Walnuts are a classic tree nut widely used in many of the world’s cuisines. The edible kernel rests inside a hard, woody shell that must be broken open before eating. Walnuts are produced by a large deciduous tree with broad pointed leaves and small white blossoms.
While walnuts are considered a culinary nut, in botanical terms they are actually classed as a seed. Scientifically, a nut is a type of fruit that consists of an inner (edible) seed encapsulated by a hard shell that does not open to release the seed. Some examples of botanical nuts are hazelnuts, chestnuts, and acorns. Walnuts first shed their outer hull and then their hard inner shell as the seed germinates and grows into a tree, thus disqualifying them as a botanical nut.⁶ Harvested walnuts keep their shells until they are removed prior to eating. Pecans, Hickories, and Walnuts are all in the Walnut family. But Walnuts have their own genus. Walnuts are Juglans and Pecans are Carya.
Juglans regia, the scientific name for the Persian walnut, was given to it by the ancient Romans. Feeling that the walnut had a royal air to it, the Romans called it “Jupiter’s Nut” in Latin, after one of their gods.
The name “walnut” evolved from the Old Welsh/English “wealhhnutu” which meant foreign nut, as the walnut arrived in Britain from the European continent.²
The commercial variety of walnut is often called the “Persian” or “English” walnut, to differentiate it from other species.
Walnut trees, as a broad family, exist in the fossil record around 40 million years ago. The Persian Walnut tree came about 3.45 million years ago as a result of natural hybridization between two now-extinct species. A theory exists that these walnuts spread out across a temperate band of eurasia that included China, Central Asia, the Near East, and Southern Europe. The recent ice age would have killed much of those populations off but some walnut trees likely survived in pockets across that band. There is evidence of early humans eating wild walnuts in France between 45k and 10k years ago.²¹’²²
Humans have likely been eating walnuts ever since we left Africa for Eurasia thousands of years ago. As gatherers, we would have seen the wild walnut trees as an excellent source of calorie-rich fats and oils during our prehistoric ramblings.
The first evidence of cultivation goes back 7000 years to Central Asia, possibly in Iran or Kyrgyzstan. Walnuts may have then spread to Persia, China, the Caucuses, and Europe. The Greeks and Romans eventually caught onto walnuts and began growing them in the Mediterranean. The Romans called it “nux Gallica” in Latin, referring to the region of Galatia in Anatolia where they had become familiar with it (nux means nut in Latin). Walnut residue has been found in the amphorae of sunken Roman ships and the ruins of Pompeii.
Approximate Dates with limited accuracy:
Central Asia: 5000 BC
China: 1 AD
Southern/Central Europe: 300s BC
Walnuts’ popularity continued through the Middle Ages when they were frequently listed in medical books as having various healing properties. Cultivation spread through Europe to Britain where the nut came into our lexicon.⁸
North America has native walnuts that are edible, but the preferred Persian variety was first cultivated in California in the 1770s by Franciscan Friars. These “Mission Walnuts” , as the Persian walnuts were called, were grown in the yards of Catholic missions. Commercial walnut cultivation began in the 1840s around San Diego. In the late 1800s California’s agriculture shifted from grain and ranching more towards grapes, produce, fruit trees, etc.²³ Eventually as citrus displaced other crops in the south, cultivation migrated to the Central Valley where the most acreage exists today.⁹
California Walnut Timeline:
1770: Franciscan Friars begin limited cultivation of Persian walnuts they brought from Europe on their Catholic Missions.
1843: Commercial walnut orchard planted around San Diego
1860s: A new cultivar of soft-shelled walnut (still Persian) was imported from Chile & France and cultivation began of that type
1892: Walnut orchards have spread to 15000 acres, mostly in Socal.
1910s: Citrus planting, pests, and urbanization displace walnut production in Socal. Walnut cultivation expanded in Central Valley.
1930: California walnuts cover all of US consumption
1940: Walnut acreage in California now 130000 acres
Present: Over 225000 acres of walnut in cultivation in California
I think the spread of walnuts around the world can be described as waves. First wave 40 million years ago is the initial one. Then the first spread of Juglans regia walnuts around Eurasia after 3.45 mya. This deposited lots of trees in the temperate band of eurasia which people ate prehistorically. Then another wave of one variety popularized by Alexander the Great brought back from Persia to the Mediterranean. Then another via the Romans that spread again around the Mediterranean. Then a wave that spread the nut to California.
Botanically, the kernel of a walnut is the energy-rich food source for a growing walnut tree. As such, it’s packed with as much nutrition as can fit into the shell.
Walnuts are made up of 65% fat by weight. These fats are the omega-6 and omega-3 polyunsaturated fatty acids which have been linked to several health benefits. Omega-3 is valuable in particular because mammals cannot produce it themselves so it must be obtained through our diet.
Walnuts also contain antioxidants to prevent the fats in their kernel from going rancid in nature. These antioxidants include Ellagic Acid, Catechin, Phytic Acid, and Melatonin. Interestingly, some medieval doctors mentioned walnuts as a sleep aid, likely having to do with the presence of the sleep-associated antioxidant melatonin.
Walnuts also contain many important vitamins and minerals, including copper, folic acid, phosphorus, B6, Manganese, and Vitamin E. ¹⁶’¹⁷
Walnuts are delicious and versatile. Walnuts are nutty and flavorful with a pleasant texture, making them a pleasure to eat plain from the shell. They are commonly incorporated into foods like salads, baked goods, yogurt, stir-fries, or encrusted with bread crumbs on cooked fish. Walnuts can also be ground into a spreadable paste similar to peanut butter. Walnut butter can be added to sauces to provide body, particularly in vegan meals where dairy based fats need substitutes.
Walnuts are also commonly pressed to make an edible seasoning oil. It can be drizzled on salads or meats as a finishing oil. Walnut oil is also used lightly in cooking to impart flavor, though should be used sparingly as it burns at high temperatures.
Walnuts make appearances in some notable dishes across the world. The Turkish pastry Baklava is one, which features chopped walnuts and honey layered into filo dough. In the 1890s, a famous American hotel, the Waldorf-Astoria in New York City, began serving the “Waldorf Salad” which is a sweet creamy dish composed of apples, celery, and walnuts.
Beyond food, walnuts have proven to be useful in other aspects of life. The wood of walnut trees is valued for its fine, straight grain and is used in furniture and other luxury wood applications. As culinary varieties of walnut tree are too valuable to be cut down, it’s mostly the American Black Walnut that is used for woodworking.
Historically, the outer hulls of walnuts were used to make dyes and inks. After aging, they produce a dark brown dye that was cheap and accessible. Many indigenous Americans used walnut dyes for their clothes and Renaissance painters like Da Vinci & Rembrandt used the pigment in their art.¹⁰’¹¹
As a niche application, ground walnut shells are used in abrasive cleaning. Similar to sandblasting, crushed walnut shells provide a softer alternative to sand for stripping paint or coatings in industrial settings. This is generally where the byproduct walnut shells from culinary walnut processing are used.¹²
Walnuts dominate a family of trees called the Juglandaceae. This group contains nearly two dozen recognized species of walnuts spread across Eurasia and the Americas. It also includes hickory and pecan trees.
Of the several regional species of walnut, many are edible. Indeed, North America has several edible species and California has its own native variety. But the main commercial species is Juglans regia, or the Persian Walnut.
Within Juglans regia, farmers grow a broad variety of cultivars. In California, the most popular varieties are: Chandler, Hartley, Howard, Tulare, Serr, and Vina.¹⁹
The world produces around 3.5 million tons of walnuts each year. The bulk of that production is in China but the United States, Iran, and Turkey are also significant producers.¹³
In the United States, 99% of all walnuts grown come from orchards in California. When cultivation began much of the acreage was in the south around San Diego, but as citrus grew more popular production moved to the Central Valley. Most orchards are in the San Joaquin and Sacramento Valleys, but there is some production in the coastal valleys and Sierra foothills.¹⁴’¹⁵
Mature walnut trees have deep roots and correspondingly require deep soil. At least 5-6 feet of fertile soil is necessary to support a productive tree. Soil should also be well drained and in an area with a lower water table. ³
Before planting, the terrain of the orchard needs to be developed. The soil should first be graded as level as possible to ensure even watering. The soil should then be loosened with clods broken up. All 5-6 feet of the soil should be loosened if possible to allow the roots of new saplings to permeate the soil. Raised beds or individual mounds should be formed in which the saplings are planted.¹⁵
The classic and easiest method of keeping walnut orchards watered is through flood irrigation. Though with California’s current drought conditions, sprinklers or drip lines are now being used more to conserve water. These newer systems deliver benefits on water conservation and reduce runoff. The frequency of water application depends on the age of the orchard, precipitation, and evaporation from heat. Irrigation lines should be installed prior to planting.¹⁵’¹⁹
Temperature & Sun
Walnut trees thrive in temperate environments but can be damaged by cold. Early frosts can damage branches and late frosts can damage flowers or young nuts.³ Though frost damage is uncommon in the temperate Central Valley.¹⁹
During the winter months, however, walnut trees require a long period of cold temperatures for the nuts to be able to set in the summer. These “Chill Hours” are periods of time when the temperature is below 45F. Walnut trees need a somewhat long intermittent period of Chill Hours lasting 800-1000 hours. ⁷
Sun damage and heat can be a threat. Special reflective spray coatings can be applied to reduce the impact of hot weather spells on trees and fruits.¹⁹
Walnut flowers emerge during the spring in April and May. Walnut trees are self-fertile, so they do not require an insect or bird pollinator. The nuts grow over the summer until maturity in fall. Nuts are harvested from September to November once the hulls begin to split.
Walnuts lose their leaves in November and begin to regrow them in April once the frost is over.²⁴ Over winter, walnut trees experience a process called “dormancy” which involves losing their leaves and relying on stores of carbohydrates to survive until the spring bloom. This is an important process for walnut trees that ensures they grow healthy nuts.²⁵
Grafting is the best practice for California walnuts. Generally, Persian Walnut varieties should be grafted onto a hardy and pest-resistant rootstock like the California Black Walnut.¹⁵ Newer, hardier rootstocks are constantly being developed to improve yields.
Commercial Persian walnut trees can be grown from persian walnuts themselves, but this comes with disadvantages in California. Namely, Persian walnut rootstocks have not developed natural resistance to California’s local pests and environmental conditions. These disadvantages would weaken the tree, making it less productive and commercially risky. Grafting Persian walnut branches onto disease resistant local Californian Black Walnut rootstocks provides a hardy tree that produces commercially appealing walnuts.
New research is indicating that a dense planting of walnut saplings can increase the number of trees per acre without damaging the yield of individual trees.
It takes 5-7 years for a walnut tree to begin producing commercial-grade nuts. During the maturation period, young trees are vulnerable and should be attentively protected from pests, infections, and the elements. A tree grows 24 inches per year and usually tops out between 40 & 60 feet high. The average lifespan of a California walnut orchard is 35 years, but it can remain productive for as long as 100 years. ¹⁵’¹⁹ Most walnut trees however, do not last 100 years, succumbing to pests, losing productivity, or otherwise having their orchard land turned over.
Fertilizer should be determined based on the nature of the soil if it has any deficiencies. Fertilizers can be applied to the root zone of the tree either during the winter dormancy or in late spring after the nuts are beginning to grow. Nitrogen, phosphorus, and potassium are common fertilizer additions. ¹⁵
Orchards must be maintained year-round to ensure good health. Weeding, pruning, and spraying for pests & diseases are needed.¹⁵ Trees should be trained into what’s called a “central leader” shape, which consists of one main trunk and 4-6 lateral branches. Pruning is done every January to maintain the shape of trees. New research is showing that less pruning of the central leader shape can result in higher yields.¹⁹
A herbicidal spray is applied to orchards regularly to prevent weed growth. Orchards should be walked regularly to detect pests or other problems, with spot solutions applied accordingly.
Harvests begin in September when the green hulls split and the walnuts begin to fall from the trees. The tree is first shaken with a mechanical shaker to drop the nuts. A specialized sweeping machine then gathers the dropped nuts into rows where a harvester can come along to transfer them to a trailer.¹⁵
After harvest, any outer hulls should be removed with a scrubber. Clean walnuts are then transferred to a drier for 24hrs to remove moisture in order to prevent rotting during storage. Once clean and dry, walnuts should be held in a dark silo or stacked bins at 41F until needed.¹⁵
Walnuts destined to be sold in-shell are sized and packaged after storage. Shelled walnuts need to have their shells mechanically removed and the kernel separated from the shell using compressed air. Kernels are then sized, sorted, and packaged. ¹⁵
Like many California nut trees, Walnuts are susceptible to fungus-based blights and cankers. Infections from fungi in the Botryosphaeriaceae and Phomopsis can kill new shoots and buds and disfigure developing nuts. Trees weakened by insects, frost, heavy rain, or drought are susceptible to this bot disease. The best method to counteract an infection is pruning infected foliage and in dire situations the application of fungicides.⁴
Walnut trees are also susceptible to crown gall, a kind of bacterial infection. Crown gall manifests itself as tumor-like structures on the roots, trunk, or crown of the trees. Since the bacterium exists in the soil, trees become infected through weakness and injury. To prevent infection, keep trees well-watered and avoid injuring the roots or trunks, particularly when the tree is young.²⁰
Rodents can damage young trees. This can be mitigated by the installation of shields or paint around the young trunks.¹⁵
Like in many aspects of commerce, sustainability and productivity dovetail. With walnut agriculture, much of the drive for sustainability manifests itself as making orchards more productive. Close examination of the soil allows for more precise application of irrigation and fertilizer that reduces wastage. Selection of disease-resistant rootstocks has reduced the impact of pests and disease on walnut orchards and the need for pesticides. ¹⁸
Drip irrigation systems help save on water and liquid application of nutrients through them can also save on fertilization labor. Good pruning and preventative weed measures ensure fewer problems and greater yields with less labor.¹⁹
Finding uses for byproducts has also contributed to walnuts’ sustainability recently. The outer hulls and prunings are composted and reincorporated into orchard soil to reduce the need for fertilizer. Crushed shells are also reserved for other industrial uses to reduce wastage.
Like many healthy foods, there is demand for organic walnuts. Organic walnuts are produced on several thousand acres in California, though they require higher input costs to meet organic standards. Methods include natural fertilizers, pesticides, herbicides, and repellents.
12 – Walnut Blast Media
13 – FAO Data on Walnuts
14 – Walnuts.org
15 – Walnuts UC Davis
17 – Healthline Walnuts
19 – Jay Mahil Farmer Advice