Cover crop research review: How can it help almonds?

HorticultureSoil
Written By

Cynthia Crézé (1), Jeffrey Mitchell (1), Andreas Westphal (2), Danielle Lightle (3), David Doll (3), Mohammad Yaghmour (3), Neal Williams (4), Amanda Hodson(4), Houston Wilson (5), Kent Daane (6), Brad Hanson (1), Steven Haring (1), Cameron Zuber (3) & Amélie Gaudin (1)

  1. Department of Plant Sciences, University of California – Davis
  2. Department of Nematology, University of California – Riverside
  3. University of California Agriculture and Natural Resources – Cooperative Extension
  4. Department of Entomology and Nematology, University of California – Davis
  5. Department of Entomology, University of California – Riverside
  6. Department of Environmental Science, Policy and Management, University of California – Berkeley

Soil Mix at the time of termination at the three orchard sites.

Although cover cropping is compatible withalmond production and is often implemented in other orchard systems, thispractice has never been widely implemented in California. The potentialbenefits are recognized by growers, especially their value for pollinatorforage and soil health but operational concerns, lack of cost-benefit analyses andunclear best management practices have hampered wide adoption. As covercropping can provide significant sustainability benefits, there is an urgentneed to assess and develop feasible and beneficial cover crop systems forCalifornia almond production. Here is some insight gathered by a research team assessingthe impacts of multiple cover crop management strategies on: 1) soil health, 2)water use and dynamics, 3) bee visitation and pollination, 4) weed and pestpressure (NOW) and 5) almond yields in four orchards across the Central Valleyprecipitation gradient.

Cover crop research trialin almond:

Project website:https://almondcovercrop.faculty.ucdavis.edu

Design:

  • Threecommercial orchards in Corning (Tehama county, 2nd leaf), Merced (Mercedcounty, 16th leaf) and Arvin (Kern county, 16th leaf).
  • Oneexperimental station: Kearney (Fresno county).
  • Twocover crops: Soil Mix (2 legumes, 2 brassicas & 1 grass), Pollinator Mix (5brassicas, Project Apis M - https://www.projectapism.org/pam-mustard-mix.html)
  • Comparedto resident vegetation & to bare soil.
  • Covercrop from October to April with two termination dates tested (March to lateApril).

Insight from two seasons:

Cover crop management should be site-specificand should be tailored to the grower’s objectives and regional climaticconditions. For example, a cover crop established for bee forage could havenarrower seeding and an earlier termination than a cover crop used forsoil-building objectives. Ultimately, there are multiple designs possible. Testingdifferent designs or mixtures on a few rows will help you find the best optionfor your orchard. Here are a few general recommendations and resources to guideyour cover cropping decisions. Keep in mind that cover cropping has proven tobe a long-term investment for which benefits may only be visible after severalyears.

Expected MainBenefits

Slightly higher almond yields were obtained inthe mature orchards after one year of cover cropping: in Merced, +225 lbs/acrecompared to resident vegetation/weeds and +217 lbs/acre compared to bare soil.In Kern county, yields in the cover cropped plots were +94 lbs/acre compared tobare soil. These yield increases could be attributed to the potential benefitsdetailed below.

Building Soil Health:

California almondproduction is faced with multiple challenges, often tied to degraded andcompacted soils, which are frequent in almond orchards across the state. Researchersare quantifying cover crop soil services across different soil healthcomponents: physical,such as aggregate stability and water infiltration rate, chemical including major nutrients and chemical properties suchas pH and CEC, and biological includingamounts of soil microbes, soil food web composition and microbial activities.The evaluation will occur across 3 seasons. As soilhealth will start to shift, cover crop productivity may also increase therebycreating a higher soil-building potential. This idea of a “virtuous cycle” mustbe kept in mind when cover cropping: although results may not be visible withinthe first year, the activation of plant-to-soil feedback processes can provide long-termsupport for your orchard soil.

Pollinator mix at the time of termination at the three orchard sites.

Soil physical properties:

After a first cover-crop season,soil physicalanalyses indicated improved aggregate stability trends at all research sitesand for both mixes, and especially for the Pollinator Mix at the Kern countyorchard, which was heavily compacted. Biologicalanalyses indicated no statistical changes six months after cover crop termination. Early information suggested that considerableimprovements in water infiltration can be found during the second cover cropseason, with higher capacity for rainfall to penetrate the soil during theestablished cover crop, thus suggesting reduced risks of runoff.

Nitrogen credits:

Organic sources of N can offset syntheticfertilizer inputs in the spring if N release from the cover crop issynchronized with tree N demand. When investigating the seasonal pattern of Nrelease from the cover crop, researchers found that C and N turnover from vetchwas very rapid. A total of 90% of N was released within 4 weeks of terminationunder adequate moisture regimes. Almond trees efficiently captured this N,which stabilized at about 30% in existing leaves, and reached 43% in new leaves(Project No 97-AB to 99-AB, A. Berry). A seeded cover crop can contribute moreN to your orchard than resident vegetation: in Tehama county, the cover cropscontained on average 5.5% N compared to 3.7% N in the resident vegetation andhad +4140 lbs/acre more biomass production. In Merced county, the seeded covercrop had 3.1% N compared to 2.4% N in the resident vegetation and had +1535 lbs/acremore biomass. Compared to bare orchards, the N contribution of the cover cropamounted to +82 N lbs/acre in Merced county and +126 N lbs/acre in Kern county.

Providing bee forage :

Research shows that flowering cover crops cansupport bees by providing habitat and nutrient resources before and during almondbloom (Lundi et al., 2017). However, in California, cover crop floweringoften peaks after the start of almond bloom. Planning the timing of cover cropflowering with tree bloom is difficult due to yearly weather changes and mowing.However, early planting (in September-early October) followed by one or two fallirrigations in dry years can help. In the cover crop mixes tested, Brassica speciesappeared more attractive to bees than other species. The cover crop did notdetract from bee visitation in the trees or significantly impact almond nutset.

Controlling weed pressure:

Winter cover crops can be aneffective weed management tool in California perennial crops, with the caveatthat good cover crop establishment is essential. Poor establishment due tolower rainfall and reduced light in mature orchards can substantially reduceweed suppression benefits. Research results suggest that cover crop competitioncan reduce overall weed diversity. Both cover crop mixes effectively reducedwinter weed populations compared to the standard herbicide treatment.

Suppressing parasitic nematodes:

Brassica species’ capacity as abiofumigant has been well-demonstrated. However, the biotoxic activity ofbrassica and mustard cover crops remains variable and relatively low comparedto commercial fumigants. The two cover crop mixes (Soil Mix & PollinatorMix) were evaluated for their biofumigation potential, as well as other speciesto identify options and confirm host potential. Greenhouse assays show thatwhile most species tested suppressed root lesion nematodes, the mostsuppressive cover crop species were sunn hemp, Balansa clover and medic. Theleast suppressive was lupine. Cover crops that suppress the root lesionnematode did not necessarily suppress ring nematodes. There was largevariability in the suppression potential of the different clover types.However, rose clover was unique in that it effectively suppressed both nematodepopulations. Compared to the clovers, sunn hemp was less suppressive to thering nematode. Ultimately, the best-suited cover crop species for biofumigationwill depend on your orchard’s pre-existing nematode population. It’s thereforeimportant to soil sample before selecting your cover crop species.

Orchard alleyway in a cover cropped orchard at the time of harvest. There were no observable cover crop residues at that time. As such, cover cropping did not interfere with harvest operations.

Potential challenges

Residues at harvest:

Cover crop termination post-bloomusing mowing and herbicide was effective; there was no observable cover crop residuesat harvest, includingpotentially problematic species like vetch. As such, cover cropping did notinterfere with harvest operations. Conditioners (for example, StickJack byJackrabbit) can be used right before harvest to help remove any branch residuesor other material. For growers who do not have access to a conditioner,additional mowing in the summer can be very effective to accelerate thebreak-down of residues.

Interference with sanitation and NOW pressure:

Cover cropping does not seem to interfere withand can even facilitate NOW sanitation by improving trafficability in thewinter for shaking and mowing. Soil cover can provide overwintering sites forbeneficial insects and predatory mites and may also negatively impact theoverwintering success of navel orangeworm (NOW) in mummy nuts on the ground. However,this does not necessarily translate into higher densities of beneficials in thetree canopy or significant benefits. Previous research has shown mixed resultswhen comparing the impact of 10 cover crop mixes on the numbers of beneficialand parasitic wasps (Project No 1994-00 and monitoring of the BIOS project No96-BIOS, L. Hendricks). In our study, nut infestation by NOW was the same inbare and cover cropped orchards. Overall, shaking the trees for mummies isstill necessary and feasible with cover cropping. It is possible to combinecover crop mowing at termination with flail-mowing of the mummies in theorchard.

Water usage:

Previous research indicated that, althoughresident vegetation can result in up to 35% more water usage compared bare soilmanagement, the use of selected cover crop species (i.e. bromegrass) results insimilar seasonal water usage as in bare orchards (Pritchard et al.,1989). Researchers are still investigating mechanisms by which improved waterdynamics with cover cropping can secure long-term improvements for water useefficiency. This includes increased water infiltration and reduced runoff,increased water holding capacity through improved soil structure, dew moisturecapture by cover crops and the buffering of soil temperatures altering waterevaporation and water loss. If terminated on time and controlled around thetree, the competition for irrigation water can be minimal. If you are seedingearly in the Fall, prior to the first rains, irrigating (one to two-12h sets)with micro sprinklers will help accelerate germination in seeded alleyways. Therest of the winter, the cover crop can be rain-fed without additionalirrigation.

Frost risks :

A main concern is that cover croppingcould reduce soil-to-tree heat transfer and therefore, increase damage duringsensitive frost nights. In this study, although the cover crop buffered top-soiltemperatures, no ambient air temperature differences were found from 3 feet andabove, which suggests that cover cropped orchards may not experience higherfrost risks. However, this is based on one season of data. It has been shownthat winter ground cover can significantly increase surface soil temperaturescompared to bare soils in windy situations (Project No 87-O7, R. Snyder). Mowingof the cover crop and irrigating for frost control can be done anytime, if thethreat of frost occurs.

Cover cropmanagement: how to get maximum benefits and minimize tradeoffs?

Selectingthe right mix for your objectives

Keep in mind thatwhat you seed is not necessarily what you get: in this study, anidentical seed mix produced three different cover crops across the Central Valley.The Soil Mix was composed of 60% white mustard in the Tehama County orchard,whereas it was composed of 59% ryegrass in Merced. As a result, their C:Nratios varied from 10:1 to 18:1. However, it is also important to note thatcompared to resident vegetation, the seeded cover crop produced up to 300% moredry matter biomass with the same amount of water. This must be considered whenselecting cover crop species for a particular purpose in an orchard. Aside fromseed mix selection, managing the cover crop may be key in obtaining desiredbenefits. Furthermore, having a high species diversity will increase your covercrop system’s adaptability to seasonal climate variation and will improve yourchances of obtaining adequate biomass and groundcover.

  • Criteria to considerwhen choosing species:
    • Yourmain objectives: pollinator habitat, pest control, soil health
    • Orchard’sirrigation system & water demand of cover crop species
    • Soiltype & tillage
    • Biodiversity& functions of the cover crop
    • Price& availability of seeds

Seedingand soil preparation

  • Seeding time: September-November: itis preferable to seed right before the first rains in the Fall to improvetrafficability, avoid compaction and avoid seeding during mummy shake.Especially for growers interested in having bee forage prior to and duringalmond bloom, seeding early in the Fall (September-early October) is importantto ensure that the cover has enough time to get to the bloom stage.
  • Soil preparation: Especially if you arecover cropping for the first time, consider seed bed preparation to optimizethe seed-to-soil contact. This will be especially helpful for smaller seedmixes (i.e. mustard mixes). Very light disking (1-2 inches of depth) can beused for soil preparation. However, we found that disking can leave unevenground, which has to be relevelled for harvest. We found that using an orchardmulcher set at 1 inch of depth was successful at both preparing the seed bedand keeping the ground even. For no-till seeding, mow any vegetation to limitcompetition at cover crop germination.
  • Seeding width: Consider that yourorchard may have different soil conditions depending on your managementpractices. In orchards in which berms are cleaned and residues piled in the alleyways,higher C and N contents were found in alleyways compared to the berms.Similarly, the location of your irrigation wetting zone may affect soilbiological activity and create differences in soil health in your orchard. Ifyou are looking to improve water infiltration and/or soil health in yourorchard, consider where you are placing the cover crop. A wider seeding, whichoverlaps with your irrigation wetting zone and is closer to the berms has morepotential to impact your orchard’s soil health and address water infiltrationproblems.
  • Seeders: For younger orchards,we were able to use regular forage grain seeders (John Deere 750 No-till graindrill – 15 feet wide) with only one pass per middle. For mature orchards,avoiding damage to the branches is the key challenge. We used compact drills(Schmeiser Compact No-Till drill – 6 feet wide or the Great Plains No-TillSeeder NTS2511 – 11 feet wide) and did two passes to get wide cover cropseeding (up to 20 feet wide) whilst avoiding branch damage. If your seeder hasno press-wheels, drag a chain behind the drill or roll the field followingseeding to improve the seed-to-soil contact.

Standmanagement and termination

Mowing in the winter can be necessary tocontrol the cover crop stand height but this is not frequent. However, forsanitation purposes, growers will need to flail-mow the mummies during thewinter season. To favor cover crop growth and ensure flowering during almondbloom, flail-mowing either early in the season at first rains and/or late inthe season, at cover crop termination is best. Cover crops might also need tobe mowed to decrease frost risks post-dormancy. Cover crops can be terminatedby spraying herbicides, mowing or rolling. For termination, a successfulstrategy is to mow and spray with a herbicide after approximately 1 week. Covercrop termination (March-April) was successful at all research sites and left noresidues at harvest in this study.

AvailableResources: Thereare numerous ways to manage cover crops: disked green manure systems, mowedperennial systems, self-reseeding cover crops, etc… To consider other covercropping options, here are a few resources below.

Program, UCDavis: https://asi.ucdavis.edu/programs/ucsarep/research-initiatives/are/nutrient-mgmt/cover-crops

  • Jackson, L.E., Wyland,L.J., Klein, J.A., Smith, R., Chaney, W.E., & Koike, S.T. (1993) WinterCover Crops can decrease soil nitrate, leaching potential. CaliforniaAgriculture, 47 (5): 12-15. PDF available at: http://calag.ucanr.edu/Archive/?article=ca.v047n05p12
    • Mitchell, J.P.,Shrestha, A., & Irmak, S. (2015) Trade-offs between winter cover cropproduction and soil water depletion in the San Joaquin Valley, California. PDFavailable at: http://casi.ucanr.edu/files/239977.pdf
    • Pritchard,T.L., Sills, W.M., Asai, W.K., Hendricks, L.C. & Elmore, C.L. (1989)Orchard water use and soil characteristics. California Agriculture:23-25. PDF available at: http://ucmanagedrought.ucdavis.edu/PDF/Prichard%20et%20al%201989.pdf

ResearchTeam- Ateam of UC Davis, UC Berkeley, UC Riverside and UCCE researchers have been collectivelyevaluating the benefits-to-tradeoffs of cover cropping compared to conventionalbare soil and resident vegetation. This work is funded by the Almond Board ofCalifornia and is currently in its second year. For the full description of theproject, visit: https://almondcovercrop.faculty.ucdavis.edu/

Acknowledgements-

We are grateful to the Almond Board ofCalifornia for their support and funding (Project No: 18-STEWCROP7, 18-ENTO22-Wilson,18-HORT12-Hanson and 18-POLL13-Williams). We thank Project APIS and KamprathSeeds for providing the seeds for this study. This project is also financiallysupported by the Western Sustainable Agriculture Research and Educationgraduate fellowship (WSARE), the CDFA Healthy Soils program, and Annie’sSustainable Agriculture scholarships. 

More here:

To followthe on-going Almond Cover Crop Benefits-to-tradeoffs Assessment (Year 2 of 3),visit https://almondcovercrop.faculty.ucdavis.edu/.

Shareyour experience!

Take theCalifornia-wide Almond Growers’ Survey at https://almondcovercrop.faculty.ucdavis.edu

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