Fruit & Nut Research and Information Center
Fruit & Nut Research and Information Center
Fruit & Nut Research and Information Center
University of California
Fruit & Nut Research and Information Center

Walnut Fact Sheet

  • Family: Juglandaceae
  • Genus: Juglans
    • Commercially important species:
    • English or Persian walnut (nuts); Juglans regia
    • Eastern black walnut (timber); J. nigra
    • Northern California black walnut (rootstock); J. hindsii
    • Paradox (rootstock); J. regia X J. hindsii
  • Related species: Pecan - Carya illinoinensis
  • English or Persian Walnut Juglans regia
  • Description: Deciduous tree; chambered pith; silver-grey bark; large pinnate leaves with (5)7-9 leaflets; irregularly dehiscent husk; monoecious; dichogamous; male flower = catkin, female flower = pistillate spike with 2(-5) flowers; wind pollinated; bearing habit terminal or lateral; generation time 2-7 years.
  • Origin: Central Asia, Himalayas to E. Europe
  • History of cultivation: Millennia
  • Site requirements: Deep fertile soil, warm temperate climate, water availability. (Rain-free growing season reduces blight incidence.)

Cultivation in California

  • History: Late 1700s Spanish missionaries bring hard- shelled walnuts to CA; 1860s-1870s introduction of "soft- shelled" walnuts and first commercial orchards; 1940- 1960 industry moves to northern CA.
  • Yield: 2 - 3 ton/acre (CA average 1.25 ton/acre)
  • Cultivars: Franquette, Hartley (most common), Payne, Vina, Chico, Howard, Sunland, Chandler (most common in new plantings), Tulare (new).
  • Rootstocks: Paradox, highly recommended and vigorous; J. hindsii, old standard; J. regia, recommended only where blackline disease is epidemic.
  • Propagation: Common - grafting or budding on seedling rootstock. Under investigation - rooting cuttings to produce clonal rootstock and own-rooted cultivars.
  • Spacing: Standard = 30' X 30' (48 trees/acre), high density 24' X 24' (76 trees/acre), hedgerow 11' X 22' (180 trees/acre)
    Irrigation: Flood, solid set sprinkler, microsprinkler
  • Training System: Modified central leader
  • Nutrition: Deficiencies-nitrogen below 2.3%, potassium below 0.9%, zinc below 15ppm; Toxicities -boron over 300 ppm, sodium over 0.1%, chlorine over 0.3%.
  • Harvesting: September-October when 95% nuts hullable. Trees are shaken, nuts swept into windrow, collected, hulled, dried to 8% moisture.
  • Marketing: Quality standards and marketing policies determined by Walnut Marketing Board. One large cooperative (50%) and many independent handlers (50%) market walnuts. 35% exported. 1/3 marketed in shell, 2/3 marketed shelled.
Production Problems
  • Environmental: sunburn; cold injury; nutrient deficiency/toxicity; water stress
  • Insect/Pest: Codling moth (Cydia pomonella), Navel orangeworm (Amyelois transitella), Walnut husk fly (Rhagoletis completa), aphids, scales and mites; nematodes (Pratylenchus vulnus)
  • Disease: Blight (Xanthomonas campestris); blackline (cherry leafroll virus); root and crown rots (Phytophthora spp., Armillaria mellea); deep bark canker (Erwinia rubrifaciens); crown gall (Agrobacterium tumefaciens).

Research

  • Development of new cultivars through breeding and genetic engineering; Replacements for methyl bromide; Codling moth control; Blight control; Clonal propagation; Harvest molds
    • Integrated Pest Management for Walnuts. (1993) University of California Division of Agriculture and Natural Resources. Pub. 3270
    • Walnut Orchard Management. (in press) UC Division of Agriculture and Natural Resources. Pub. 21410
    • Maintaining the Competitive Edge in California's Walnut Industry: Trends, Issues and Challenges. (1994) Agricultural Issues Center, UC Davis

    Prepared by Dr. Gale McGranahan 1995

References
Page Last Updated: February 6, 2011
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