Seeding into Moisture from Irrigation or Rainfall
Some growers, particularly in the San Joaquin Valley and in the desert valleys of Southern California, preirrigate then sow into moist soil. Preirrigation of fine- textured soils such as clay loams and clays should be done early enough in the fall to allow time for the topsoil to dry sufficiently to permit seedbed preparation and sowing before rain begins in mid to late November. Preirrigation can be done later on loam and fine sandy loam soils that drain more quickly. One advantage of preirrigation is that weeds germinate before seeding and can be removed by tillage during seedbed preparation. Preirrigation can also provide ideal soil-water content in the seedbed so that uniform germination begins soon after seeding.
Seeding into a dry seedbed and then irrigating to germinate the seed is a second option. In the Central Valley, early seedings, in mid-November to early December, are more successfully germinated by irrigation. Irrigating early seedings assures warm soil temperatures at germination, while the risk of rainfall immediately after irrigating is relatively low. Significant rainfall after irrigating prolongs standing water and creates poor aeration around the seed, retards seedling growth, and may lead to seedling dis- ease. Irrigating for germination is more successful on fine sandy loam and loam than on silt loam, clay loam, and clay. The advantage of irrigating to germinate seed rather than waiting for rainfall is that rainfall is unpredictable. However, in years when rain- fall during December is insufficient for germination and the field is not irrigated to germinate the seed, emergence will be late, the production season will be shorter, and yields will be lower.
Seeding into a dry seedbed and waiting for rain is a third alternative. Seed retains its viability in dry soil for an extended time (several months), and stands will generally be adequate once rainfall induces germination. In California’s Central Valley production region and similar areas, as long as seed germinates by the end of December, losses in yield potential will be minimal. This alternative is cost-effective in areas where surface water is unavailable and groundwater is expensive to pump or is of undesirable quality.