Challenges: Perspectives of Expert Growers

Cover Cropping in an Era of Water Scarcity

A significant barrier to orchardists using cover crops includes concerns about water consumption by the cover crops and uncertainty of how this balances out against soil benefits (Gaudin et al., 2020; Ory & Iles, 2021) particularly as the ongoing drought in California has made water supplies unreliable and expensive. Amongst the concerns are questions about competition for water between the trees and cover crops and concern over the establishment of cover crops during drought. On the other hand, cover crops can increase soil organic matter and water infiltration, a problem with which many growers with compacted clay soils struggle.

Increasing soil water holding capacity

Almond grower Kim Gallager of Erdman Farms says through her experience, she has learned that contrary "to the mindset out there that if you're planting a cover crop, well, you're wasting water…we learned here that's not how it works, [on bare soil] radiation is happening [the water] is just evaporating, and you're not getting any benefit at all".

In a study to assess how winter cover crops (grown from late fall to spring) would affect soil moisture and field evaporation compared to bare ground in almond orchards in California's Central Valley, researchers found no significant differences (DeVincentis et al., 2022). These results suggest that farmers can adopt winter cover crops without altering irrigation plans and management practices. However, the timing of winter crop termination is vital since extra evapotranspiration will deplete soil moisture when temperatures increase in spring. Other studies have found that cover crops can improve the hydraulic properties of soils, including significantly higher water retention in soils, but changes may take several years (Çerçioglu et al., 2019). Clippings from mowed cover crops can act as a mulch and reduce evaporation from the orchard floor, possibly offsetting earlier water use (Ingels et al., 1994). Nick Edsall from Bullseye Farm in Yolo County finds that mulch left after mowing the cover crop in spring "keeps moisture in [the soil] and provides weed suppression."

Similarly, walnut grower Daniel Unruh of Unruh Farming in Colusa County uses a roller to terminate his cover crops, creating a thick mat of residue which keeps the soil covered and prevents evaporation from the soil surface. He says of his water use to grow cover crops, "yes, you use some moisture to grow the crop. I don't know if growing that cover crop out there is worse or if it's better than having bare ground. But one thing I'm certain of is that if I need water out there, I'm going to turn the pump on to try to keep something living out there because I think that's valuable. I think evaporation is harder on your soil than something growing over it to cover entirely."

Moisture utilization by the cover crop can also be counterbalanced by the improved infiltration and reduced evaporative losses that occur in no-till and minimum tillage systems (Unger & Vigil, 1998). Some research indicates that cover crops decreased the soil water content in the first soil layers due to the increase of transpiration compared to conventional tillage. However, overall water storage was significantly improved due to the increase in soil permeability and porosity (Celano et al., 2011).

Kirk Pumphrey grows 156 acres of almonds at West Wind Farms in Yolo County. He says of his experience planting cover crops: "Initially feedback was, you know, water is precious, and it's going to use [up] the water, on the surface, that sounds like there's some logic to that. But the benefits far outweigh whatever minuscule amount of water that cover crop uses". Kirk has been motivated by the monetary savings he has found with cover-cropping, which keeps enough moisture in the ground to eliminate some irrigation events in his orchards. "I want to convert that to dollars. Every grower on some device should know what it costs per irrigation for power, and you can come up statistically [to see] you'll save X volume of water. If you get it down to a dollar and cent number, now it's got power… that's how I look at it. So if you've got 28 to 30 irrigation events per year and it costs you $57,000 in power, if you can eliminate two, you can figure that out pretty quick. Not only saving the water, but you've also more than covered whatever it costs [to put in] that cover crop, whatever groundwork you've done, planting it, and chopping it back down."

Water competition

Timely termination of cover crops may help prevent water competition between the cash crop and cover crops. Seth Cooley, a Dixon walnut grower at Juneco, noted, "it seems less beneficial to grow a cover crop during the summer in an orchard because the cover crop wouldn't thrive being shaded out." He also added he thinks "[cover crops during summer] would compete with the water for the trees, to what extent I'm not sure, you know, maybe it's only 10%, but you know, we are always trying to conserve as much water as we can". This observation is backed up by research. Summer-active cover crops (such as perennial clovers) compete directly with orchard trees, using 20 - 25% more water than a bare floor in a mature orchard (Prichard et al., 1989).

Establishment challenges

Cover crop growing in Walnut orchard at Full Belly Farm in Yolo County. Photo by Joanna Normoyle.

Peter Hunter, prune and olive grower of Longview Ranch in Yolo County, says, "To me, cover cropping is kind of a [gamble] because, especially with our irrigation strategy, which is all drip, if it doesn't rain, you lose your crop. it's not like you're on solid set [irrigation system]…where if it doesn't rain, you can turn on the water and germinate stuff". He has found that he probably has “one out of three successful years, strictly because of the weather."

Some growers have found that the number of previous cover cropping years can influence the ability to get the cover crop seeded. Nick Edsall of Bullseye Farms has found he needs rain "on fields that have never been cover cropped [in order to use the no-till drill]" whereas on previously cover cropped orchards there is sometimes enough root mass that they can get the no-till planter in before the rain. Edsall uses a multi-species seed mix and has found that with a lack of rainwater and ability to irrigate, the two or three species in the mix that are more drought tolerant have taken over. In particular, he has found that grains grow better than larger seeded crops when there is less water.

Paul Muller of Full Belly Farm in Yolo has concluded that he has to grow cover crops like a crop and cannot be dependent on fall rains. As a result, he generally plans on irrigating them in the fall. Despite being a difficult thing to do, he thinks it is necessary for the establishment and the benefits that come from cover crops. 

Water infiltration

Several growers were first drawn to try cover cropping to address issues they noticed in their soils. Dave Mostin, a walnut and pear grower for Kelsey Creek Orchards in Lake County, had a particularly troublesome area of soils that suffered from standing water after irrigation. He saw an immediate change the first year after planting cover crops and not cultivating the soil. Three years later, the standing water was non-existent, and he also saw improvements in soil organic matter. Kirk Pumphrey of Yolo County also noted that "wherever there is a cover crop, we never have standing water." Cooley of Solano County observed, "we would actually have, … significantly better water infiltration from rainfall, even if we had just a five-foot strip. I think cover crops are extremely efficient at providing channels for water to go down and using that moisture for its benefit."

Research confirms these observations; cover crops enhance water infiltration of the soil by stabilizing soil aggregates and creating root channels, this can help mitigate runoff of irrigation and rainwater, increasing the stored soil moisture available to the tree crop (Blanco-Canqui et al., 2015; Joyce et al., 2002). 

Orchard with bare soil has poor structure causing water ponding (L). Orchard with cover crops has good structure and infiltration (R). Photo Z. Kabir.