What Is Redroot Pigweed?

QUESTION: What is redroot pigweed? I keep hearing about it at gardening club meetings, but I’m not sure what people are talking about. — Harvey T.

ANSWER: Redroot pigweed is a widespread summer annual weed plant. The seeds germinate in summertime or whenever soil is sufficiently moist and the temperature is 70 to 85 degrees Fahrenheit. These seeds are the reason the plant is so prolific. Every redroot pigweed plant can produce tens of thousands of seeds, and those seeds remain able to reproduce for a decade or even longer.

When we say redroot pigweed is widespread, that’s almost an understatement. The vigorous plants grow across the continental United States, spreading from New York all the way across to California.

Here are the basics on redroot pigweed. Then, coming up in the next section, we’ll delve into how to fight back against this invasive weed.

Redroot Pigweed: The Basics

Botanical Name: Amaranthus retroflexus L.

Height: Between one foot and six feet tall

Soil: Pigweeds need less than one inch of soil to grow and can also do well in compacted soil. They like fertile, rich soil that’s high in nitrogen, phosphorus, and potassium. Where the pH balance is low, redroot pigweed will have reduced growth. Because it likes sunny, fertile conditions, redroot pigweed is often found in agricultural fields or other places where the soil has been disturbed. These include ditch banks, gardens, landscaped areas, orchards, pastures, roadsides, and vineyards.

Life Cycle: First appears in springtime, going to seed at the end of summer or beginning of fall before dying.

Flowering: From June to November, redroot pigweed clusters of green flowers form small bloom spikes at the end of branches.

Redroot Pigweed Toxicity

Information about toxicity to humans or small pets is not available, but there is lots of information out there about the toxicity of redroot pigweed to livestock. Redroot pigweed contains lots of oxalates and nitrates. Once consumed, the oxalates cause kidney tubular nephrosis, which is fatal. Livestock are at risk of death from eating redroot pigweed when they consume large amounts. The toxicity response has been seen in cattle, goats, horses, pigs, and sheep. 

Here are the symptoms of redroot pigweed toxicity.

• Abdominal distension (seen in pigs)
• Knuckling at fetlock joints
• Loss of control over body movements
• Muscle weakness

Affected animals may develop nitrate toxicity. The effects of nitrate toxicity on livestock include abortions, accelerated heart rate, rapid breathing, weakness, and sometimes death.

If your livestock are susceptible to redroot pigweed toxicity, you can help fight against it by supplementing their feed with forage low in nitrates, especially for livestock that are pregnant.

Tips for Controlling Redroot Pigweed

Adjusting Planting Date: Plant redroot pigweed early in May so the pigweed will emerge after the crop, giving the crop seedlings a head start to get established and stake a claim.

Crop Rotation: Practice crop rotation, using small grains, to suppress pigweed. Find out more in our article Crop Rotation [https://www.gardeningchannel.com/crop-rotation/].

Dark Tillage: Lower the emergence of redroot pigweed by 50 to 75 percent with night tilling.

Flaming: When pigweed plants are an inch and a half or shorter, flaming can be very effective.

Mulching: Use a layer of mulch three inches deep around all your plants to suppress the seed germination of redroot pigweed.

Predation: You can enlist the help of insects that feed on the seeds of redroot pigweed. These include northern field cricket and some types of ground beetles. Mice and other mammals can eat the redroot pigweed seeds, too,

Rotary Hoeing: If you get to them before they exceed a quarter of an inch tall, redroot pigweed seedlings are easily controlled by rotary hoeing.

Soil Solarization: Heat up the soil by covering the area in plastic sheeting, and the soil can reach temperatures exceeding 120 degrees Fahrenheit. That’s high enough to kill both weeds like redroot pigweed as well as diseases the soil might contain. For more information, check out our article Soil Solarization: A Method of Killing Weeds and Diseases Using the Sun.

After reading this article, you now understand what redroot pigweed is, how its toxicity works, and how to control it in your garden. You’re ready to keep your space free of redroot pigweed for seasons to come.

Learn More About Redroot Pigweed

https://blogs.cornell.edu/weedid/redroot-pigweed-3/

https://www.canr.msu.edu/weeds/extension/pigweeds-redroot-pigweed-smooth-pigweed-and-powell-amaranth

https://extension.usu.edu/pests/ipm/ornamental-pest-guide/weeds/w_redroot-pigweed

https://www.gov.mb.ca/agriculture/crops/weeds/red-root-pigweed.html

https://horticulture.oregonstate.edu/weed/redroot-pigweed