Common Weed on the Lamb This Year

Denise McWilliams, NDSU/UM Extension Crop Production Specialist (from the MN Crop Newsletter)

One weed that seems to be getting more and more elusive to herbicides is common lambsquarter. This annual weed can flower any time between June and September and will set the individual one-seeded fruit within clusters in dense spikes on terminal or axillary inflorescences. The tiny, black or brown, shiny seed (around 72,450 per plant) are creative, smart, proliferation capsules as they are polymorphic (exhibit different dormancy states). The black seeds require some cold or other dormancy breaking treatment to germinate but the brown seed require only water, oxygen and a suitable temperature for germination (hence some of the multiple flushes seen this year). In fact, seed germination is enhanced as the seeds age (even after 38 years, 7% of lambsquarters seed have been found to germinate; a 26 year study found most of the seed viable). The minimum temperature for common lambsquarters germination is 36F and the maximum generally is around 95F. The optimum for a real flush of plants is around 68F. Alternating temperatures actually promote more germination (as seen this year) and the manner in which light affects germination varies by seed age, genotype, nitrate availability and temperature. Many varieties of lambsquarters exist. Check the leaf shapes and inflorescences to determine what you have. The plant is highly variable and may be simply a group of many different, but similar species (inclusion of the pitseed goosefoot occurred for many years under the guise common lambsquarters) such as the family relation of the mapleleaf goosefoot. The taproot on the plant can easily branch allowing this adaptable weed to proliferate even under the heat. Also look for red or light green striations on the stems that occasionally occur with this weed, very similar to another, further removed, family relation in the Goosefoot family (Chenopodiaceae), the kochia. This weed easily bides its time underground, waiting patiently until tillage pulls the small seed up to the surface for germination. If common lambsquarters is running you ragged on fields this year, make sure a comprehensive management plan and rotation is in the plans for next year.

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@tc.umn.edu
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture, jeanne.ciborowski@state.mn.us
Cindy Tong, Department of Horticulture, University of Minnesota, ctong@extension.umn.edu
Production Editor: Suzanne Wold, Research Specialist, University of Minnesota, woldx018@tc.umn.edu

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Last Revised August 10, 2000.
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