Palmer amaranth is a relatively new weed for the upper Southeast. It has been dubbed ‘Super Weed’ in both agricultural and mainstream publications.
Long-time North Carolina State weed scientist Alan York says there are good reasons to call Palmer pigweed ‘Super’.
1.) Heat and drought tolerance: Palmer amaranth is native to the Sonoran desert of the southwestern U.S. Cotton and a few other popular crops grown in the Southeast like hot weather — the Super Weed doesn’t just like hot weather, it thrives on it.
“North Carolina growers who have glyphosate-resistant Palmer amaranth have seen their cotton droop over in this record hot weather we’ve been having this summer. The Super Weed just keeps growing— smiling at you,” says York.
When daytime temperatures get higher than 90 degrees most crops grown in the Southeast start to feel bad and begin to shut down. The optimum temperature for photosynthesis in Palmer amaranth is 108 degrees F.
2.) Deep and diverse root system: Probably due to its native habitat, Palmer amaranth has a deep growing tap root. It also has a very extensive set of fine roots extending from the taproot. The combination allows Palmer amaranth to explore the soil better than target crops and allows the Super Weed to get moisture when crops can’t do it.
3.) High photosynthesis rate: Palmer amaranth fixes carbon better than any crop plant grown in the Southeast. Its photosynthesis rate is about twice the rate of soybean and cotton and as least as much or more than corn.
Puts on rapid growth
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.
Organic matter makes difference
Pre-plant and pre-emergence herbicides for cotton work well on soil with up to about 5 percent organic matter. As the organic matter gets higher than about 5 percent, the effectiveness of these herbicides drops off rapidly. Valor is the exception. It will work up to around 10 percent organic matter.
Cotton growers in the Blacklands of North Carolina, where most soils are greater than 5 percent organic matter, will have a real problem if Palmer amaranth spreads to that area. With the current technology, growers must depend heavily on soil-applied herbicides to control Palmer amaranth. Options for growers in the Blacklands will be very limited.
Don’t make the same mistake twice!
The use of glufosonate tolerant cotton was fairly widespread in North Carolina in 2010 and some experts contend as much as 65 percent of cotton in the Tar Heel state next year will have the glufosinate tolerant gene in it.
“The first thing you need to understand, if you’re planting glufosinate-resistant cotton, is that you are putting out Ignite and Ignite is not Roundup. Timing will be critical and to get the kind of control you need, you have to get this herbicide out before Palmer amaranth gets more than 3-4 inches tall.
“There’s always going to be somebody who claims they killed pigweed 14 inches tall with Ignite. Well, good for you — you’re lucky, but the optimum size for glufosinate to kill pigweed is still 3-4 inches,” York says.
“Perhaps the most important thing growers need to keep in mind when they plant glufosinate tolerant cotton is that over a relatively short period of time we took Roundup Ready, the best weed management technology we ever had, and we blew it. Glufosinate technology is good, and there is nothing coming down the pike for a few years, so we cannot afford to blow this one by over-using it,” York adds. “We really need to focus on a strong resistance management program in glufosinate tolerant cotton”.