Walnut

The walnut is a nut-bearing tree in the genus Juglan in the family Juglandaceae. Two main species are found in temperate regions around the world. The Persian or English walnut, Juglans regia, is grown commercially for its fruit—nuts that have a sweet taste and are used in a variety of foods. There are a number of scientific studies on the nuts that indicate that they are beneficial to heart health and possibly improve cognitive function. These fruit have a high concentration of the fatty acid alpha-linolenic acid, also known as fish oil. China is the largest producer of these nuts in the early 21st century, with a large number also being grown in the United States, almost entirely in California. The black walnut tree, Juglans nigra, sometimes grown as an ornamental, is native to the United States and is found in forests in the eastern half of the country. Both tree species are prized for their lumber, which is used in making furniture and gunstocks. The trees can be problematic for gardeners, since they produce chemicals known as juglones that inhibit the growth of many other plants underneath them. The name of these trees derives from Roman terminology. The Romans thought that the nuts looked like testicles and named them “glands of Jupiter” or Juglans, for short. Jupiter was the king of the gods worshipped by the Romans. Juglans regia translates to “royal nut of Jupiter.” The species name of the black walnut is more pedestrian, with the species name of nigra indicating the blackness of the fruit and bark. Walnut trees were designated Juglans by Swedish naturalist Carl Linnaeus in 1753.

History

Walnuts have a long and storied history in human civilization. There is evidence that they were consumed up to 17,000 years ago, and they were considered of great medicinal value during ancient Mediterranean times. The physicians Dio- scorides and Galen both extolled the virtues of walnuts as a medicine.

Persian walnut trees were disseminated around the world by English traders, leading to the alternate name of English walnut. They are thought to be native to central Asia. The trees were introduced to California in 1867 from Chilean trees that originated in Spain. Black walnuts are native to the eastern United States.

Plant Culture

The trees grow best in deep soil that is fertile and moist. Walnut trees have deep taproots, along with a lateral network of more shallow roots. The degree of lateral

Walnuts (Ghettog76/Dreamstime.com)

root production depends on the soil type. More fibrous roots are produced in sandy soils. This root system enables walnut trees to tolerate drought, although the deep taproot can hamper transplantation of them. This type of tree can tolerate some degree of flooding, although the trees usually succumb if they are flooded for 90 days or more. High temperatures can damage fruit production, although varieties differ greatly in their degree of heat resistance. Commercial growers in California have applied reflective materials to the tops of the trees to protect against the deleterious heat from sun rays. Such treatment can reduce temperatures by about 10 percent.

Walnut breeders are constantly creating new varieties. There were more than 400 varieties of black walnut introduced over a 100-year period. The trees have a great degree of genetic variability, and new growers of walnuts are advised to select varieties that grow well in their area. Persian walnuts are frequently grafted onto rootstocks that are better suited for the geographic area in which they are grown. In the 21st century, black walnuts were primarily used as rootstocks upon which to graft Persian walnuts.

Left to grow to their full potential, the trees commonly reach 75 to 100 feet. Individuals in forest settings can grow to 150 feet. Black walnut trees in the eastern United States have been found with trunks as wide as 5 feet. In their natural setting, walnut trees are frequently found on stream banks or slopes that face north or east. The seedlings do not tolerate shade and generally grow in clearings in the forest.

The trees have flowers that are monoecious, with each tree having both male and female flowers. The male flowers are greenish-yellow, hairy catkins that develop on growth from the previous year. Female flowers are spikes of up to eight flowers that produced on the current year’s growth. They generally appear before the male flowers. The flowers are produced in the mid- to late spring concurrently with leaf production. Normally, another walnut tree is required to pollinate the flowers, so that fruit is produced. Walnut trees are self-fertile to some degree and will not produce a large crop of fruit unless at least one other tree is nearby.

Nuts ripen in the fall and drop to the ground after the leaves have fallen. With black walnuts, optimal nut production begins when the trees have reached 30 years old. They generally produce large amounts for another century. In commercial orchards, tree shakers are used to remove the nuts from the trees. With the fruit of wild trees, people compete with squirrels to gather them from the ground. New black walnut trees in the wild commonly grow from fruit that have been buried by squirrels and left behind.

When the trees are grown as landscape specimens, the fruit production can be considered undesirable, since its fall creates a mess underneath the trees. Home owners are generally advised against growing walnut trees. They are deemed best for parks and campuses, which have plenty of space for the trees to spread. Their use as street trees is also usually not recommended.

Properties of the Nuts

Walnuts are grown commercially for their edible fruit—nuts contained within a dark, hard husk. In the late 20th and early 21st centuries, there was a plethora of scientific studies on health benefits of the nuts. Walnuts were the first food for which the U.S. Food and Drug Administration (FDA) allowed a health claim to be made. Supportive but not conclusive research indicated that a diet containing one-and-one-half ounces daily, as part of a diet low in saturated fats and cholesterol, might reduce the risk of coronary heart disease. The European Union’s European Food Safety Authority has also made a health claim about walnuts.

One ounce of walnuts is about 14 walnut halves. Nutrition information is presented as amount per one-ounce serving. The nuts from Persian walnuts contain 18 grams of fat with 13 grams of it being polyunsaturated fat. This high concentration of polyunsaturated fats rather than monounsaturated fat makes walnuts unique among nuts. These nuts possess a high concentration of alpha-linolenic acid compared to other plants. Persian walnuts contain 2.5 grams of this compound per serving, while black walnuts have only one-fifth as much alpha-linolenic acid. This polyunsaturated fatty acid is more commonly known as fish oil. Humans are incapable of synthesizing this type of fatty acid, and such essential fatty acids must be obtained from food. There is evidence that this type of oil is broadly beneficial to human health. The nuts are also high in protein and have been found to contain 4 grams per ounce of protein along with 2 grams of fiber.

Studies with walnuts as a component of human diets have indicated that they help to regulate cholesterol levels, resulting in a cholesterol profile that is more beneficial to human health. In people with type 2 diabetes, a diet rich in walnuts was found to improve the flow of blood. These nuts have also been studied for their anticarcinogenic functions and their ability to promote cognitive function. When consumed in moderation as part of a so-called Mediterranean diet, the nuts were found not to cause weight gain in the people studied. Some people have an allergy to walnuts and should avoid their consumption. Contact with the leaves can cause dermatitis of the skin.

 
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