Whether you drive north, south, east, or west, chances are that you will see quite a number of fields infested with common lambsquarters. This weed grows across the world, and it is among the top 10 worst weeds in the world. Common lambsquarters is widely distributed throughout North America. Except for extreme deserts, common lambsquarters can be found in all states in the U.S. It is competitive with more than 40 crops and a problematic weed in several crops grown in the Pacific Northwest (Figure 1). Despite the huge tracks of land occupied by this weed, it hasn’t developed resistance to a lot of herbicides. I was shocked to learn that in the U.S., common lambsquarters has developed resistance to only two herbicide sites of action (group 2 and 5) with the majority being group 5 resistance. This is way less compared to other common or problematic weeds of small grains such as Italian ryegrass (resistant to herbicide groups 1, 2, 9, 10, 15, and 22), wild oat (resistant to herbicide groups 1, 2, 3, and 15), and kochia (resistant to herbicide groups 2, 4, 5, and 9). For more information, view the 2020 Take Action Herbicide Classification Chart (pdf) in pdf format.
Figure 1. Common lambsquarters in spring wheat.
As I read more about common lambsquarters, one thing became clear; this weed is a survivor! This weed is versatile in a lot of ways. Here are a few characteristics that make common lambsquarters a successful weed in many cropping systems.
Leaf surface
Common lambsquarters’ leaf surface is super waxy (Figure 2), meaning herbicides don’t stick well or long enough to allow herbicide uptake and kill the plant. A lot of herbicides need to be mixed with conditioners or surfactants to be effective on this weed.
Figure 2. Common lambsquarters leaves are covered with a grayish waxy layer. This interferes with the retention and efficacy of postemergence herbicides. Photo credit: Andrew Kniss, University of Wyoming.
Seed production
Just like many annual broadleaf weeds, common lambsquarters produces an incredibly high number of seeds. This weed can produce more than 70,000 seeds per plant.
Different seed types on the same plant
One of the most important characteristics of this weed is its ability to produce different kinds of seeds on the same plant: light-colored (brown) and dark (black seeds) (Figure 3).
Brown or light-colored seeds germinate readily once they are provided with water, even if temperatures are as low as 32 F. It is thought that about 3% of seeds produced on a common lambsquarters plant are brown or light-colored. However, we still don’t fully understand how growing conditions may affect the proportion of brown or light-colored seeds. It is not uncommon to see common lambsquarters emerge as late as the fall and with a few warm days, these late-emerging plants can produce a few seeds and add to the seedbank. The ability to emerge at different times during the growing season makes it difficult to prevent common lambsquarters from producing seeds.
Black or dark-colored seeds on the other hand tend to be more dormant and persist in the soil for decades. These differences in seed size and color (polymorphisms) play a major role in selection and adaptation across different environments. It allows common lambsquarters to spread its seeds in space and time.
Figure 3. Common produces seeds of different sizes and colors (polymorphism). About 3% are brown (light-colored), which germinate readily. The remainder are black, which are more dormant and stay viable in the soil for decades.
Figure 4. Common lambsquarters emerged in small grains planted in in fall 2021. Photo taken on November 12, 2021.
Light-sensitive seeds
Common lambsquarters produces prodigious numbers of light-sensitive seeds. These seeds accumulate in the soil where they will not germinate until the soil is disturbed and they are exposed to direct sunlight. The ecological/practical advantage here is clear; by germinating when there’s direct sunlight, the seedlings have less competition and a high chance of growing to produce seeds. Although most agronomic weeds behave in a similar manner, we have seen over the years that this is more prevalent in common lambsquarters. In various laboratory research, we simulated the effect of direct sunlight (e.g., after tillage or burndown herbicide application), shade (e.g., under crop canopy or presence of cover crops), or darkness (e.g., buried seeds) on the germination of different agronomic weeds (Figure 5). One thing was clear; common lambsquarters was very sensitive to the composition of light and this contributes to the success of this weed.
Figure 5. Common lambsquarters seeds are very light-sensitive. When grown under artificial light to simulate seeds in darkness, under shade or direct sunlight, germination is significantly suppressed under shaded conditions compared to other broadleaf weeds like kochia. This behavior enables lambsquarters seeds to emerge when there’s less competition.
There’s still a lot more we don’t know about this weed but what we know so far points to the fact that understanding the biology and ecology of common lambsquarters (and other problematic weeds) will be important for successful ecological management.