Juglans is a plant genus, containing species known as Walnuts, which is placed in the Juglandaceae family. They are deciduous trees, 10–40 meters tall (about 30–130 ft), with pinnate leaves 200–900 millimetres long (7–35 in), with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya), but not the hickories (Carya) in the same family.
|Juglans major |
Morton Arboretum acc. 614-47*1
The 21 species in the genus range across the north temperate Old World from southeast Europe east to Japan, and more widely in the New World from southeast Canada west to California and south to Argentina.
EtymologyThe word walnut derives from Old English wealhhnutu, literally "foreign nut", wealh meaning "foreign" (wealh is akin to the terms Welsh and Vlach; see Walha and History of the term Vlach). The walnut was so called because it was introduced from Gaul and Italy. The Latin name for the walnut was nux Gallica, "Gallic nut". The Latin name, Juglans (Latin iūglāns, -andis), is etymologically *dyew-*glandi- "Jove's acorn" (Proto-Indo-European *dyew- "sky, sky god", cf. Greek Zeus, Latin Iūpiter "sky-father"): figuratively, an acorn fit for a god.
TaxonomyThe genus Juglans is divided into four sections.
Sections and species
- Juglans sect. Cardiocaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Fruits borne in racemes of up to 20. Nuts have thick shells. Northeast Asia.
- Juglans sect. Juglans. Leaves large (20–45 cm) with 5–9 broad leaflets, hairless, margins entire. Wood hard. Southeast Europe to central Asia.
- Juglans sect. Rhysocaryon. (The black walnuts) Leaves large (20–50 cm) with 11–23 slender leaflets, finely pubescent, margins serrated. Wood hard. North America, South America.
- J. australis Griseb. (J. brasiliensis Dode)—Argentine Walnut, Brazilian Walnut
- J. boliviana (C. DC.) Dode—Bolivian walnut, Peruvian walnut
- J. californica S.Wats.—California Black Walnut
- J. hindsii (Jepson) R.E.Smith—Hinds' Black Walnut
- J. hirsuta Manning—Nuevo León Walnut
- J. jamaicensis C.DC. (J. insularis Griseb.)—West Indies Walnut
- J. major (Torrey) Heller (J. arizonica Dode, J. elaeopyron Dode, J. torreyi Dode)—Arizona Black Walnut
- J. major var. glabrata Manning
- J. microcarpa Berlandier (J. rupestris Engelm.)—Texas Walnut or Little Black Walnut
- J. microcarpa var. microcarpa
- J. microcarpa var. stewartii (Johnston) Manning
- J. mollis Engelm.—Mexican Walnut
- J. neotropica Diels (J. honorei Dode)—Andean Walnut, Cedro Negro , Cedro Nogal , Nogal , Nogal Bogotano
- J. nigra L.—Eastern Black Walnut
- J. olanchana Standl. & L.O.Williams—Cedro Negro, Nogal, Walnut
- J. olanchana var. olanchana
- J. olanchana var. standleyi
- J. peruviana Dode—Peruvian Walnut
- J. soratensis Manning
- J. steyermarkii Manning—Guatemalan Walnut
- J. venezuelensis Manning—Venezuela Walnut
- Juglans sect. Trachycaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Fruits borne in clusters of 2-3. Nuts have a thick, rough shell bearing distinct, sharp ridges. Eastern North America.
- J. cinerea L.—Butternut
The Eastern Black Walnut (J. nigra) is a common species in its native eastern North America, and is also widely cultivated elsewhere. The nuts are edible, and though they are often used in expensive baked goods, the persian walnut is preferred for everyday use due to its easier to extract nutmeat. The wood is particularly valuable.
The Hinds' Black Walnut (J. hindsii) is native to northern California, where it has been widely used commercially as a rootstock for J. regia trees. Hinds' black walnut shells do not have the deep grooves that are characteristic of the eastern black walnut (J. nigra).
Japanese Walnut (J. ailantifolia) is similar to Butternut, distinguished by the larger leaves up to 90 cm long, and round (not oval) nuts. The variety cordiformis, often called the heartnut has heart-shaped nuts; the common name of this variety is the source of the sectional name Cardiocaryon.
The Butternut (J. cinerea) is also native to eastern North America, where it is currently endangered by an introduced disease, butternut canker, caused by the fungus Sirococcus clavigignenti. Its leaves are 40–60 cm long, the fruits are oval, the shell has very tall, very slender ridges, and the kernel is especially high in fat.
- J. × bixbyi Rehd.—J. ailantifolia x J. cinerea
- J. × intermedia Carr.—J. nigra x J. regia
- J. × notha Rehd.—J. ailantifolia x J. regia
- J. × quadrangulata (Carr.) Rehd.—J. cinerea x J. regia
- J. × sinensis (D. C.) Rehd.—J. mandschurica x J. regia
- J. × paradox Burbank—J. hindsii x J. regia
- J. × royal Burbank—J. hindsii x J. nigra
PhylogenyA study of sequenced nuclear DNA from the External Transcribed Spacer (ETS) of ribosomal DNA (rDNA), the Internal Transcribed Spacer (ITS) of rDNA, and the second intron of the LEAFY gene taken from at least one individual of most of the species of Juglans has supported several conclusions:
- The genus Juglans is monophyletic;
- Sect. Cardiocaryon is sister to Sect. Trachycaryon;
- Sect. Juglans is sister to Sect. Cardiocaryon and Sect. Trachycaryon together;
- Sect. Rhysocaryon is monophyletic and sister to Sect. Juglans, Sect. Cardiocaryon, and Sect. Trachycaryon together;
- Sect. Rhysocaryon, the "Black Walnuts", contains two clades:
- one comprises the more northerly species J. californica, J. hindsii, J. hirsuta, J. major, J. microcarpa, and J. nigra;
- the other comprises the more southerly species J. australis, J. boliviana, J. jamaicensis, J. molis, J. neotropica, J. olanchana, J. steyermarkii, and J. venezuelensis
- J. olanchana var. standleyi seems to be more closely related to J. steyermarkii than to J. olanchana var olanchana, suggesting that J. olanchana var. standleyi might be better understood as either a separate species or a variety of J. steyermarkii.
Cultivation and uses
Walnuts are light-demanding species that benefit from protection from wind. Walnuts are also very hardy against drought.
Interplanting walnut plantations with a nitrogen fixing plant such as Elaeagnus × ebbingei or Elaeagnus umbellata, and various Alnus species results in a 30% increase in tree height and girth (Hemery 2001).
When grown for nuts, care must be taken to select cultivars that are compatible for pollination purposes; although some cultivars are marketed as "self fertile" they will generally fruit better with a different pollination partner. There are many different cultivars available for growers, offering different growth habit, flowering and leafing, kernel flavour and shell thickness. A key trait for more northerly latitudes of North America and Europe is phenology, with ‘late flushing’ being particularly important to avoid frost damage in Spring. Some cultivars have been developed for novel ‘hedge’ production systems developed in Europe and would not suit more traditional orchard systems.
FlowersThe leaves and blossoms of the walnut tree normally appear in spring. The male cylindrical catkins of the Walnut tree are developed from leafless shoots from the past year, they are about 10 cm in length and have a large number of little flowers. Female flowers appear in a cluster at the peak of the current year’s leafy shoots.
FruitSome fruits are borderline and difficult to categorize. Hickory nuts (Carya) and Walnuts (Juglans) in the Juglandaceae family grow within an outer husk; these fruits are technically drupes or drupaceous nuts, and thus not true botanical nuts. Tryma is a specialized term for such nut-like drupes.
Nuts and kernels
Two-thirds of the world export market and 99% of the US commercial production of English walnuts is grown in California's Central Valley and in Coastal Valleys, from Redding in the north to Bakersfield in the south. Of the more than 30 varieties of J. regia grown there, Chandler and Hartley account for over half of total production. In California commercial production, the Hinds' black walnut (J. hindsii) and the hybrid between J. hindsii and J. regia, Juglans x Paradox, are widely used as rootstocks for J. regia cultivars because of their resistance to Phytophthora and to a very limited degree, the oak root fungus. However, trees grafted on these rootstocks often succumb to black line.
In some countries immature nuts in their husks are preserved in vinegar. In England these are called "pickled walnuts" and this is one of the major uses for fresh nuts from the small scale plantings. In Armenian cuisine, walnuts are preserved in sugar syrup and eaten whole. In Italy, liqueurs called Nocino and Nocello are flavoured with walnuts, while Salsa di Noci ("Walnut Sauce") is a pasta sauce originating from Liguria. In Georgia, walnuts are ground along with other ingredients to make walnut sauce.
Walnuts are heavily used in India. In Jammu, India it is used widely as a prasad (offering) to Mother Goddess Vaisnav Devi and, generally, as a dry food in the season of festivals such as Diwali.
The nuts are rich in oil, and are widely eaten both fresh and in cookery. Walnut oil is expensive and consequently is used sparingly; most often in salad dressing. Walnut oil has been employed in oil paint, as an effective binding medium, known for its clear, glossy consistency and non-toxicity.
Manos and Stone studied the composition of seed oils from several species of the Rhoipteleaceae and Juglandaceae and found that the nut oils were generally more unsaturated from species which grow in the Temperate zone and more saturated for species which grow in the Tropical zone. In the northerly-growing section Trachycaryon, J. cinerea oil was reported to contain 15% linolenate (the report did not specify whether the linolenate was the alpha (n-3) or gamma (n-6) isomer, or perhaps a mixture), 2% of saturated palmitate, and a maximum concentration of 71% linoleate. In the section Juglans, J. regia nut oil was found to contain from 10% to 11% linolenate, 6% to 7% palmitate, and a maximum concentration of linoleate (62% to 68%). In the section Cardiocaryon, the nut oils of J. ailantifolia and J. mandshurica were reported to contain (respectively) 7% and 5% of linolenate, 2% of palmitate, and maximum concentrations of 74% and 79% linoleate. Within the section Rhysocaryon, the nut oils of the U.S. native black walnuts J. microcarpa and J. nigra were reported to contain (respectively) 7% and 3% linolenate, 4% and 3% palmitate, and 70% and 69% linoleate. The remaining results for black walnuts were: J. australis contained 2% linolenate, 7% palmitate, and 61% linoleate; J. boliviana contained 4% linolenate, 4% palmitate, and 70% linoleate; J. hirsuta contained 2% linolenate, 5% palmitate, and 75% linoleate; J. mollis contained 0% linolenate, 5% palmitate, 46% linoleate, and 49% oleate; J. neotropica contained 3% linolenate, 5% palmitate, and 50% linoleate; and J. olanchana contained only a trace of linolenate, 9% palmitate, and 73% linoleate;
ShellsThe walnut shell has a wide variety of uses. Eastern black walnut (J. nigra) shell is the hardest of the walnut shells, and therefore has the highest resistance to break-down.
- Cleansing and polishing: Walnut shells are mostly used to clean soft metals, fiberglass, plastics, wood and stone. This environmentally friendly and recyclable soft grit abrasive is well suited for air blasting, de-burring, de-scaling, and polishing operations because of its elasticity and resilience. Uses include cleaning automobile and jet engines, electronic circuit boards, and paint and graffiti removal. For example: In the early days of jet transportation, crushed walnut shells were used to scour the compressor airfoils clean, but when engines with air cooled vanes and blades in the turbine started being manufactured this practice was stopped because the crushed shells tended to plug up the cooling passages to the turbine, resulting in turbine failures due to overheating.
- Oil well drilling: The shell is used widely in oil well drilling for lost circulation material in making and maintaining seals in fracture zones and unconsolidated formations.
- Flour made from walnut shells is widely used in the plastics industry.
- Paint thickener: Walnut shells are added to paint to give it a thicker consistency for "plaster effect" ranges.
- Explosives: Used as a filler in dynamite.
- Cosmetic cleaner: Occasionally used in soap and exfoliating cleansers
Husksdye that is used for dyeing fabric and for other purposes. The dye does not require a mordant and will readily stain the hand if picked without gloves.
Woodburls (or 'burrs' in Europe) are commonly used to create bowls and other turned pieces. Veneer sliced from walnut burl is one of the most valuable and highly prized by cabinet makers and prestige car manufacturers. Walnut wood has been the timber of choice for gun makers for centuries, including the Gewehr 98 and Lee Enfield rifles of the First World War. It remains one the most popular choices for rifle and shotgun stocks, and is generally considered to be the premium – as well as the most traditional – wood for gun stocks, due to its resilience to compression along the grain. Walnut is also used in lutherie, i.e. making stringed musical instruments. The wood of the Butternut and related Asian species is of much lower value, softer, coarser, less strong and heavy, and paler in colour.
In North America, forestry research has been undertaken mostly on J. nigra aiming to improve the quality of planting stock and markets. In some areas of the US black walnut is the most valuable commercial timber species. The Walnut Council is the key body linking growers with scientists. In Europe, various EU-led scientific programs have studied walnut growing for timber.
Traditional Chinese medicinal useWalnuts are considered to be a herb in Traditional Chinese medicine. They are said to tonify kidneys, strengthen the back and knees, warm and hold qi in lungs and help kidneys to grasp the qi, moisten the intestines and move stool. It is believed to stop asthma and is prescribed to be taken between bouts of asthma, but not for acute asthma. It is also used by the elderly to relieve constipation.
Parkland and garden treesWalnuts are very attractive trees in parks and large gardens. Walnut trees are easily propagated from the nuts. Seedlings grow rapidly on good soils. The Japanese Walnut in particular is known for its huge leaves, which have a tropical appearance.
As garden trees they have some drawbacks, in particular the falling nuts, and the releasing of the allelopathic compound juglone, though a number of gardeners do grow them. However, different walnut species vary in the amount of juglone they release from the roots and fallen leaves - J. nigra in particular is known for its toxicity, both to plants and horses. Juglone is toxic to plants such as tomato, apple, and birch and may cause stunting and death of nearby vegetation. Juglone appears to be one of the walnut's primary defence mechanisms against potential competitors for resources (water, nutrients and sunlight), and its effects are felt most strongly inside the tree's "drip line" (the circle around the tree marked by the horizontal distance of its outermost branches). However, even plants at a seemingly great distance outside the drip line can be affected, and juglone can linger in the soil for several years even after a walnut is removed as its roots slowly decompose and release juglone into the soil.
Walnut as wildlife food plantsWalnuts are used as food plants by the larvae of some Lepidoptera species. These include:
- Brown-tail (Euproctis chrysorrhoea)
- the Coleophora case-bearers C. laticornella (recorded on J. nigra) and C. pruniella.
- Common Emerald (Hemithea aestivaria)
- Emperor Moth (Pavonia pavonia)
- The Engrailed (Ectropis crepuscularia)
- Walnut Sphinx (Amorpha juglandis)
In California and Geneva (Switzerland), ravens have been witnessed taking walnuts into their beaks, flying up to 60 feet or so in the air, and dropping them to the ground in order to crack the shells and eat the nut inside.