Walnut Orchards and Cover Crops: Telling the Story of the Understory

By Melina Sempill Watts

Photo by Kandi Manhart

In Colusa, Calif., Daniel Unruh owns and operates a walnut orchard, where his ongoing use of cover crops on Columbia silty clay loam has given his orchard floor a soil organic matter that varies from 3-4 percent. Unruh started at a lesion nematode count of over 5,000 in 2012; in 2018, that population was zero, garnering attention from other growers. In partnership with Glenn County Resource Conservation District, Colusa County Resource Conservation District and the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), Unruh invited growers to come and see his orchard and its soil.

In addition to growers from Colusa and Glenn Counties, guests came from as far as San Luis Obispo, Stockton and Los Angeles, drawn by Unruh’s success with cover crops and a chance to see a rainfall simulation by NRCS West Regional Soil Health Specialist Kabir Zahangir.

Part of the draw of cover crops is how they help the orchard’s soils by increasing absorption. When the soil stays covered, the surface of the soil stays moist a lot longer,” Unruh said. Cover crops also help soils fare better during rainfall. “When rainfall hits bare ground at full speed, it causes surface crusts to form as the heavy impact separates the clay that then settles on site and seals, making it more difficult for soils to absorb water afterwards. Cover crops reduce rainfall impact intensity, preserving and increasing soil structure and thus soil permeability.”

Zahangir’s rainfall presentation helped showcase the stark contrast between soil in Unruh’s orchard and soil from a field with conventional farming practices. By using a tray of soil from each area, Zahangir revealed how Unruh’s darker, richer soil fare infinitely better against conventionally farmed soil.

Unruh explained to participants that trees are most productive when they live in a healthy ecosystem on farm, and this means encouraging biodiversity in soil to create optimal growing conditions. He describes grasses as living pumps that “pump carbon from the atmosphere into carbon in the soil,” and explained how grass creates soil structure. “The grass roots that grow won’t go down the same root hole. They have to find an easier route, creating pore space in soil,” Unruh said. “Microbes live off both living and decaying roots, and exude the glues that create functional soil structure.”

On Unruh’s orchard, “Soil biology creates our nutrients, rather than fertilizer.” In 2018, Unruh used zero synthetic fertilizer for his walnut crop. Unruh’s one certainty: focusing on soil health is the answer.

Photo by Kandi Manhart

Unruh calls what he does, “Farming the sun,” and champions holistic practices, emphasizing that the beautiful flowers on his cover crops spurred healthier bee and pollinator populations. Unlike almonds, walnuts use air movement to pollinate and thus do not require bees to pollinate. Nevertheless, a healthy population of pollinators is beneficial for biodiversity, for other food crops and is also key to biological control of pest bugs.

“The healthier your soil, the healthier your plants,” Unruh said. He advised growers starting out with cover crops to take their time, saying it’s “Not a situation where you have to do the whole thing before next year. Implement little steps. Try it. Go for it with new ideas or…bingo moments.”

Photo by Kandi Manhart

Unruh described his current brassica-dominant cover crop mix while discussing the benefits and difficulties of other mixes. He’s in favor of veering towards a wide array of seeds to mimic biodiversity, allowing different plants to do different things for soil health. He said that a triticale and rye mix can be beneficial in excessively compacted soil, as their high root pressure allows them to easily dig through the soil. If a farmer is looking to open up their soil, he suggests Daikon radishes. He likes Phacelia, bell beans and peas for nitrogen improvements. To determine the right mix of cover crop seed types, Unruh recommends trying the free smart mix calculator from Green Coverseed in Nebraska.

“Not one size fits every farmer,” Unruh said. “Know your situation and understand what is going on in your soil.”

If you are interested in learning more about soil health or receiving information about Glenn County Resource Conservation District’s field days, held in partnership with the USDA-NRCS and UC Cooperative Extension, please contact Melina Watts via email at melina[at] or by calling the Glenn County RCD’s office at 530-934-4601 x5.

Melina Watts is the soil health coordinator for Glenn County RCD. Visit for more information.

Tags: Cover Crops, walnut, orchard

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