Redroot pigweed, Amaranthus retroflexus
Also known as redroot amaranth, rough pigweed, common amaranth, or careless weed.
Weed identification:
Redroot pigweed is a summer annual weed that can reach heights of 0.6 to 1.8 meters. It has a thick and heavily branched stem that often develops a distinct reddish coloration at the base, a characteristic used in identifying this species.
- Seedlings have narrow and oval cotyledons, often with a reddish tinge on the underside. The hypocotyl is also reddish. As the plant grows, the first true leaves that emerge are broader than the cotyledons and may have a slightly wavy margin.
- Young plants have fine hairs covering the leaves and stems that become less noticeable as the plant matures. These plants have alternate leaves that are oval to lanceolate, with wavy or slightly serrated edges. The leaves are dark green on top and sometimes reddish underneath, mainly along the veins. Long petioles, which can also exhibit a reddish hue, attach the leaves to the stem. The stems are ridged and covered in fine hairs.
- Mature plants have more robust stems and can support extensive branching. This weed has a deep, thick taproot, which is often red colored. This kind of root system makes the weed highly drought-resistant and difficult to remove once established. Its aggressive growth habit enables it to outcompete crops for water, nutrients, and sunlight, reducing yields in agricultural systems. Also, it produces small, green flowers densely packed into spiky clusters, which appear at the tips of stems and leaf axils. These inflorescences contain stiff, sharp bracts, making them rough to the touch.
- Flowering begins in mid-to-late summer and responding to decreasing daylight hours. A single redroot pigweed plant can generate up to 250,000 small, black seeds. These seeds are enclosed in tiny and hard capsules. It can remain viable in the soil for decades, making redroot pigweed a difficult to control species.
Redroot pigweed
Image credit: Robert Vidéki, Doronicum Kft., Bugwood.org
Redroot pigweed at seedling stage
Image credit: Phil Westra, Colorado State University, Bugwood.org
Biology and life cycle:
- Growth habit: Redroot pigweed has an aggressive growth habit, mainly in disturbed soils. The plants grow quickly and maturing within a single growing season as it is a summer annual plant. The extensive root system allows it to access moisture and nutrients deep in the soil, making it highly resistant to drought. The roots system also contributes to its difficulty in removal once it matures. The plant begins its life cycle as a seedling, emerging as soon as environmental conditions are favorable. In the vegetative phase, the plant exhibits a vigorous growth pattern, with leaves developing rapidly. The stem grows erect and becomes very branched, what allows the plant to spread out and shade the soil around to compete with other plants. The leaves are alternately arranged and ovate to lanceolate in shape and grow larger as the plant matures. The plant grows quickly, often reaching 0.6-1.8 meters in height, and can continue branching, what allows it to form a dense canopy that shades the ground below.
- Emergence dynamics: Its emergence is triggered by soil temperature and moisture conditions. The seeds typically germinate in spring when soil temperatures exceed 50°F. However, the optimal temperature range for germination is between 75°F and 85°F. The seeds have high germination rate, mainly when exposed to light and favorable moisture conditions. The quick germination allows the plant to gain an early advantage over competitors. The first true leaves emerge after the cotyledons, and quickly expand, starting the plant process of photosynthesis and rapidly increase in size. As the plant grows, it becomes highly competitive, shading out neighboring plants and limiting their access to sunlight and space.
- Pollination: Redroot pigweed is a dioecious plant, which male and female flowers are on separate plants. The male flowers are small and green, typically found in loose, branched clusters at the top of the plant. These flowers are wind-pollinated, releasing large amounts of pollen that are carried by air currents to the female plants. The female flowers are located in dense, spiky clusters at the tips of the stems and branches. Female flowers are capable of producing large quantities of seeds once pollinated. Pollination is largely a wind-driven process, and cross-pollination between different plants is necessary for seed production. Flowering begins in mid- to late summer, and the plant is sensitive to daylength changes, which trigger the onset of flowering.
- Seed production: This species is highly prolific in seed production; a single plant can produce up to 250,000 seeds. The seeds are tiny, dark, and encased in a hard and protective capsule. After pollination, the female flowers mature into these dense clusters of seeds, which are dispersed by wind, water, or machinery. Once the seeds mature, they fell, and can travel long distances. Seed dispersal occurs when the seeds are carried by the wind, because their small size and light weight. The inflorescences are at the top of the plant, facilitating the effective release and dispersal of seeds into the surrounding area. Human activity can further contribute to the spread of Redroot pigweed, as seeds can be carried in soil or by machinery.
- Seed viability: The seeds also have a higher seed bank longevity, with capability of remaining viable for until 40 years in the soil. Because of the seed longevity, Redroot pigweed can re-establish itself year after year, even if no mature plants are present during a given growing season. The seeds can remain viable in the top 1 to 2 inches of soil, but they can also survive deeper in the soil profile, mainly in undisturbed areas. The viability of seeds is influenced by soil temperature, moisture, and the depth at which seeds are buried. Some seeds may germinate in the first year, while many will remain dormant for several seasons, until have favorable conditions for germination.
Resistance
In 2025, 52 cases of herbicide resistance have been reported worldwide in Redroot pigweed. The resistance has been observed to PSII, ALS, PPO, and EPSPS inhibitors, with resistance to PSII herbicides being the most frequent. In Colorado, a case of resistance to atrazine was reported in 1982.
Redroot pigweed stem
Image credit: Bruce Ackley, The Ohio State University, Bugwood.org
Redroot pigweed seeds
Image credit: Lynn Sosnoskie, University of Georgia
Integrated weed management strategies based on Colorado cropping systems:
Prevention
- Purchasing crop seeds free of weed seeds
- Cleaning agricultural machinery
- Controlling plants along borders and fences
Cultural
- Cultural practices that enhance crop competitiveness against Redroot pigweed are also essential.
- Crop rotation, which helps break the weed’s life cycle and disrupt its growth is an example. The use of herbicide-resistant cultivars can provide more flexibility in herbicide options to improve the control.
- Select crop varieties that are more competitive and optimize planting density to promote rapid canopy closure can help suppress Redroot pigweed growth by reducing light availability.
Mechanical
- Tillage can help bury seeds and delay germination, but Redroot pigweed has a persistent seed bank and can germinate when conditions become favorable.
- Shallow tillage can disrupt emerging seedlings, and in-season cultivation can effectively remove young plants before they establish and compete with crops. However, relying solely on mechanical methods may not be sufficient due to the plant’s high regenerative capacity.
Biological
- Biological control methods have limited effectiveness against Redroot pigweed. While some seed predators, like beetles, may contribute to reducing the seed bank, their impact is generally not sufficient for effective control.
- Grazing by sheep and goats can help manage young redroot pigweed plants in non-crop areas or during fallow periods, but the plant’s rapid growth and high seed production make it challenging to control through biological means alone. Therefore, biological control is best used as a complementary strategy within an IWM approach.
Chemical
- Chemical control of Redroot pigweed involves the use of pre-emergence and post-emergence herbicides, depending on the growth stage of the weed. Pre-emergence herbicides, like atrazine and metolachlor, are applied to preventing seedling establishment.
- Post-emergence herbicides are applied after redroot pigweed has emerged and are effective when the weed is actively growing. Glyphosate and PPO inhibitors, like fomesafen, are commonly used. However, frequent reliance on herbicides led to herbicide resistance, making it essential to rotate modes of action and integrate other control methods.
- To manage herbicide resistance, it is essential to rotate herbicides with different modes of action and combine chemical control with the other strategies mentioned above.