Cover Cropping and Frost in Orchards and Vineyards

A successful harvest in orchard and vineyard crops begins with a viable spring bloom and fruit set. Given the importance of this stage of crop development, spring frosts during bloom and early fruit set pose a legitimate concern. The potential contribution of cover crops to increased frost risk at canopy height is uncertain but is often mentioned as an issue of concern by growers considering winter cover crops. Here we review the latest science on cover crops and frost risk in orchards and showcase grower strategies for managing spring cover crop growth.

Risks Associated with Spring Frost Events

Spring frosts pose significant challenges to temperate fruit and nut orchards and can substantially reduce yields (Snyder and Paulo de Melo-Abrea, 2005). In the winter months, orchard trees have a high tolerance to low temperatures because they are in a dormant state and have little to no vegetative growth. However, as vegetation begins to develop during the spring months, the reproductive organs in tree flowers become susceptible to freezing temperatures. For example, in almonds, frost damage is highest at petal fall when temperatures below 30° F can damage young nuts (Connell, 2011), but frost can cause damage any time after tree dormancy through spring (Wauters et al., 2020).

Effects of Cover Crops on Frost Risk in Orchards – What Does the Research Say?

Cover crops may limit the amount of solar radiation that reaches the soil surface during the day, limiting the amount of radiation absorbed and possibly reducing soil-to-tree heat transfer at night (O’Connell and Snyder, 1999). Therefore, many farm managers worry that cover crops will exacerbate the cold temperatures associated with spring frosts and increase the risk of frost damage to trees and vines.

However, only a relatively small body of field research addresses the effect of cover crops on orchard temperatures, and the outcomes of this research are mixed. One study found that cover crops generally tend to reduce soil temperature compared to bare, moist soil (Snyder and Connell, 1993), suggesting that cover cropping could indeed reduce heat transfer from the orchard floor at night and increase the risk of frost damage. Two additional studies further indicated that vineyards and citrus orchards with some level of ground cover, either from cover crops or weeds, tend to have lower temperatures at the vine cordon height of 4 ft (Donaldson et al., 1993) or the tree canopy height of 5 ft (in citrus orchards) than vineyards and orchards with bare soil (Donaldson et al., 1993; O’Connell and Snyder, 1999).

On the other hand, Snyder and Connell (1993) also found that the surface temperature of an orchard floor with ground cover is higher than bare, dry soil. There is also evidence that in windy conditions, cover crops can significantly increase soil surface temperatures compared to bare soils (Project No 87-O7, R. Snyder, cited in Creze et al., 2019). These findings suggest that the effect of cover crops on orchard temperatures may be influenced by background climatic conditions and in some instances cover crops may actually decrease frost risk relative to bare soils.

Recent research from UC Davis also suggests that the air temperature three to five feet above the ground is not reduced in almond orchards with cover crops compared to bare soil, suggesting that the presence of a cover crop may not affect the temperature at the canopy level where the damaging effects of frost are the most important (Crézé et al., 2019). However, this finding is based on one season of data collection and longer-term data are needed to determine if this result is consistent across seasons. A 2023-2025 study at UC Davis is collecting data to better understand the effects of cover crops on spring nighttime temperatures in orchards.

Cover Crop Management Solutions

Given these mixed results, the current University of California Cooperative Extension recommendation is to monitor weather patterns in high frost risk areas and low-mow cover crops (i.e., mow the cover crop to less than two inches) if there is a risk of frost overnight. Many of the expert cover crop growers in the Expert Cover Crop Growers database follow this recommendation and describe keeping an eye on the weather and mowing ahead of cooling events to limit frost damage in their orchards. A winegrape grower in Napa said, “For the frost damage we just keep an eye [on it] and mow before we think it could be an issue.” Similarly, another grower explained, “We try to maintain the cover crop regularly to minimize frost damage.”

Mowing ahead of a frost event can increase soil temperatures and reduce the risk of frost damage, but the trade-off is that low mowing will likely terminate the cover crop. A higher mow may save the cover crop but may also not sufficiently increase soil temperatures to mitigate frost risk. Paul Muller, at Full Belly Farm in Yolo County, explains his approach to balancing this trade-off in walnuts, almonds, and mixed fruit orchards.  In the spring, when the trees begin to push and bloom, he looks at the weather. If the cover crop is tall and rank, he’ll mow it high—roughly six to eight inches—to manage frost damage. From his perspective, if you mow the cover crop too short, you’ll kill some of the species. The goal is to get through the walnut bloom period without the ground cover being too high, while also allowing for some regrowth after mowing. In contrast to the traditional belief is that the lower you mow down the cover crop during tree bloom, the better off you will be in terms of frost protection Muller feels he does not gain much by mowing tightly. Rather, he benefits more by retaining leaf surfaces on the cover crop that continue to push sugars into the soil. His goal is to manage optimal coverage on the orchard floor and reduce evaporation.

He also recommends rolling the cover crop down during the bloom period as it provides good mulch, and spring rain will infiltrate much better. The goal here is to optimize the amount of surface area that can absorb sunlight, while also keeping the ground covered. To ensure his mower can access the tree rows, he waters the orchard middles using a hanging micro-sprinkler (hanging droppers). He grows cover crops in the tree rows as well as middles and uses an articulating mower that can weave in and out of the trees. There is no berm in the tree row, so he is able to cross mow.

Another option to balance the trade-off between mowing to mitigate frost risk and terminating the cover crop is to select cover crop species that regrow after mowing. This includes most grasses and clovers like berseem clover (Giambalvo et al., 2011). These species can regrow later in the spring and provide soil coverage during heavy spring rain events that might otherwise limit orchard access by machinery (Wauters et al., 2020). However, as one winegrape orchard manager in Napa explained, a strategy of mowing shortly before an expected frost event may not be feasible in a large-scale vineyard, where it is harder to get through the whole vineyard fast enough. In these situations, a good alternative may be to select low-growing cover crops such as clovers that can reduce the need to mow (Wauters et al., 2020).