The Benefits of Planting a Perennial Cover Crop in California

The Rise, Fall, and Revival of Cover Cropping

Planting cover crops is a sustainable agricultural technique that has stood the test of time. Manuscripts from over 3000 years ago indicate that ancient civilizations around the world planted cover crops to improve soil fertility and crop yield. Native Americans employed the “The Three Sisters” strategy which took advantage of the symbiotic relationship between tall corn stalks, nitrogen-producing beans, and the large, low growing leaves from squash that retained soil moisture and inhibited weed growth. Even the first U.S. president George Washington, who was a farmer by trade, categorized his crops into two groups: “plants grown to eat and sell” and “plants grown to replenish the soil.”

But, after World War II, industrial plants that had been manufacturing ammonia for munitions were repurposed to create nitrogen–rich fertilizers for agriculture. By 1950, the annual capacity of ammonia production had skyrocketed to 2.6 million tons. Only a small percentage of farmers continued to grow cash crops organically.

Soon however, research about the negative effects synthetic chemicals had on human health and the environment began to emerge. In 1962, Rachel Carson, published Silent Spring which documented the environmental harm caused by the chemical pesticide DDT. This book started an environmental movement that led to drastic changes in U.S. policy around agricultural practices and led to the founding of the U.S. Environmental Protection Agency (EPA).

In 1983, Congress passed the Agricultural Productivity Act, which eventually resulted in the Sustainable Agriculture Research and Education (SARE) program. The SARE program gave farmers and researchers funding to test sustainable ways to fertilize crops, control pests, and improve crop yields. The projects funded by SARE brought the practice of planting cover crops back into popularity.

The Challenges Facing California’s Perennial Crop Systems

Since the 1980’s, California’s crop mix has shifted such that perennial orchards and vineyards now cover about half of all irrigated farmland. California is the primary or sole producer of many perennial cash crops in the U.S. including grapes, almonds, and pistachios. These three crops alone generated over $10.9 billion in cash receipts for California in 2022. Although perennial crops can offer a higher return on investment than annual crops in terms of time and money, these systems also pose several unique challenges to sustainable farming.

Negative conditions, like the accumulation of pathogens and soil-borne pests, can compound over time which can impact productivity, especially when crops are not rotated every year as in the case of fruit and nut trees.

Climate change also poses a new challenge to growers. California has a Mediterranean climate, characterized by cool, wet winters and warm, dry summers. In the summertime, the state’s widespread irrigation infrastructure allows growers to use water to prevent heat stress responses in crops, however this increases water costs. Research suggests that if climate change continues for the next few decades, Mediterranean climates will face longer, hotter summers, which will impact crop yield, leading to food scarcity, economic decline and job losses. 

Planting Cover Crops in Perennial Agriculture Systems

California has followed a similar trend in sustainable farming practices as the rest of the nation. In the early 1900s, Californian farmers were using cover crops in vineyards to reduce soil erosion and improve water penetration. Since the resurgence of sustainable farming practices in the late 1980s, a wide range of perennial cover crop species, including cereal rye, radishes, clover, grasses, and buckwheat, were identified for use in agricultural regions across California. Each species has its own unique benefits, so it is important that growers carefully consider all options when choosing which cover crops to plant.

Cover crops mitigating soil erosion from wind and water.

One of the primary benefits to planting cover crops is mitigating soil erosion. In California, long hot summers and droughts caused by climate change dry out the soil, making it more susceptible to wind erosion. Conversely, heavy rain, snowmelt, or irrigation runoff can displace layers of loose, unprotected soil, washing away sediment and nutrients. Farmland that is frequently tilled is particularly susceptible to erosion as soil is loosened and broken up.

Cover crops help prevent erosion in multiple ways. The above-ground shoot systems provide coverage so that the soil is protected from wind and rainfall. Underground, roots bind the soil together and increase water infiltration, preventing the top layer from forming a crust. Perennial grasses, like California Brome and purple needlegrass are especially good for this because of their extensive, fibrous root system which physically holds soil in place and prevents nutrients from being washed away. As a result, water runoff can be reduced by up to 90% and sediment concentration in runoff water can be reduced by 30-45%.

Cover crops with shallow root systems, like perennial ryegrass are adept at preventing erosion in rocky landscapes. They are less heat and drought resistant compared to cover crops with deeper roots, like tall fescue. However, this isn’t necessarily a negative. When perennial plants die (naturally over years or due to environmental conditions) the plant residue contributes to the soil organic carbon (SOC) and releases nutrients. In fact, some of the most fertile lands on Earth were created under perennial grasslands, where organic matter had accumulated over centuries.

Legumes fix nitrogen while other cover crops sequester it for future use.

Nitrogen is a vital element for crop growth because it is an essential part of chlorophyll, the green pigment that enables plants to capture sunlight for photosynthesis. It also is a building block for proteins and other molecules that contribute to a plant’s metabolism. Before ammonia-based synthetic fertilizers were developed, growers used legumes as cover crops to replenish the soil’s nitrogen.

Legumes like white clover have a symbiotic relationship with bacteria called rhizobia. These bacteria live in the legume root nodules and convert nitrogen gas from the atmosphere into ammonia and other nitrates which can be used by plants. White clover is a particularly popular cover crop because it contains more of its nitrogen in its roots than other legumes, making it easily accessible to other plants.

Legumes are often planted alongside other cover crops, like barley, which act as a nurse crop while the legume strand is establishing itself. Nurse cover crops also take up nitrogen produced by the legumes to prevent it from leaching away into the groundwater. After a harvest or when the cover crop dies, the nitrogen returns back to the soil to be taken up by the next crop.

Cover crops can serve as natural weed suppressors and pest control.

Many cover crops aid in weed and pest control because they compete for resources like water, sunlight and nutrients. Some cover crops are particularly adept at killing the competition.

For example, barley produces a toxic alkaloid, called gramine, that protects it from certain pathogens, fungi, and parasites. White clover and strawberry clover have also been shown to significantly reduce the number of pests found on vegetable crops without reducing crop yields. This is because cover crops provide a habitat for and attract and natural predators of insect pests. This reduces the cost of pesticides for farmers and exposure to potentially harmful chemicals. It can also increase consumer trust in the product.

Barley and clover can also suppress weed growth because they are fast, low growing crops which can outcompete weeds for ground space. However, it is important that farmers carefully choose cover crops that do not compete with their cash crops for space or nutrients. For example, Oakville bluegrass, a hybrid perennial grass cover crop, has promise for vineyards because it begins its growth period in the fall and becomes dormant in early spring. Therefore, it will not compete with grapevines for resources, which start growing in the spring.

Polyculture farming increases biodiversity and reduces the need for synthetic pesticides and fertilizers.

When synthetic fertilizers and pesticides were popular, monoculture farming also became the norm to increase planting and harvesting efficiency. However, planting large areas with a single species created multiple negative feedback loops in the environment. Monoculture farming attracted fewer pollinators and birds that would feed on harmful insects. Pests and diseases were able to spread through the crop more easily because there were no other plants or predators to limit them. All the plants would also be competing for the same resources as each other, reducing soil fertility. This led to a vicious cycle of needing more synthetic fertilizers and pesticides which would spread into the air and water systems.

Instead, polyculture farming naturally creates a healthier farm ecosystem. Cover crops boost biodiversity on a farm both above and below ground. Perennial crops increase microbial and fungal diversity in the soil as compared to annual crops where the soil is repeatedly tilled and microbial communities are disrupted. The soil microbiome is important for nutrient cycling and retention so planting multiple plant species can alter these dynamics. Therefore, it is crucial to understand how different crops can affect microbial communities. 

For example, certain cover crops like legumes are particularly beneficial due to their symbiotic relationship with nitrogen-fixing microbes, which can reduce the need for synthetic fertilizers. Other cover crops, like grasses, can play a more complementary role by contributing SOC and plant residues which can be metabolized by microbes.

Flowering cover crops like birdsfoot trefoil and white clover can attract pollinators like bees and butterflies. These crops can be beneficial even when planted on the margins of farmland, buffer strips, hedgerows, or other regions that are unfit for cultivating cash crops. Planting crops that flower at different times of the year can also attract different pollinator species and further increase biodiversity.

Cover crops can reduce and remove carbon emissions.

Perennial cover crops provide long-term carbon dioxide (CO2) capture and storage capacity to farms. Plants capture atmospheric CO2 emissions through photosynthesis and store it as SOC. Cover crops also help sequester carbon, with one study estimating that cover crops are sequestering 5.5 Mg of SOC per year in the U.S alone. This number will change depending on a number of factors including crop rotation, cover crop type, soil tillage and the climate.

Perennial cover crops reduce the need for tillage. Disturbing the soil through tilling reduces SOC in the upper soil layer (0-15cm depth) which is why no-till systems have become popular. Once a perennial cover crop has been established, there is no need to till or reseed for several years, allowing SOC to accumulate.

The Economic Benefits of Planting Cover Crops

One of the biggest hurdles growers face when deciding whether to plant cover crops is calculating whether the investment will be economically beneficial. Unfortunately, there is no single answer. Each grower must look at their individual needs to understand whether planting cover crops will be worth the cost. However, with careful planning and research, planting cover crops can have many economical benefits.

Investing in Perennial Cover Crops Can Reduce Long-term Costs 

Perennial cover crops are particularly cost effective because they only need to be planted every 5-15 years, decreasing the costs for fuel and labor. And although some non-native perennial cover crops may initially increase the need for irrigation, once established they are able to increase the soil’s capacity to retain water. 

When the weather is hot and arid, cover crops shade the soil, reducing the soil’s temperature and water evaporation from the soil’s surface. The roots of cover crops also significantly increase water infiltration by increasing soil porosity and leading to the buildup of SOC as they decay overtime. A study in the San Joaquin Valleys showed that water infiltration rates were 2.8 times higher than bare ground, which could significantly reduce the need for irrigation.  Scientists estimate that cover crops can reduce water costs by 4-14% and increase crop yields in drought years. During stormy seasons, cover crops create a physical barrier that softens the impact of rain droplets falling on soil, reducing water runoff by 40-87%. They also increase soil’s moisture after they are terminated, allowing cash crops to access water from the mulch. 

As mentioned earlier, cover crops can also reduce the need for harmful chemical pesticides. It is estimated that U.S. farmers spent $12.7 billion in 2022 on herbicides but the right cover crop can reduce weed growth by 90-100%.

The government also has created programs to incentivize cover cropping by strengthening crop insurance, which is currently the most expensive farm subsidy. As climate change worsens, the cost of insurance programs rise. In May 2023, Senator Sherrod Brown (D-OH), Representative Sean Casten (D-IL) and Representative Elissa Slotkin (D-MI) introduced the Conservation Opportunity and Voluntary Environment Resilience (COVER) Act, which gave farmers $5/ acre savings off of their crop insurance bills if they planted cover crops. This concept has enjoyed bipartisan support, with 78% of farmers also in favor.

Investing in Perennial Cover Crops can Create New Revenue Streams 

Since plants naturally sequester carbon dioxide, growers who plant cover crops can receive payments in the form of carbon credits. A carbon credit is a certificate of proof indicating that a certain amount of carbon dioxide has been permanently sequestered. Growers who plant cover crops must measure their soil to get an accurate reading of how much SOC has been stored. The credits they receive can be sold in carbon markets, such as IndigoAg and Tuttera, to buyers looking to offset their own emissions. In 2023, the price of credits averaged at about $7 per ton of sequestered carbon and that price is projected to increase to as much as $25-30 by 2030.

Having perennial cover crops also increases the amount of stover that agricultural farms can sell to biofuel manufacturers. As the nation transitions to clean energy, the demand for biofuels is likely to increase. In June 2023, the USDA announced that they would invest $500 million from President Biden’s Inflation Reduction Act towards increasing the availability of domestic biofuels through building infrastructure and increasing availability of higher blends of ethanol and biodiesel made from agricultural products. Once a cover crop has been established, farmers need less stover to replenish the soil’s nutrients and organic matter. One study shows that farms can sustainably remove up to 1.8 tons more stover per acre than they could without cover crops. This not only provides additional revenue, but helps with our nation’s transition to clean energy and sustainability as well.

Laying seeds for the future

When it comes to perennial cover crops, the greater the investment, the greater the reward. As long as growers carefully weigh the costs and benefits and manage the cover crop system appropriately, they have the potential to reap huge rewards. The cost savings from using less water, reduced need for synthetic chemical pesticides and herbicides, increased revenue from stover, and reduced insurance will likely cover the costs of investing in a perennial cover crop system. Plus, the benefit to the environment in terms of healthier soil, increased biodiversity, and reduced carbon emissions helps us move towards a cleaner planet and a more sustainable future.