Good irrigation management is a major factor in quality walnut production.
Kernel size and pellicle color are key indicators of walnut quality. Good size nuts with light color pellicle bring higher prices for growers.
UCCE Integrated Orchard Management Specialist Bruce Lampinen said starting irrigation too early in the season is a common mistake and can make an orchard sensitive to water stress later on. Putting on more water than the trees need is most likely responsible for much of the walnut pellicle darkening, Lampinen said. He noted that Chandler is generally less sensitive to wet conditions than Howard or Tulare.
Nut quality issues can be associated with both current growing season conditions and previous year conditions.
Factors that can affect walnut quality include variety, leaf loss or shading, overly wet orchard conditions or lack of water, disease or insect damage. Management decisions, including hedging and shaking nuts with green hulls and not removing hulls immediately, also impact nut quality.
Lampinen’s studies showed that walnuts with quality problems, such as shriveled kernels or bronze pellicles, came from spurs receiving limited direct sunlight each day. He noted most shrivel and pellicle color issues occurred in lower, shaded positions where light levels were lowest.
If an orchard is approaching its yield potential, some quality problems are likely to occur in lower canopy due to shading-related issues.
Lampinen said in order to keep lower-canopy positions alive and productive, at least a half-hour of direct sunlight is required. As an orchard gets above about 70% midday canopy light interception (or yields above 3 tons per acre), these lower positions will be continually shading out due to lack of light, and this results in quality problems.
Vigorous shoot growth following mechanical hedging operations can result in a dense canopy edge, which increases shading and quality problems.
Sunburn from leaf loss causes darkening of the pellicle on the sunburned side of the nut. Sunburn is also usually worse on excessively wet trees.
Quality impacts due to previous year conditions include peewee nuts.
Peewee nuts are not related to late bloom, but are likely from buds that formed in low light position the previous year and had limited carbohydrate reserve. If these positions are then exposed to sunlight due to hedging or pruning, they only have sufficient resources to produce small nuts. A less extreme case of peewee nuts are medium-size nuts with brown adhering hulls and may have discolored pellicles.