Reseeding annual legume cover crops can contribute significant nitrogen to tree crops. We seeded several cover crop demonstration trials in walnut and peach orchards in 2007 and 2008 in northern California. Results showed the various nitrogen fixing clover mixes sampled in May 2009 from above ground biomass ranged from 31 pounds to 250 pounds of nitrogen (N) per acre averaging 92 pounds N per acre.
Of course, not all of this nitrogen will be available to the tree. Some of the nitrogen will be lost to volatilization and some of it will only be released slowly to the crop. With good cover crop management, however, growers can expect the accumulated biomass of the cover crops to deliver a consistent amount of nitrogen each year. They can achieve these results by allowing for reseeding of the annual legume cover crop year after year by mowing after seed maturation in late May or June. Our demonstration sites indicated that some of these orchards in 2009 did not need any additional nitrogen applications and had leaf values with adequate nitrogen based on July leaf samples.
Reseeding annual cover crops are only suitable for no-till or non-cultivated orchards. They are planted in the fall, grow during winter and early spring during the rainy season, and mature their seed by late spring or early summer. A primary advantage of planting this kind of cover crop in high rainfall areas is better fall and winter orchard access due to firmer ground. Early fall rains can make it difficult to harvest in walnut orchards cultivated for weed control or incorporation of a winter green manure cover crop.
Growers can select from a large number of clover varieties for their reseeding annual cover crop seed mix. For most growers, we recommend planting mixes of Subterranean clovers (subclovers, imported from Australia) only. These varieties are more forgiving: They will re-grow more easily after low mowing; reseed easily; and not get as tall as many other clovers. Other common varieties that produce good quantities of nitrogen include rose clover, crimson clover, and Persian clover. These clovers must be managed carefully to ensure good reseeding. Bur and barrel medics (also called burclover) are often included in annual cover crop mixes for their hardiness and copious reseeding. Medics should not be planted in peach orchards because they are attractive to Lygus and Calocoris insects. Seed companies that carry orchard cover crop seed will sell mixes developed in-house or made to order.