Common lambsquarters is a rapidly growing annual broadleaf plant that emerges mostly in mid to late spring as well as throughout the summer. It often grows well in disturbed areas and is known to be a big contributer to yield loss in corn and soybeans.
Common lambsquarters is a summer annual weed with one of the earliest emergence periods, having 25 percent emergence before spring tillage. The remaining seedlings will emerge in th late spring as well as throughout the summer. Seedlings of common lambsquarters are characterized by a white mealy coating, with the underside of the leaves having it in higher density than the upside. This coating disappears as the leaf matures. Cotyledons are arranged opposite and are spear-shaped, while the true leaves are egg-shaped or triangular with an alternate arrangement. The edges of these leaves are mostly smooth, with some being slightly toothed or lobed. Lower stems of the common lambsquarters seedlings are purplish-pink or green and are grooved.
Mature plants do not have the mealy coating throughout, but remain characterized by having it on new growth. Mature plants are easily confused with oakleaf goosefoot, but oakleaf goosefoot has more oval-shaped leaves and deeper wavy-toothed leaf edges. Common lambsquarters has triangular, egg-shaped or lance-like leaves with stalks about half as long as the leaf itself. Some leaves may become purplish. The shape of the mature plant is pyramidal and highly branched, ultimately reaching a heighth of between 2 and 6 feet. The foliage of the older plant is crowded with clusters of its dull green flower spikes.
Common lambsquarters flowers from early summer to fall. The day-length does not effect flowering. Its gray-green flowers are clustered at the tips of the plant's stems and branches and are located throughout the plant. These flowers become brown and black seeds harbored inside a thin, papery shell. Each plant produces upwards of 100,000 seeds, ensuring its return the following year. The most common methods of dispersal is through animal consumption and equipment transfer.
Common lambsquarters has the ability to grow in almost any environment, making it especially dangerous in highly-fertile cropland. The overall damage is determined by the weed density. Common lambsquarters achieves control of an area through seed longevity and carpeting of an area that has been propogated by its previous generations. It competes heavily with surrounding crops for nutrients, sunlight, and space. Its stems have been known to clog up combines, hampering corn and soybean harvests. Common lambsquarters is also a host for a number of crop diseases, including alfalfa mosaic virus, bean yellow mosaic virus, and soybean mosaic virus.
Seed production for common lambsquarters can be effectively controlled through mowing and tillage. Common lambsquarters is very sensitive to mechanical damage and will not survive uprooting. Tilllage will be very effective if done at peak emergence and repeated. Stirring the top 1-2 inches of soil will control the emerging seedlings if control measures are taken during first 4 to 6 weeks after planting. Rotating away from summer annuals and planting winter grain or perennial forage will also help break up the life cycle of this weed. Allowing desired crops to establish a dense canopy before emergence of common lambsquarter will stunt the weed and reduce the density of the seedlings, allowing for much less seed production overall.
Preemergence herbicides are known to be highly effective in corn and soybeans. Acetochlor is able to control common lambsquarters in the early season but should be tank-mixed with another effective preemergence herbicide. For maximum effectiveness, preemergence applications should be followed by postemergence herbicide. There are some postemergence herbicides useful in corn, but POST herbicides in soybeans are not recommended. In the case of populations germinating later, PRE and POST may be necessary. All treatments exercise the most control in populations less than 6 inches tall. Metolachlor, dimethenamid, and flufenacet do not effectively control this weed. Widespread triazine resistance has been documented in areas where mechanical control of weeds is rarely used. ALS-resistance has been identified as well as cultures of common lambsquarters throughout the US having low-level resistance to glyphosate. Most weed scientists agree that repeated use of Roundup Ready crops and glyphosate in the same fields will exacerbate this problem. In exercising late planting or no-till farming practices, a burndown herbicide is highly effective. 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.