Adapted from the article “Planting a successful annual reseeding cover crop” by Janine Hasey, UCCE Farm Advisor, Sutter, Yuba, Colusa Counties and Mark Cady, Community Alliance with Family Farmers in the August 2008 Sacramento Valley Walnut News.
Three cover crop systems used in walnut orchards are winter green manure crops that are mowed or cultivated in spring, annual reseeding legumes or grasses, or perennial sods. This article focuses on annual reseeding cover crops although the planting guidelines are similar for successfully establishing any seeded cover crop.
Annual reseeding cover crops are planted initially in the fall and managed during the spring and early summer to allow plants to naturally reseed. They can only be maintained in non-cultivated orchards. Once seed has fully matured in early to mid-June, the annual reseeding cover crop is mowed. If managed properly, the cover crop will re-seed annually and re-establish the following fall and winter so costs incurred will be initial seed and planting costs only.
Why are annual reseeding cover crops so well suited to orchards in our area? A primary advantage to an annual reseeding cover crop in high rainfall areas such as the Sacramento Valley is better fall and winter orchard access due to firmer ground. Early fall rains can make it very difficult to harvest in walnut orchards cultivated for weed control or incorporation of a winter green manure cover crop. Other advantages may include weed suppression, less runoff and reduced labor and diesel costs from mowing. Allowing resident vegetation to grow during the winter with mowing in the spring and summer has many of these same advantages but may have less biomass than a seeded cover crop.
For any fall-seeded cover crop, the best results will be achieved with the earliest possible planting date. Any time in October to early November is suitable for planting a cover crop in the Sacramento Valley. By December, soil temperatures are too low to provide quick and consistent germination while competition from resident vegetation becomes more of a problem. A cover crop can be planted in young non-producing walnuts in October. In producing orchards, plan to seed just after harvest but before significant leaf fall for best stand establishment. Make sure the seed and equipment are lined up before walnut harvest begins.
Cover crops can be seeded with various planting equipment including a no-till drill, a standard grain drill or a broadcast seeder. No-till drills can be rented from seed suppliers or Resource Conservation Districts. Grain drills are standard equipment for most field crop growers. Broadcast seeders are less available, though a fertilizer spreader can be used with some difficulty. In a pinch or on very small acreage, a worker with a large belly grinder on the tailgate of a pickup truck will suffice. Small seeded legume mixes are typically seeded at a rate of 25 to 30 lbs per planted acre.
The kind of ground preparation required depends on the seeding equipment. Whatever the method, you will want to have a flat, level surface that is ready for harvest without any further ground work. A no-till drill requires little or no ground preparation and will plant directly into most surfaces. A contact herbicide treatment (not a pre-emergent) applied at or before seeding will prevent weed competition.
Both grain drill and broadcast seeders require a soft surface into which small cover crop seed can be placed. Work up the top two inches of soil with a harrow or disk until the surface condition is fairly fine. Plant the seed right away. Seed should be buried in just the top quarter- to half-inch of soil. A no-till or a standard grain drill can place seed fairly precisely. If you broadcast the seed onto soft ground, a single pass with a ring roller should move seed and soil around enough to cover the seed. Normal fall and winter rainfall will be sufficient for germination and winter growth. A light irrigation may be needed by late November if there hasn’t been sufficient rainfall for seed germination.
Common problems with cover crop seeding often involve poor timing. Early rain can stop you at any step of the seeding process. If rain falls after ground preparation but before seeding, weed seeds get a head start on the cover crop and the ground can seal up making it impossible to bury the seed with a roller or a standard grain drill. This is a good reason to have your seed and equipment lined up before walnut harvest is over.
A subterranean clover mix with varying maturity times is preferred by many growers because it grows lower to the ground so doesn’t interfere with other cultural practices, it can be mowed more often if needed without decreasing seed production, and it fixed some atmospheric nitrogen. Another cover crop mix that performs well but grow much taller was a nitrogen fixing legume mix with several clovers including Persian clover. These lower growing legume mixes were preferred to those that included the grass soft chess (Blando brome) mainly due to the taller grass but also due to potential benefit of nitrogen fixation. Supplemental nitrogen is needed in mature walnut orchards and should be applied as a broad band in the tree row rather than broadcast or clover seed production will decrease over time.
For more information, see ANR Publication 21627, “Cover Crops for Walnut Orchards”.