Redroot Pigweed


Amaranthus retroflexus

Redroot pigweed
Redroot pigweed. Photo courtesy of University of Delaware. Licensed under CC BY 2.0.
Redroot pigweed is a summer annual broadleaf plant that thrives in open areas with plenty of sunlight. Characterized by its pinkish base base and taproot, It is a close relative to the dangerous Palmer amarath weed and has been known to reduce yield by 10 to 30 percent in some cases.
Germination for redroot pigweed requires a fairly high optimal temperature, which prevents emergence in the early spring and late summer through fall. They do, however, continue to germinate throughout the summer in a wide variety of soil types. Cotyledons are long, narrow, lance-like, and usually have reddish undersides. The stem is often reddish as well. The redroot pigweed's first true leaf is egg-shaped and much wider than the preceding cotyledon. The leaves grow in an alternate pattern with hairy leaf stalks. The veins on each leaf are reddish on the underside, and the tips of the leaves are notched.
Redroot pigweed seedling
Redroot pigweed seedling. Photo courtesy of Phil Westra. Licensed under CC BY 3.0.
Mature plants generally grow to about 3 feet tall but have capabilities of growing to be almost 10 feet. The stems are course and covered with hair. The llower leaves are diamond or egg-shaped, while those at the top are more lance-like. The size of the leaves vary from 1/2 to 7 inches long with prominent veins.
Redroot pigweed infesting an onion field
Redroot pigweed infesting an onion field. Photo courtesy of Howard F. Schwartz. Licensed under CC BY 3.0.
The redroot pigweed flowers June through November along the branches. Individual plants have both male and femail flowers. They cluster together to form a large, dense, stiff, spikelike terminal cluster between the leaf stalks and the stem. Sometimes these spikes have an overall reddish tint. Seeds are round and a glossy reddish brown to black. Each plant can produce tens of thousands of seeds annually. Seeds can remain in the soil for 10 years or more before producing a plant.
Redroot pigweed flower cluster
Redroot pigweed flower cluster. Courtesy of Lynk media. Licensed under CC BY 3.0.
Pigweed is very aggressive and has been known to greatly reduce crop production if left unchecked. It quickly utilizes available moisture but is able to tolerate drought well, usually thriving in hot temperatures. The weed can also act as a host for insect pests and plant viruses. Redroot pigweed is toxic to animals when consumed in large quantities.
Pigroot seeds in the soil can be controlled by maintaining a healthy and desireable crop population that can shade the soil, discouraging colonization. Frequent mowing, tilling, and mulching will be very effective against seedlings.
Most preemergence herbicides used for broadleaf plants will be effective for controlling redroot pigweed. In various crops, postemergence applications of mecoprop, bromoxynil, 2,4-D, or atrazine will effectively control pigweed. There have been reported populations of redroot pigweed that are triazine-resistant. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.