by Robert Marcos
In 2022, University of California Davis published the results of a three-year study on covers crops, which was carried out on ten commercial farms and research sites in California’s Central Valley. The study examined the impact of winter cover cropping on soil health and water retention in irrigated agricultural systems with a focus on almond and tomato crops, which are two of the most common crops grown in the region.1
Three cover crop systems were included in the study and then compared with adjacent control fields that were left bare, at the same site. The systems included: 1) a cover crop in processing tomato fields; 2) a cover crop planted in between rows of almond trees; and 3) allowing whatever native vegetation was available to grow in between the almond tree rows. The planted cover crops were a mix of legumes, grasses, and brassicas.
The results were impressive. Researchers found that the cover crop fields had higher levels of soil organic matter, soil nitrogen, and microbial activity, indicating improved soil health. In addition, the cover crop fields had higher levels of water infiltration and retention, meaning that they were better able to hold onto water during periods of drought or water stress. The researchers found that the cover crops did not compete with cash crops for water, and that the same amount of water used in the control fields without cover crops was able to support the same amount of crop yield in the cover crop fields. In one case in Davis, there was heavy rainfall at one point during the study. The water loss via evapotranspiration was greater in the bare control plot, showing that use of cover crops improved water retention.
The study provided important evidence of the benefits of winter cover cropping in California’s Central Valley, particularly for improving soil health and reducing water usage in agricultural systems. The findings suggest that cover crops can help farmers make more efficient use of their water resources, potentially reducing the need for additional irrigation, and providing environmental benefits such as reduced erosion and improved water quality.
Top 5 Cover Crops for use in the Western US
For the Western United States – including the arid regions of California, Arizona, and Nevada, the best nitrogen-fixing cover crops are selected for their drought tolerance and ability to thrive in either high heat or mild winters.2
- Cowpea: Best For: Summer heat in low-elevation deserts. Highly drought-tolerant with a taproot that can reach up to eight feet deep to access water. It thrives when temperatures exceed 100° F and can fix roughly 100–175 lbs of nitrogen per acre. The ‘Iron Clay’ variety is widely recommended for use in the Southwest.
- Alfalfa: works best for long-term soil restoration. Often called the “queen of forages,” alfalfa is a perennial legume with deep roots that break up subsoil and reach nutrients deep in the earth. It is one of the most powerful nitrogen fixers, capable of producing 250–500 lbs of nitrogen per acre.
- Crimson Clover: Used as winter cover in the Southwest or for spring cover in the north. A fast-growing annual that establishes quickly in the fall to provide winter protection. It is frequently used in mixtures with radish to improve soil structure while fixing roughly 70–150 lbs of nitrogen per acre.
- Hairy Vetch: is excellent as a winter-hardy coverage and weed suppression.
Why It Works: It grows slowly in the fall but resumes vigorous growth in the spring, creating a thick mat that smothers weeds. It is known for high nitrogen fixation (over 100 lbs per acre) and performs well in the cooler, non-agricultural environments of the West. - Lablab: works best during the summer-to-fall transition in Arizona. Lablab is specifically noted for its performance in the hot weather of central Arizona. It produces high biomass and can contribute 50–200 lbs of nitrogen per acre. Unlike some other summer legumes, it continues vegetative growth late into the year without flowering immediately, offering more flexible termination dates for growers.