HPIPM:Redroot pigweed

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Caryophyllanae
Order	Caryophyllales
Family	Amaranthaceae
Genus	Amaranthus

Scientific Name

Amaranthus retroflexus

Common Name

redroot pigweed

Redroot pigweed (Amaranthus retroflexus)

Compiled by Mary Rumph, Montana State University, Powder River County Extension

and Marjolein Schat, Montana State University from the following sources:

http://plants.usda.gov/java/profile?symbol=AMRE

http://www.css.cornell.edu/weedeco/WeedDatabase/index2.html

Walsh, Roberta A. 1993. Amaranthus retroflexus. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/plants/forb/amaret/all.html [2008, October].

Identification and Life Cycle

Redroot pigweed (Amaranthus retroflexus) is an introduced annual in amaranth family (Amaranthaceae). Other common names for redroot pigweed include redroot amaranth, careless weed, and rough pigweed. It has an erect stem, 1 to 6.6 feet tall, that is commonly freely branched. Leaves are 0.8 to 3.9 inches long. Inflorescences are usually densely crowded. There are often additional dense clusters of flowers in the axils of upper leaves. The fruit is a utricle. Rough pigweed has a taproot that can grow 3.9 to 95 inches deep.

Habitats

Rough pigweed grows in cultivated fields, gardens, orchards, fallow land, stream valleys, shores, prairie ravines, roadsides, fence rows, and waste places. Its grows in dry to moist conditions.

Impacts

Important weed of agronomic crops. Redroot pigweed supports several important crop insect pests, including black cutworm larvae (Agrostis ipsilon), green peach aphid (Myzus persicae), tarnished plant bug nymphs (Lygus lineolaris), cabbage looper (Tricoplusia ni), bean leaf skeletonizer (Autoplusia egena), the fleabeetle (Systena frontalis), and European corn borer (Ostrinia nubilalis).

Biology and Ecology

Rough pigweed regenerates from seed each year. Plants can result from germination of newly released seed, or from germination of seed carried over in the seedbank from previous years. Seeds may germinate any time soil moisture is adequate during the growing season. Redroot pigweed emerges mostly in late spring and early summer, but sporadically throughout the growing season. It tends to flower as days become shorter, and sets seeds over a relatively short period as the plants die. Maturation generally occurs from Aug to early October but early maturation may be triggered by protracted drought. A large plant (e.g., 4 ft) can produce over 100,000 seeds. Seeds can remain viable after passing through animal digestive tracts. Seeds can survive in the soil for up to 20 years. Many of the seeds remain on the plant until early winter.

Management Approaches

Rough pigweed is difficult to eradicate when once established, so management should be aimed at preventing establishment.

Biological Control

There are no biological control agents available for red root pigweed.

Mechanical and Cultural Control

Because redroot pigweed seedlings are very small and fragile, stirring of the top 1-2 inches of soil 2 to 4 times within the first month following tillage is highly effective at removing most of the individuals that will emerge during the season. Dense planting helps control this weed in crops that will tolerate high density. Cultivation reduces longevity of rough pigweed seed, apparently by increasing soil aeration, exposing the seeds to light, and generally improving conditions for germination. High soil temperatures favor germination and reduce rough pigweed seed survival.

Chemical Control

There have been multiple reports of redroot pigweed resistance to photosystem II inhibitors in the United States, and a few reports of resistance to ALS inhibitors. For a complete list of reports of resistance and more information on specific cases, please see http://www.weedscience.org/Summary/USpeciesCountry.asp?lstWeedID=16&FmSpecies=Go

For herbicide recommendations for specific grain rotations and weeds in Montana, please see the MSU Herbicide Chooser Tool.

Examples of herbicides that can be used to manage redroot pigweed

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Herbicide Active Ingredient trade name	Mode of Action	Product per Acre	Application Time or Growth Stage
Alfalfa
Imazethapyr	Inhibition of acetolacetate synthase ALS (acetohydroxyacid synthase AHAS)
*Pursuit		3 - 6 ounces	Apply when pigweed is 4 to 8 inches tall. Apply when the seedling alfalfa is in the second (2nd) trifoliate stage or larger. Apply to established alfalfa in the fall, in the spring to dormant or semi-dormant alfalfa or between cuttings. Any application should be made before significant alfalfa or clover growth or regrowth (3 inches).
Dry Peas and Lentils
Imazamox	Inhibition of acetolacetate synthase ALS (acetohydroxyacid synthase AHAS)
*Raptor		4 ounces	See label for specific bean and pea crops. Apply postemergence prior to bloom stage but after dry beans have at least one fully expanded trifoliate leaf and dry peas have at least 3 pairs of leaves. Delay application until majority of pigweed is 3 inches tall.
Grass Grown for Seed
Dicamba	Action like indole acetic acid (synthetic auxins)
*Clarity		8 - 64 ounces	Apply 8 - 16 fluid ounces per acre on seedling grass after the crop reaches the 3 - 5 leaf stage. Apply up to 64 fluid ounces on well-established perennial grass. For best performance, apply when weeds are in the 2 - 4 leaf stage.
Small Grains
Carfentazone	Inhibition of protoporphyrinogen oxidase (PPO)
*Aim EW		0.5 - 1 ounce	Apply Aim EW alone or as a tank mixture with other herbicides to emerged and actively growing weeds. Apply to small grains in all tillage systems from prior to planting up to jointing. For optimum performance, make application to actively growing weeds up to 4 inches tall. See label for tank mixture information.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=redroot%20pigweed&Start=1&results=8