Rough Pigweed (Amaranthus retroflexus) is an annual weed found in agricultural systems and disturbed landscapes globally. It is a member of the Amaranthus genus. As a summer annual, it completes its life cycle within a single growing season, but its aggressive nature and prolific seed production make it a management challenge for growers. The plant thrives in various climates and soil types, making it one of the most common and troublesome weeds.
Identifying Characteristics and Taxonomy
Rough Pigweed is classified within the Amaranthaceae family and is characterized by its robust, erect growth habit, often reaching heights of between 3 and 6 feet. The stout, sometimes reddish stem is typically covered in fine, dense hairs, giving the plant its common name. This stem is often freely branched and anchored by a thick, pink or reddish taproot that can extend deep into the soil profile.
The leaves are alternate on the stem and possess a distinct diamond, oval, or ovate shape with slightly wavy margins. Prominent veins are visible on the underside of the leaves, which may also have a hairy texture. These physical features help distinguish it from other pigweed species.
The plant is monoecious, meaning individual plants bear both male and female flowers, which are small and greenish. These flowers are densely clustered into thick, bristly spikes that form a large terminal panicle at the top of the plant. The flower clusters contain rigid, spine-tipped bracts that are noticeably longer than the flower parts, contributing to the rough texture of the mature seed head.
Agricultural Impact and Distribution
Amaranthus retroflexus is considered one of the world’s most problematic weeds, having established itself in over 60 different crops across more than 70 countries. Its success as a weed stems from its highly efficient C4 photosynthetic pathway, allowing for rapid growth, especially in hot, high-light conditions. This fast growth enables it to quickly outcompete cultivated plants for essential resources like light, water, and soil nutrients, particularly nitrogen.
The weed’s competition can cause significant crop yield reductions, even at low densities. Economic thresholds for this weed are extremely low, with estimates suggesting that as few as 0.2 to 0.5 plants per 10 square feet emerging with the crop can cause a measurable yield loss. It is a major issue in row crops such as corn, soybeans, cotton, and sugar beet, where its tall, branching structure can shade out and physically interfere with crop development.
A single Rough Pigweed plant can produce an immense number of tiny, shiny black seeds, with mature individuals yielding up to 300,000 seeds. These seeds form a long-lived seed bank in the soil, maintaining a high percentage of viability, sometimes up to 98% in dry storage. This high viability and seed production mean that infestations can persist for many years, with seeds germinating in multiple flushes throughout the growing season. The plant is native to central and eastern North America but is now naturalized throughout most temperate regions of the world, flourishing in disturbed habitats like cultivated fields and gardens.
Practical Control Strategies
Managing Rough Pigweed infestations requires an integrated strategy that combines cultural, mechanical, and chemical approaches to target the weed’s entire life cycle. Preventative measures focus on minimizing seed production and reducing the viability of the soil seed bank. Crop rotation is a useful tactic, especially incorporating perennial forage crops or fall-seeded small grains, which can suppress pigweed emergence and limit its ability to set seed.
Cultural control also involves maximizing the competitiveness of the cash crop by ensuring a quick and dense canopy closure, which shades the soil. Since pigweed seeds generally only emerge from the top inch of the soil, the use of thick organic or synthetic mulches is an effective way to block the light necessary for germination.
Mechanical Control
For mechanical control, hand-pulling is a viable option for small-scale operations, especially when the seedlings are young before they develop a deep taproot. Tillage can be used to stimulate a flush of germination, with a subsequent tillage pass destroying the newly emerged seedlings, effectively depleting the seed bank near the soil surface. Mowing should be timed carefully to remove the flower heads before the seeds mature, preventing the reintroduction of seeds into the soil.
Chemical Control
Chemical control typically relies on a program utilizing both pre-emergent and post-emergent herbicides with different sites of action. Pre-emergent residual herbicides are applied to manage the existing seed bank before the weed emerges, providing a clean start for the crop. Post-emergent applications must be made when the plants are very small, ideally less than four inches tall, for maximum effectiveness. Due to the prevalence of herbicide resistance in some Amaranthus species, careful selection and rotation of chemical compounds are necessary to prevent the evolution of resistant populations.