Dixon Ridge Farms owner Russ Lester, who has farmed walnuts in California’s Yolo and Solano counties since 1979, knows there are multiple avenues to achieve more efficient production.

From an inventive irrigation system for his trees to the use of walnut shells to generate electricity, this grower-processor has embraced practices to help ensure the longevity of his operation.

Based at Winters, Dixon Ridge Farms produces, processes, and markets organic walnuts in domestic and international markets. Lester also buys, processes, and markets walnuts from other organic growers.

Dixon Ridge Farms’ 400 walnut acres were certified organic in 1991, yet Lester began using integrated pest management practices and reduced tillage much earlier.

“We try to go beyond what is required for organic certification. We are looking at long-term sustainability,” Lester said.

Irrigation systems at Dixon Ridge Farms were adapted to work with planted ground cover in walnut orchards. After experimenting with various systems, Lester moved the plastic lines off the ground, lacing the lines through lower tree branches. He used low pressure sprinkler heads hanging down to deliver the water.

Larger drops from the sprinkler heads reduce evaporation loss. The system can achieve 60-100 percent coverage with this system. With hoses and sprinklers off the ground, the ground cover is easier to manage with less damage from harvest equipment.

Lester says cover crops are a key part of the farm’s pest management system, providing habitat for predatory insects. His orchard cover crop mixture was developed with help from a University of California researcher and a seed expert. The cover crop is a food source for insect predators until pest insect levels are high enough to provide food.

Different legumes are part of the vegetation mix, he says, since a longer flowering period ensures a better food source. The determinant legumes die back after going to seed and become mulch, holding soil moisture. By harvest time, most of the mulch has decomposed.

Lester says Botryospaeria fungal disease is a major disease in his orchards. In addition to controlling a scale that vectors the disease, diseased wood is pruned and chipped.

According to Lester, a UC Cooperative Extension trial showed a pathogen reduction as the wood decomposed in the field. The chips do not interfere with harvest activity, he notes, since the moist environment on the orchard floor helps with decomposition.

Lester plans to compost the walnut hulls to reduce pathogen levels before returning the hulls to the orchard floor.

Ten years ago, Dixon Ridge Farms became the first on-farm user of a 50-kilowatt biogas powered generator to convert walnut shells into energy. Before then, shells were shipped to a traditional biomass plant for energy conversion.

With their own farm facility and subsequent upgrades in 2012 and 2014, Lester now produces all the energy needed to operate the processing plant, and achieves carbon negative status by returning part of the carbon in the walnut shells to the ground.

Lester’s upgraded Biomax 100 units add energy with fuels to their walnut drying facility, plus generate electricity. Each of the two units produced 850,000-kilowatt hours of electricity annually, producing $136,000 in electricity and $34,000 in heat to offset propane use in the facility’s dryers.

The Biomax 100 equipment converts shells into combustible gases through a process called pyrolysis. Lester describes it as capturing gas given off when a log is ignited in a fireplace. The pyrolysis gases are filtered and used for energy.

The by-product of the process, a sand-like ash called biochar, is a stable form of carbon which Lester returns to the soil. Besides carbon, biochar is 4 percent nitrogen. The biochar, compost, and nitrogen from the legume cover crop supply the nutrients needed for the trees.

“With this system, we can sequester carbon in the soil plus return part of the nitrogen to the orchard,” Lester said.