Top 5 Ways Pigweed Spreads – And What You Can Do About It

It sometimes seems that even the very act of farming can spread Palmer amaranth, or pigweed, across U.S. farmland. The aggressive weed can produce a million seeds from a single plant. It can infest farms through seeds carried on second-hand farm equipment, by combine harvesting, in livestock feed, or even by being blown into farms from roadside mowing.

Individual strategies to control the weed range from pesticides to pressure washing of farm equipment to hand-weeding or even burndown of infested areas. By themselves, however, these strategies are only partially successful. Because completely avoiding contamination is practically impossible, weed experts are focusing instead on educating farmers on how to resist the spread of the weed through multiple modes of treatment.


Let’s take a closer look at how pigweed is spread, and ways to resist pigweed’s growing chokehold on American farmland.

How a “Prohibited Noxious Weed” Becomes a Major Threat

Just last summer, under a new emergency rule, pigweed was labeled a “prohibited noxious weed seed” in Wisconsin. In North Dakota, pigweed also was first seen in the summer of 2018, and has already been called “a major threat to North Dakota crop production” by North Dakota State University.

In fact, pigweed has spread already through a majority of the continental U.S. Andrew Kniss, a weed researcher at the University of Wyoming, has been tracking pigweed’s infestation in his personal blog, and the number of affected states at present numbers 30, ranging from California to Massachusetts, and literally dozens of states in between.

What may be surprising is the many ways in which the seed can invade farms. In June 2018, Wisconsin Weed Science, a communications program under the University of Wisconsin at Madison made note of two ways that pigweed is introduced:

  • Contaminated seeds. Contaminated conservation or pollinator seed mixes were among the means of spreading detailed by the university, which also said that “equipment moving from Palmer amaranth infested areas can also transport seeds.”
  • Second-hand equipment. Mark Renz, a University of Wisconsin-Madison weed specialist, speaking at an Arlington Agricultural Research Station agronomy-and-soils field day, theorized that out-of-state feed, farm machinery or both entering Wisconsin may be responsible for the state’s surge in Palmer amaranth infestation.

This is not the result of carelessness. Each pigweed seed is as small as the head of a pin. A small vial of only a few milliliters’ volume can contain thousands of seeds.

Even a single escaped seed falling to the ground and germinating could produce one million seeds per plant – or literally a billion in just a few years’ time.

There are more sources of infestation than were described by the University of Wisconsin:

  • Livestock feed. If drought-stricken farmers buy livestock feed from an area with pigweed infestation, it can pass into farmland. As far back as 2012, Purdue University speculated that “the source of these weed populations may be from cattle manure spread over fields, with feed rations that contained cotton seed or cotton seed hulls infested with the palmer seed.”
  • Custom combining. Custom combining creates an opportunity for pigweed infestation, especially in the Dakotas and the high plains. It’s typical for four to eight combines in a crew to go from one farm to the next. If any of those farms have Palmer, the combines will very likely move them.
  • Roadside mowing. Drive down any county road in Illinois, Indiana, or Iowa, for example, and you’ll see roadside grassy areas full of pigweed and other noxious weeds. As mowers cut back the grass they can redirect seeds back into farmland. To make matters worse, when roadside ditches flood – which is not uncommon – wherever the water settles is also where the weeds settle.

Some local governments are taking action to minimize the spread of pigweed through these means. The Minnesota Department of Transportation has listed Palmer amaranth on its “Prohibited: Eradicate” list of invasive species. Among the guidelines offered by the state to prevent establishment of the weed are to “use fire to improve the environment for competing native plant community,” and the use of pre-emergent and post-emergent herbicides.

“If seed is present, do not mow,” the state guidelines read.

“No Pigweed Left Behind”: Tactics from Academia

As mentioned earlier, universities are taking measures to ensure that pigweed can be controlled as much as possible.

In 2018, Ohio State University, together with the United Soybean Board and the Ohio Soybean Council, launched the “No Pigweed Left Behind” campaign. The program lists several precautions for farmers to prevent infestation. These precautions are part of a multi-mode strategy, and include:

  • When purchasing used equipment, know where it has been previously, and avoid purchasing of combines from Palmer-infested areas.
  • Scout recently seeded areas for the presence of the weed. (According to the program, the Ohio Department of Agriculture will test seed lots for Palmer seed.)
  • Include residual herbicides to control early-emerging Palmer plants.
  • Scout fields starting in mid-July for Palmer that escaped herbicide programs. Plants without mature seed should be uprooted or cut off just below soil and removed from field, and then burned or buried at least a foot deep or composted.
  • Do not run combines through Palmer patches discovered during harvesting.

Pennsylvania State University, through its Penn State Extension, also offers considerable advice. Their strategies include:

  • Plant only certified clean crop seed.
  • Use integrated management practices to aggressively control weeds. (This includes, Penn State says, decreasing row widths for faster canopy closure and shade formation. Palmer amaranth seeds need light to germinate.)
  • Manage infested fields with no-till if possible, leaving any potential seeds near the soil surface.
  • Use residual herbicides (pre and post) during the growing season.
  • Apply effective herbicides to plants less than four inches tall.

The Need for Multi-Mode Planning

What’s clear is that a multi-mode approach is necessary to control the spread of pigweed. This combines hand weeding in the early stages and applying both pre- and post-emergent herbicides – in addition to being cautious about combine harvesting and carefully cleaning equipment purchased where pigweed is already established.

There are problems, of course, associated with each of these tactics individually.

In the case of a moderate infestation, hand laborers may be able to pull out pigweed prior to harvest. But there comes a point at which the economics prohibit that type of work. Eventually it could cost more to remove all the weeds than the crop is worth.

Additionally, pigweed has become resistant to some herbicides. Ohio State University acknowledges that “most populations of Palmer in Ohio are resistant to glyphosate” and that the weed “will not be controlled by burndown or post-emergence applications of glyphosate alone.”

One herbicide that is seeing considerable success in areas where glyphosate may not work is pyridate. This chemical is a selective herbicide for post-emergence control of actively growing annual broadleaf weeds including Palmer amaranth. Pyridate has been shown to provide up to 100% control even on herbicide-resistant strains.

Because pigweed is herbicide-resistant, treatment must be part of a planned program. Let’s use corn as an example. A good plan would be to use pyridate in the early stages, when the corn may have six leaves. After the initial application, it may be best to return with pyridate, mesotrione, and atrazine to kill any smaller weeds that are present.

Pigweed is a stubborn plant that can wreak havoc on farms. With proper planning, however, farmers can resist the spread of pigweed and get on with the business of agriculture.