competition, combine soil applied and post-emergence herbicides and incorporate cultivation practices after post-emergence herbicides to eliminate surviving plants and reduce the risk of herbicide resistance.

The control and spread of palmer amaranth is not just the agriculture industry’s problem, but it is everyone’s responsibility.  Palmer amaranth’s seed is a small inconspicuous seed that is easily spread in manure, equipment, contaminated feed and seed mixes.  Manure is one of the primary pathways for spread of this weed into new areas because the seed can pass undamaged through the guts of an animal, especially cattle.  The small seed is potentially difficult to remove from other seeds and can easily contaminate feed sources. 

In addition to feed sources, palmer amaranth can contaminate seed mixes used for restoration projects, cover crops or wildflower mixes.  Palmer amaranth infestations have spread extensively in Midwest states, due to contaminated seed mixes for reclamation seedings. This same issue is happening in small-seeded cover crop and wildflower mixes, which could be a potential source to spreading this weed into the state of Wyoming.

So, for 2018, be on the lookout for palmer amaranth plants within your cultivated lands, gardens and disturbed soil sites. If suspected populations are found, notify your local weed and pest office, then terminate the plants prior to seed set.  Only through everyone’s diligence can we keep this weed from spreading further into Wyoming.

For more information, contact Jeremiah Vardiman at 307-754-8836.