Planting
Nearly all rice fields in California are planted with certified seed. Certified seed ensures varietal purity, high germination, and freedom from weed seed, including red rice. Red rice, a serious weed in the southern United States and South America, has been virtually eliminated in California through seed certification and water seeding.

Before planting, seed is soaked in water for 24 to 36 hours (fig. 16), drained for 18 to 24 hours, and planted by airplane directly into the flooded paddy (fig. 17). Soaking initiates germination and displaces air in the seed coat, increasing seed weight by approximately 25 percent so it will sink to the soil surface. Unsoaked seed may float on the water surface and distribute unevenly in the field. Planting presoaked seed speeds germination and seedling emergence by two days, compared to planting dry seed. A disadvantage of pregerminated seed, however, is that once germination has begun, the seed must be planted. A prolonged soak/drain period reduces seed viability and may cause complete mortality from overheating or oxygen deprivation. If the soak/drain period has resulted in radicle emergence, mechanical damage to the seedling may occur at planting.

	Figure 16. Before planting, rice seed is soaked in bins for 24 to 36 hours, then drained for 18 to 24 hours.

Seed is planted at approximately 150 pounds per acre (dry weight basis). Low seeding rates may provide an inadequate buffer against stand losses from seedling disease and other stand establishment problems. High seeding rates may provide overly dense stands, increasing the severity of diseases, interplant competition for nutrients, and lodging. The number of seeds per square foot from seeding rates of 150 pounds per acre may vary from 45 to 60 because seed weight differs by as much as 25 percent from the smaller- to the larger-seeded varieties. Optimum yields may be obtained from stands of 15 to 20 established seedlings per square foot in the seedling stage. Stands below five to seven seedlings per square foot may require reseeding.

Figure 17. Soaked and drained rice seed is seeded by air into a flooded paddy, where it will sink to the soil surface.

Water management
Rice in California is grown primarily in a continuously flooded, flow-through system. Water is supplied in a series from the topmost to the bottommost basin, and is regulated by irrigation boxes placed in the levees between basins. However, outlets at the bottom of the system must be blocked during the growing season to reduce water movement for long periods in order to reduce pesticide release. Alternatively, water may be supplied to each basin via a head ditch or recirculated from the lowest point of the system.

Levees constructed from soil are spaced at intervals of 0.10 to 0.25 feet elevation differences to maintain uniform water depths within each basin. Precision leveling with laser-guided equipment permits the formation of large rectangular basins (fig. 18). Levee spacing may depend more on wind protection than on slope, since wave action in large expansive basins may cause seed burial by soil or seedling drift. Forming levees at 90-degree angles to the prevailing wind direction and restricting basin size to 20 acres or less helps control water depth and minimize wind damage. Rice levees may be formed each year where rice is rotated with row crops or in fields with steeper slopes; permanent levees are more common in rice-only cropping systems and where fields have little or no slope. Field roads doubling as levees are often constructed around large fields to improve access and help contain water.

Figure 18. Precision leveling with laser-guided equipment permits the formation of large rectangular basins.

Water depth in a rice field is controlled by rice boxes (weirs) placed in levees (fig. 19). Depth is increased or decreased by adding or removing "flash" boards in the boxes. Regulations to control pesticide movement into public waterways from rice fields mandate by law that the flow through outlet boxes be stopped for specific periods following the application of certain pesticides. Therefore, it is imperative to have an irrigation system that is easily controlled either within a field or within a recirculating water system.

Figure 19. A rice box or weir placed in a levee helps control water depth in a rice field. Photo by Jack Kelly Clark

The goal of early season water management is to establish a vigorous, healthy, weed-free crop stand. Shallow water promotes rice growth and root anchorage, but also favors weed growth. Deep water (7 to 8 inches) delays early season growth and tillering, but also greatly inhibits barnyardgrass and watergrass, the most competitive weed species. A continuous flood of 4 to 5 inches generally provides good stand establishment and, coupled with a herbicide, good weed suppression. Drainage at any time during stand establishment may stimulate the germination and growth of new weeds. Early drainage may also delay rice heading.

Water depth is commonly raised to 8 inches or more soon after panicle initiation, submerging the reproductive organs and acting as a heat sink to buffer against cold night air temperatures. Exposure of the developing panicle to temperatures of 55°F or lower at 10 to 15 days before heading may significantly increase floret sterility (blanking).

Growers greatly reduce or eliminate water inflow about five weeks before harvest, allowing water in the field to subside in preparation for drainage. Very flat fields may be difficult to drain. In these fields, the construction of temporary internal drains in each basin may be necessary before planting. In poorly drained fields, it may be necessary to cut channels through the levees to remove excess water.

The timing of drainage is critical, since residual moisture must be available throughout grain filling, but the soil should be dry enough at harvest to support heavy equipment. Draining too early results in incompletely filled kernels that break or crack in the harvester and produce low milling yields. Draining too late results in muddy soils that reduce combine efficiency. Generally, fields are drained when the panicles are fully tipped and golden. Rice variety, soil type, and lateness of the season influence drainage strategy. Inclement weather at maturity can delay harvest, but water management systems with proper slopes and good drainage provide the best insurance for an efficient harvest.

Typical seasonal water delivery for California rice has been estimated to be as high as 6 to 7.5 acre-feet in continuous, flow-through systems. However, 3 to 3.5 acre-feet are required for evapotranspiration, and in most rice soils 0.5 to 2 acre-feet go to deep percolation. The balance flows through the field and may be reused many times within irrigation districts before it is returned to public waters. Although rice is flooded throughout most of the growing season, net water use is similar to that of pasture, alfalfa, cotton, and several tree and vegetable crops.