The Growing Threat
of Palmer Pigweed

pigweed header
Many of us who garden and meticulously care for our lawns know the headache of weeds competing for the natural resources our flowers and grass need to thrive. Weeds like crabgrass, wild morningglory, lambsquarters, quackgrass, and dandelions can be such a pain to clear from your yard. Imagine if they grew up to three inches (7.62 cm) a day—weeding would not be relegated to a weekend activity!
pigweed growth
pigweed growth

Palmer amaranth, commonly referred to as Palmer pigweed, does just that and continues growing up to 10 feet (3.048 m) tall with the circumference rivalling a baseball bat. By contrast, the average dandelion only grows to 18 inches (45.72 cm) in height. In the right conditions, Palmer pigweed can grow so rapidly that if farmers aren’t monitoring their fields closely, it can quickly take over, stealing essential nutrients from crops. Not only can Palmer pigweed populations reduce the yield of a soybean field by up to 78 percent, it can have devastating effects on farm equipment due to its size.

In addition to rapid growth rate, Palmer pigweed has a few other characteristics that can lead to its troublesome reputation.

While a typical lawn weed like dandelion generally produces about 2,000 seeds per plant, each individual female Palmer pigweed plant can produce up to nearly 1 million seeds. Multiply that million by just four or five plants spread throughout an acre (0.404686 ha) planted with an average of 35,000 crop seeds. That field can quickly become inundated with Palmer pigweed seed which, given the weed’s accelerated growth rate, will rapidly compete with any crop planted there.

pigweed seed
pigweed seed

Additionally, most Palmer pigweed seeds flourish best in the top inch of soil and can remain viable underground for up to two years. Dandelion seeds, on the other hand, need certain temperature and soil moisture to remain viable. Deep tillage, the practice of tilling more than 12 inches (30.48 cm) deep, has been effective in reducing Palmer pigweed emergence. However, this should only be done if Palmer pigweed populations are very severe and tillage is not planned for the near future, as follow up tillage can bring buried seed to the surface. There is one other important down-side to consider when using deep tillage to deal Palmer pigweed. It is inconsistent with the no-till and conservation tillage practices many farmers follow today and may not be the right solution for all farms.

Finally, Palmer pigweed has separate male and female flowers that cross-pollinate, increasing genetic diversity and the potential for development of herbicide resistance. For example, glyphosate was initially effective in killing Palmer pigweed, but over time certain populations of the weed have grown resistant to it. Note that, although some Palmer pigweed populations are glyphosate-resistant, there are other herbicides that are still effective in killing Palmer pigweed like 2,4-D and dicamba.

Like gardeners battling dandelions in their fields, farmers can always hand pick and burn Palmer pigweed plants. While this is a very effective way to deal with a Palmer pigweed problem, it is often labor and time intensive, taking the farmer and their resources away from their crops.

Farmers need to constantly be on the lookout for Palmer pigweed in their fields. Through certain crop protection techniques, farmers can help reduce the risk of Palmer pigweed spreading throughout their fields. Regular crop scoutingcrop rotation practices, and managing the weeds present in non-crop areas surrounding the field, all give farmers a leg up in the fight against Palmer pigweed.

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