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Walnuts – Juglans spp.

WALNUT (JUGLANS SPP.) TAXONOMY

Walnuts are members of the relatively small Juglandaceae family, containing about 60 species, 21 of which are placed in the genus Juglans. Nuts from all species are edible, although none are as large and easily cracked as the Persian or English walnut, Juglans regia L.

Persian Walnut Cultivars
‘Hartley’ is an old, terminal bearing but productive cultivar, and as of the mid 1990′s, was still the major cultivar in terms of acreage. It produces large nuts with light kernels and attractive shells, and leafs out late enough in spring to avoid walnut blight. Other “Payne-type” cultivars popular in California are ‘Chandler’, ‘Serr’, ‘Vina’, ‘Ashley’, ‘Tehama’, ‘Pedro’, ‘Sunland’, and ‘Howard’.

Carpathian Walnut Cultivars
These are cold hardy cultivars of J. regia which can be grown outside of California, in the Pacific Northwest or the mid-Atlantic and southeastern USA. ‘Cascade’ has large nuts with up to 56% percent kernel, and is rated #1 by the North American Nut Grower’s Association.

ORIGIN AND HISTORY OF THE WALNUT (JUGLANS SPP.), HISTORY OF CULTIVATION

J. regia has its origins in eastern Europe, Asia Minor, and points eastward to the Himalayan Mountains. However, there are native Juglans in North, Central, and South America, Europe and Asia. Although native to Europe, it probably was not utilized there until improved forms were imported from Persia. Romans spread cultivation throughout southern Europe. The species came to the new world with English settlers, and to California via missionaries. Today, walnut production is almost entirely located in the San Joaquin/Sacramento valleys of California, where over 5,000 growers and 52 processors (marketers) make up a highly organized and productive industry.

WORLD AND UNITED STATES WALNUT (JUGLANS SPP.) PRODUCTION

World (2004 FAO) – 1,491,152 MT or 3.3 billion pounds. Walnuts are produced commercially in 48 countries on 1.6 million acres. Yields average about 2000 lbs/acre.

Top 10 countries
(% of world production)
1. China (28%) 6. Romania (2%)
2. USA (20%) 7. France (2%)
3. Iran (11%) 8. India (2%)
4. Turkey (8%) 9. Egypt (2%)
5. Ukraine (5%) 10. Serbia & Montenegro (2%)

United States(2004 USDA) – 296,000 MT or 652 million lbs of in-shell nuts. Virtually all are produced in California on about 200,000 acres. The industry value was $375 million, Yield averages over 3000 lbs/acre, and prices paid to growers are 50 to 79 ¢/lb.

For the most up to date statistical data on United States and World production numbers please refer to the following two websites:

World: The Food and Agriculture Organization of the United Nations Statistics Division (FAOSTAT). FAOSTAT

United States: The United States Department of Agriculture National Agricultural Statistics Service (USDA Ag Stats). USDA Ag Stats

WALNUT (JUGLANS SPP.) BOTANICAL DESCRIPTION

Plant
A medium to large tree (to 100 ft in nature, 20-50 ft in cultivation) with spreading crown. Leaves are compound, composed of 7-9 leaflets, which have prominent, herring-bone venation. Leaflets are ovate, with pointed tips and smooth margins. The shuck of Persian walnut does not split into 4 regular sections at maturity as it does in pecan and hickories; it splits, but irregularly.

Flower
All Juglans are monoecious, bearing male and female reproductive organs on separate flowers on the same tree. Although J. regia is self-fertile, it is heterogamous, either protandrous or protogynous depending on cultivar. Catkins (male inflorescences) are borne laterally on 1-yr wood, and pistillate flowers are borne terminally or laterally (newer cultivars) on current season’s wood in spikes of typically 2-3 flowers.

 

 

Walnuts are monoecious and similar to pecan with respect to flowering. Catkins of eastern black walnut, produced laterally on one-year-old wood.

 

Persian walnut female flowers produced terminally on current season’s growth.

Pollination
Walnuts are similar to pecans in that the time of pollen shedding does not always overlap well with the time of female flower receptivity to pollen. Hence, although most walnuts are self-fertile, they sometimes require another cultivar for pollination since the timing of the functions of male and female flowers is different. This condition is referred to as dichogamy (see pecan page for description). Most walnuts are protandrous. As with most catkin-bearing species, walnuts are wind pollinated.

Fruit
A nut. Nuts are borne singly or in clusters of 2-3 on shoot tips. A green, fleshy shuck surrounds the nut, which splits irregularly at maturity. The shell is rough, wrinkled or furrowed, and thin. Nuts are ovoid to round, ½ -2″ in diameter, containing two kernels separated by a thin, papery central plate extending from the inner layer of the shell.

 

Nuts of the ‘Hartley’ (left) and ‘Chandler’ (right) Persian walnut

 

Walnut kernels are the cotyledons of the embryo, as in pecan, but are more deeply furrowed and rounded than pecan

WALNUT (JUGLANS SPP.) GENERAL CULTURE

Soils and Climate
The best soils are deep, well-drained silt loams with pH 6 to 8, as found in central valleys of California.
As the dominance of the California industry suggests, Persian walnuts are best adapted to Mediterranean climates, with dry, hot summers and mild winters. Cold hardiness is a major limiting factor Persian walnut. In California, trees are considered cold hardy to 12-15°F. Carpathian cultivars are far more cold hardy and quicker to acclimate than Persian cultivars. Chilling requirement is 400 to 1600 hr. With the need for late leafing characteristics to avoid walnut blight, cultivars with higher chill requirements have been produced.

Propagation
Common methods are 1) whip grafting, and 2) ring or patch budding. This is commonly done in spring on 1-yr seedling rootstocks in nurseries, but some growers prefer to plant ungrafted stocks and graft in the orchard, after the rootstock becomes established.

Rootstocks
Persian walnut seedlings (J. regia) are the most popular rootstock worldwide, and in areas where blackline disease is a problem. ‘Manregian’ is the selection most tolerant of blackline disease. Trees usually lack vigor and yield efficiency in California when propagated on this rootstock, so Northern California black walnut (J. hindsii) is the most common stock for Persian walnut in California. ‘Paradox’ (J. hindsii x J. regia) is a hybrid of Persian and Northern California black walnut, and generally superior to its parents in several traits. Therefore, ‘Paradox’ is the most preferred rootstock in California, but high variation among seedlings and susceptibility to blackline disease limits its use somewhat.

Planting Design, Training, Pruning

Traditional orchards are usually planted at densities of 50-70 trees/acre initially, and thinned to 30-50 trees/acre (30×30′ or 40×40′) as crowding occurs over time. Pollinizer cultivars are planted in solid rows (cross-wind) at selected intervals, usually 10%. Hedgerow orchards of lateral bearing cultivars can be planted at higher densities (100 trees/acre) and do not require thinning of trees since regular hedging and topping maintains trees size. Traditionally spaced trees are trained to a modified central leader, allowing 4-5 scaffolds to develop on the central leader before its removal.

WALNUT (JUGLANS SPP.) HARVEST, POSTHARVEST HANDLING

Maturity
Persian walnuts are harvested at the beginning of shuck split, when the seed coat is a light tan color (market preference).

Harvest Method
For Persian walnut, trunk or limb shakers are used depending on tree size. A windrow machine places the nuts into narrow rows to be picked up by a sweeper. Nuts are collected in large bins and taken to the processing plant. The process is analogous to that used in pecan (see pecan chapter for photos).

Postharvest Handling
Freshly harvested Persian walnuts are removed from hulls and dried in forced-air dryers at 100-110°F until 8% moisture content is achieved. In-shell nuts are bleached and sold fresh, or shelled and marketed as kernels.

Storage
Dried nuts can be stored for about 4 months at room temperature before becoming rancid, but last 1-2 yr when stored in the freezer.

THE WALNUT (JUGLANS SPP.)’S CONTRIBUTION TO DIET

Walnuts are marketed primarily as shelled kernels (76%), with the remainder in-shell. Much of the shelled kernels are processed into baked goods, candies, cereals, and other snack foods. Per capita consumption of walnuts is 0.6 lbs/year.

Dietary value, per 100 gram edible portion

Persian Walnut
Water (%) 3-5
Calories 651
Protein (%) 14.8
Fat (%) 64
Carbohydrates (%) 16
Crude Fiber (%) 2.1
% of US RDA*
Vitamin A 0.6
Thiamin, B1 24
Riboflavin, B2 8.1
Niacin 5.0
Vitamin C 4.4
Calcium 12
Phosphorus 48
Iron 31
Sodium
Potassium 9.6

* Percent of recommended daily allowance set by FDA, assuming a 154 lb male adult, 2700 calories per day.