Field Guide     Weed Management   Pigweed  Pigweed

Pigweed (Amaranthus retroflexus L.)

Crop Impacts: Corn and soybean

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About pigweed:

Pigweeds are an annual plant that germinates by the release of their seeds starting in late winter until summer. There are several species of pigweed that are very similar in looks, control methods and locations they can be found. The 3 most common species are Redroot, Green and Smooth Pigweed. They all can be found in the USA along with southern Canada, and are typically all referred to as “Redroot Pigweed.” This is due to the fact that they are hard to tell apart before they flower. It is quite common to find all species infesting the same areas. Overall, pigweed is an aggressive weed that is known for being extremely invasive in grain crop fields, contributing to the reduction of their yield. The reason why they are so aggressive is because they can produce between 10,000 to 30,000 seeds per plant, and can lie dormant in the soil for up to 40 years. If pigweed species are consumed in large concentrations in pastures or silage it can poison livestock. Pigweed can also be hosts for pests in crop fields, such as cucumber mosaic virus, European corn borer, flea beetle, strains of Fusarium, tarnished plant bug, and the green peach aphid.

There are two species of pigweed not listed below that are not as common as Redroot, Green or Smooth pigweed. These less common varieties are Prostrate Pigweed and Tumble Pigweed. Prostrate and Tumble Pigweed do not have terminal spikes, and both are smaller in stature. On top of the varieties of pigweeds listed here, there are several other weeds that lie in the pigweed family. In total there are about 900 species of pigweed, which include two very common weeds called Water Hemp and Palmer Amaranth.

Family: Amaranth Family (Amaranthaceae)

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Pigweed Scouting and Prevention:

General description

Each variety of pigweed listed below has a red to pink taproot with a green to reddish stem. All varieties can be quite simply branched, or with many branches, standing as tall as 1.5m in height. The top half of a mature pigweed plant has a rough texture with some hairs. The leaves range from shiny to dull green or reddish green in colour, are long-stalked and alternate along the stem. The margins of these leaves are smooth with a broader base, pointed tip and are oval to diamond in shape. The green flowers pigweed plants produce sits in small clusters that form bristly spikes in the leaf axis, or at the top of each plant. The very small, dark brown to black seeds are flattened and somewhat rounded. Flowering of pigweed plants starts in July, and the seeds will mature over several months.

Making the varieties of pigweed even harder to tell apart, they have been known to create hybrid varieties with one another. Hybrid pigweed is typically sterile with oddly shaped flower spikes that stay green much longer after non-hybridized pigweed have turned brown.

Common locations

  • - Soybeans fields
  • - Corn fields
  • - Cultivated fields
  • - Gardens
  • - Pastures
  • - Grain fields

Prevention

Prevention of pigweed is less expensive and less time-consuming than trying to control it. Make sure when you are seeding a new area you are doing so with certified weed-free seeds. If there is an infested area on your property, be sure to drive around, instead of through it. Finally, make sure to give all equipment that has been in infested fields a good clean, to make sure no seeds are transferred.

Pigweed Control:

It is recommended to use a combination of both chemical and cultural control methods for taking control of pigweed on your land. Due to its difficult-to-control nature and its multiple flushes of germination, combined residual control is often necessary.

Cultural Control

Do not panic if you find pigweed on your land. First, it is important to know that pigweed seeds germinate in sunlight and high temperatures in the soil, that reach an average of 15˚C. Here are a few general suggestions to protect your crops from weeds:

  • - Make sure you have high-yielding varieties of plants that are adaptable
  • - The field should be planted in narrow rows with high plant population, as soon as ideal soil and weather conditions are met
  • - Scout fields on a regular basses
  • - Through land preparation by harrowing and plowing a minimum of two times a season is key
  • - If cultivating land, do so at night as this will reduce, but not eliminate, the chances of pigweed seeds germinating.
  • - Putting sorghum in your plant rotation has the capability of reducing the emergence and growth of pigweed plants.

Chemical Control

There are a few herbicides that the pigweed population is resistant to. Atrazine in southern Ontario and ALS herbicides (Group 2) in at least 7 countries do not work on Green and Redroot Pigweed. Smooth Pigweed is resistant to Atrazine and ALS in eastern United States, but it has not been recorded as resistant in Canada. Distinguishing herbicide resistant and non-resistant pigweed cannot be done visually. With crop and herbicide rotation, along with other cultural control practices, will help stop the spread of herbicide resistant pigweed.

Trying to control older and larger plants is much more difficult and time consuming, therefore timely application is critical. Herbicides will be ineffective if applied when the plant is under stress. The herbicides will have more effect right before or right after rain due to the fact that the roots of the plants are taking in water at the time, which in turn means they will be taking in the herbicide as well. It is important to know that most herbicide labels do not differentiate between the different varieties of pigweed. Due to the fact that pigweed is a broadleaf weed, the use of any broadleaf weed killers will work sufficiently. Using non-selective herbicides containing the chemicals glyphosate or glufosinate-ammonium works well. They may only be used in areas where you wish to clear out all plants. This is because non-selective herbicides kill any plant they come in contact with. Herbicides with the active ingredient 2,4-D or dicamba are selective meaning they will kill the pigweed and not the desired plants.

Varieties:

Redroot Pigweed

Description

Redroot Pigweed is the most commonly known variety of pigweed. It is also regularly referred to as common pigweed. The first leaves that grow from this particular variety are rounded. Redroot Pigweed has a noticeably shorter, thicker and compacted inflorescence, that only extends a short distance above the first branches of the panicle. The seed of Redroot Pigweed are the first of the three pigweeds to mature.

Latin / Alternative Redroot Pigweed names:

  • - Amaranthus retroflexus L.,
  • - Amarante à racine rouge
  • - Common Pigweed
  • - Green amaranth
  • - Redroot
  • - Rough Pigweed
  • - Tall Pigweed
  • - Amarante réfléchie
  • - Armarante pied rouge

Green Pigweed

Description

The first leaves of Green Pigweed are typically tapering and somewhat pinches toward the end. Green Pigweed has slightly less hair than other varieties of pigweed. The flowers, also known as spikes in this particular variety of pigweed, are typically longer and narrower with minimal but longer branches. Green Pigweed has a habit of germinating and growing much faster, and in greater competitiveness, than Redroot and Smooth Pigweed.

Latin / Alternative Green Pigweed names:

  • - Amaranthus powellii S. Wats.,
  • - Amarante de Powell
  • - Green amaranth
  • - Powell's amaranth
  • - Powell's Pigweed

Smooth Pigweed

Description

The first leaves that grow from this particular variety are rounded. Smooth Pigweed has terminal spikes that are typically lax, and very narrow with several short lateral branches. The seed of Smooth Pigweed are the last of the three pigweeds to mature.

Latin / Alternative Smooth Pigweed names:

  • - Amaranthus hybridus L.
  • - Amarante hybride
  • - Green amaranth
  • - Prince's-feather
  • - Wild beet
  • - Amarante verte
  • - A. hypochondriacus L.

Additional Pigweed Resources

http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/smooth_pigweed.htm

https://gobotany.newenglandwild.org/species/axyris/amaranthoides/

http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/redroot_pigweed.htm

http://www.ipm.ucdavis.edu/PMG/WEEDS/pigweeds.html

http://www.oisat.org/pests/weeds/broad_leaf_weeds/pigweeds.html

http://science.jrank.org/pages/278/Amaranth-Family-Amaranthaceae.html

http://www.omafra.gov.on.ca/english/crops/facts/01-009.htm

http://www.gardeningknowhow.com/plant-problems/weeds/prostrate-pigweed-control.htm

http://extension.psu.edu/pests/weeds/weed-id/redroot-pigweed