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.
- 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).
- 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