Walnuts

Growing up in Southern California, I was lucky enough to attend a childcare program that was built on the grounds of a former walnut grove. Scattered throughout the property, there were dozens of ancient walnut trees, great for climbing, tire swings, and more delicious walnuts than any of us kids could possibly have eaten. But we sure tried!

Walnut shell and husk (Böhringer Friedrich) CC BY-SA 2.5

Inside a walnut (Kkchaudhary11) CC BY-SA 4.0

People have been growing walnut trees longer than any other food tree. Nearly 10,000 years ago, in ancient Persia, walnuts were grown for members of the royal family. Traded along the Silk Road, and then via sea trade, Persian walnuts made their way to Rome, where they were called Jupiter’s royal acorn, and to England, where the name was changed to English walnuts, even though they were not being grown commercially in England at that time. In the 1700s, missionaries brought walnuts to California.

Types of walnut trees

There are actually several different trees that qualify as walnut. They are all members of the Juglans genus. The familiar English walnut is only one of four types of walnut tree:

• Persian or English walnut (Juglans regia) - these trees grow faster than the others, with thiner shells, making it the walnut tree of choice for commercial production
• Black walnut (Juglans nigra) - native to eastern North America; valued for both nuts and extremely hard lumber, they are also grown as shade trees
• Butternut (Juglans cinerea) - also known as “white walnut”
• Heartnut (Juglans ailantifolia) - also known as “Japanese walnut”; these trees produce heart-shaped fruits

There are also many different walnut cultivars, most of which are hybrids of the English walnut.

The walnut tree

Walnut is a deciduous hardwood. It is also one of the few trees with a true taproot. [Most tree roots are fibrous.] Walnut trees can take 5 or 6 years before they produce fruit. When selecting a site for a walnut tree, keep in mind that a mature walnut tree can reach 40 to 80 feet, in both height and width, and it can live 50 to 250 years!

Walnut trees, like avocado trees, are monoecious, which means they produce both male and female flowers. Male walnut flowers are catkins that look like hanging cat tails. The female flowers are spiky and short. If you slice open a walnut twig, you will see a series of tan chambers, called pith. This is different from the white pith found in citrus rinds.

​Fruits of the walnut tree

​Unlike chestnuts, which are both botanical and culinary nuts, the common walnut isn’t a nut at all. You may be surprised to learn that walnuts are actually a type of stone fruit. [I know! I was surprised, too!] This means that the walnuts you enjoy eating are a form of fleshy fruit, known as a drupe. Almonds are also drupes. Surrounded by a thick, green rind, the walnuts you see in the grocery store are not what they look like when they are still hanging in the tree. That rind is actually the fruit of a walnut tree, but you wouldn’t want to eat it. It’s nasty.

Walnut fruit (George Chernilevsky) Public Domain

Walnut trees, like citrus and many other fruit and nut trees, tend to produce heavy crops one year and a light crop the following year. Known as alternate bearing, these fluctuations allow trees to recover from heavy production years.

Toxic walnut

You have probably heard that walnut trees put out toxins that make it impossible to grow other plants nearby. This is only partly true. Many plants use a type of chemical warfare, called allelopathy, to reduce competition. Walnut trees produce do produce toxins that can cause some other plants to wilt. Tomatoes, potatoes, eggplant, peppers, blackberries, raspberries, blueberries, and asparagus do not grow well when planted near a black walnut tree, according to the University of Illinois Extension. At the same time, according to the PennState Extension, onions, beets, squash, melons, carrots, parsnips, beans, yarrow, stonecrops, and corn can all be grown near a walnut tree without any problems. In fact, in commercial walnut groves, a type of agroforestry, called alley cropping, is used to plant other crops, such as corn, between the rows of walnut trees.

Propagating walnut trees

While you can certainly buy a bare root walnut tree, there are other ways. You can plant a raw walnut in the ground, or, if you know of someone with a walnut tree, you can use air layering. Layering is a form of vegetative propagation. Strawberry runners are an example of layering. The nice thing about air layering is that the parent plant continues to feed and care for the newly developing plant, since they are still attached to one another. To air layer a walnut, pull a stem down until it touches the ground at what would have been a leaf node. Instead of developing into a leaf, that bud will start putting out roots.

Pruning walnut

Walnut trees can produce nuts on the same spurs for several years. Because of this, mature walnut trees do not require renewal pruning. The only pruning needed is occasionally thinning branches to maintain overall shape and good health. Young walnut trees are trained using the modified central leader system. In this method, a single, strong shoot is encouraged up the central line of the tree. Two or three lateral branches, spread evenly around the tree, both vertically and horizontally, are allowed to grow. All other branches are removed. Eventually, there can be five to seven lateral branches in place before the central leader is removed.

Walnut pests and diseases

Walnuts are susceptible to an astounding number of pests and diseases. Luckily, walnuts are rugged trees that rarely need assistance in fighting off these foes. It’s still a good idea to know what your tree might be up against. Many varieties of scale insects, including walnut scale, frosted scale, European fruit lecanium scale, San Jose scale, Kuno scale, and Italian pear scale may be found on walnut. Walnut husk flies, aphids, southern fire ants, walnut twig beetles, fall webworms, Pacific flathead borers, navel orangeworms, false chinch bugs, redhumped caterpillars, American plum borers, and Mediterranean fruit flies prefer walnut, as do tortricid moths, such as the light brown apple moth, which can cause leaf roll of walnut. A type of eriophyid mite, called the blister mite, will also attack walnut trees, as will European red mites and webspinning spider mites. Codling moth larvae will burrow into the nut meat of English walnuts, starting in April. You can monitor your trees for many of these pests by using pheromone traps.

Diseases, such as crown gall and walnut blight can be prevented and treated with Bordeaux mixture or fixed copper. Walnut trees may also become infected with anthracnose, armillaria root rot, phytophthora root and crown rot, branch wilt, and several canker diseases. If that weren’t trouble enough, squirrels, voles, pocket gophers, rats, and deer will try to get at as much of your walnut crop as they can.

While many trees are treated with horticultural oils during dormancy, oils should not be used on walnut. Dormant oils are phytotoxic (poisonous) to walnut trees. Like apricot trees, walnut trees are also susceptible to Eutypa dieback. This fungal disease can kill a tree. The easiest way to avoid it is to only prune your walnut tree during summer, when there are no rains expected. Also, make sure that your sprinklers are not hitting the tree’s trunk.

Walnuts contain high levels of oils that can turn rancid. To keep walnut meats fresh, they are left in their shells and placed in cold storage. If you buy walnuts from a store and plan on using them within one month, store them in your refrigerator. Since walnuts can absorb odors, be sure to keep them away from fragrant foods, such as cabbage, broccoli, and fish. Longer storage should be done in the freezer.

Walnut trees can make a magnificent addition to your foodscape, providing decades of delicious nuts and welcoming shade from the summer sun.