4.) Fast growing: Primarily because of reasons 1-3, Palmer amaranth can grow an inch a day, even in adverse growing conditions. Most herbicides need to be applied before Palmer exceeds 4-5 inches, and with the rapid growth rate, the application window is very short.  It is difficult for growers with significant acreage to apply herbicides in a timely manner.

5.) It is prolific: York says this is probably the major reason it is so difficult to manage and probably the biggest reason it has been dubbed the Super Weed.

“If you have one growing by itself out on a ditch-bank, it may get as big as a Christmas tree and have 700,000 to 800,000 seeds. Even when growing with a crop, Palmer amaranth may have 400,000 to 450,000 seed,” York says.

6.) It is highly mobile: The glyphosate resistance trait can be moved by pollen. “If you have a susceptible female plant in your field and I have a resistant male in my field, pollen from my male plant crosses with your female plant and part of the female plant’s offspring has the glyphosate resistance trait,” York explains.

How far will that pollen move? Research in Georgia has shown pollen from resistant males can fertilize susceptible females at least 1,000 feet away. So, the resistance trait can easily be spread from field to field by pollen.

7.) Resistant Palmer pigweed populations build up quickly: If a grower has one susceptible plant in a field and lets that plant go to seed, even with 99 percent control in following years, we can build up to over 1,000 plants per acre and 42 million seed in three generations. “If that one plant is resistant and we fail to control it, we could theoretically have 10 million plants per acre in three generations. In reality, there’s not room for that many, though if you have a severe problem, it may seem like you do,” York says.

What do you do with Super Weed?

In general, regardless of what crop you are growing, if you see one or two pigweed in a field you are sure was sprayed properly with glyphosate and you intend to continue using the herbicide, carefully remove the plant from the field, losing as few seed as possible.

Then, destroy the plant so you are sure none can get back into any of your fields. 

In corn, growers have some good pre-emergence and post-emergence options. In soybeans, options are somewhat fewer, but there are some good control programs. The problem in cotton is that there are no good post-emergence options. In addition to glyphosate resistance, resistance to ALS inhibitors (such as Staple) is widespread, leaving no post-emergence options for cotton. 

A very aggressive pre-plant and/or pre-emergence program is essential to control glyphosate-resistant Palmer in cotton. In no-till cotton, York likes to start with a residual pre-plant herbicide such as Valor and follow with at least one residual pre-emergence herbicide. The residual pre-plant gives some insurance against lack of rainfall to activate pre-emergence herbicides. 

In the absence of the pre-plant or in heavily infested fields, York recommends a two- or three-way pre-emergence application. Reflex plus Direx plus Prowl has been one of the more effective treatments.

The next step in the program is Dual Magnum or Warrant put in early post with the first application of glyphosate. That is followed by a residual lay-by application of a material like Direx or similar material. In other words there is probably a place for hoods or directed sprayers.

The objective, York says, is to layer in herbicides so that a new herbicide kicks in prior to the preceding herbicide playing out.