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Juglans

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For other uses, see Juglans (disambiguation).
"Walnut Tree" redirects here. For other uses, see Walnut Tree (disambiguation).

Walnuts
Common walnut, Juglans regia
Scientific classification
Kingdom:
Plantae
Division:
Magnoliophyta
Class:
Magnoliopsida
Order:
Fagales
Family:
Juglandaceae
Genus:
Juglans

Species

See text

Walnuts (genus Juglans) are plants in the family Juglandaceae. 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.

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. The Latin name, Juglans, derives from Jovis glans, "Jupiter's acorn": figuratively, a nut fit for a god.

The 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).[1] The walnut was so called because it was introduced from Gaul and Italy. The previous Latin name for the walnut was nux Gallica, "Gallic nut".[1]

Species and classification

  • Sect. Juglans. Leaves large (20–45 cm) with 5–9 broad leaflets, hairless, margins entire. Wood hard. Southeast Europe to central Asia.
    • Juglans regia L. (J. duclouxiana Dode, J. fallax Dode, J. orientis Dode) — common walnut, Persian, English, or Carpathian walnut
    • Juglans sigillata Dode — Iron Walnut (doubtfully distinct from J. regia)
  • Sect. Rhysocaryon. Leaves large (20–50 cm) with 11–23 slender leaflets, finely pubescent, margins serrated. Wood hard. North America, South America.
  • Sect. Cardiocaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Northeast Asia, eastern North America.
    • Juglans ailantifolia Carr. (J. cordiformis Maxim., J. sieboldiana Maxim.) — Japanese Walnut
      • Juglans ailantifolia var. cordiformis — Heartnut
    • Juglans cinerea L. — Butternut
    • Juglans mandshurica Maxim. (J. cathayensis Dode, J. formosana Hayata, J. hopeiensis Dode, J. stenocarpa Maxim.) — Manchurian Walnut or Chinese Walnut.
Japanese Walnut foliage and nuts

The best-known member of the genus is the common walnut (J. regia, literally "royal walnut"), native from the Balkans in southeast Europe, southwest & central Asia to the Himalaya and southwest China. According to the UN Food and Agriculture Organization (FAO), Shahmirzad orchard in Iran is the largest in the world (700-750 ha). In Kyrgyzstan alone there are 230,700 ha of walnut-fruit forest, where J. regia is the dominant overstory tree(Hemery and Popov 1998). This is the species which is widely cultivated for its delicious nuts. J. regia is also called English walnut because English merchants marine once controlled its world commerce.

The Black Walnut (J. nigra) is a common species in its native eastern North America, and is also widely cultivated elsewhere. The nuts are edible, but have a smaller kernel and an extremely tough shell, and they are not widely grown for nut production. 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 black walnut (J. nigra).

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 ridges, and the kernel is especially high in fat.

The 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.

Hybrids
  • Juglans x bixbyi Rehd. — J. ailantifolia x J. cinerea
  • Juglans x intermedia Carr. — J. nigra x J. regia
  • Juglans x notha Rehd. — J. ailantifolia x J. regia
  • Juglans x quadrangulata (Carr.) Rehd. — J. cinerea x J. regia
  • Juglans x sinensis (D. C.) Rehd. — J. mandschurica x J. regia
  • Juglans x paradox Burbank — J. hindsii x J. regia
  • Juglans x royal Burbank — J. hindsii x J. nigra

Cultivation and uses

Walnut output in 2005

The two most commercially important species are J. regia for timber and nuts, and J. nigra for timber. Both species have similar cultivation requirements and are widely grown in temperate zones.

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.

Fruit

Some 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.[2][3]

Nuts

Inside of a common walnut nut with green outer layer visible in the top left corner

The nuts of all the species are edible, but the walnuts commonly available in shops are from the common walnut, the only species which has a large nut and thin shell. A horticultural form selected for thin nut shells and hardiness in temperate zones is sometimes known as the 'Carpathian' walnut. 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. Walnuts are also an excellent source of omega-3 fatty acids, and have been shown as helpful in lowering cholesterol. They need to be kept dry and refrigerated to store well; in warm conditions they become rancid in a few weeks, particularly after shelling. Oil paint often employs walnut oil as an effective binding medium, known for its clear, glossy consistency and non-toxicity. Flour made from walnut shells is widely used in the plastics industry. According to some source, some species of walnut produces the 14th best nut in terms of taste.[citation needed]

Two-thirds of the world export market [1][2] and 99% of the US commercial production is grown in California's Central Valley and in Coastal Valleys, from Redding in the north to Bakersfield in the south.[3] Over 30 varieties are grown there, but Chandler and Hartley account for over half of total production.[4]

In California commercial production, the black walnut (J. hindsii) is used mainly as a rootstock for common walnuts because of its resistance to oak root fungus. Black walnuts are more work to crack and remove from the shell.

One ounce of English walnuts has 18.5 grams of total fat and 2.6 grams of omega 3’s. One ounce of black walnut has 16.7 grams of total fat and .57 grams of omega 3’s.[5]

Common walnuts

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. 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.

Shells

The walnut shell has a wide variety of uses. It is commonly used as an organic abrasive, to polish and clean a number of different materials. Eastern black walnut shell is the hardest of the walnut shells, and therefore has the highest resistance to break-down. It is environmentally friendly and can be recycled.

The shells of walnuts.
  • Cleansing and polishing: Walnut shells are mostly used to clean soft metals, fiberglass, plastics, wood and stone. Uses include cleaning automobile and jet engines, electronic circuit boards, and paint and graffiti removal. This soft grit abrasive is well suited for air blasting, de-burring, de-scaling, and polishing operations because of its elasticity and resilience. 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. The problem being that 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.
  • 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
  • Shells also make an excellent dye with no need for mortant. It can produce lovely browns ranging from light to dark browns. Many people have allergic reaction to walnuts.

Husks

Walnut husks are often used to create a rich yellow-brown to dark brown dye that is used for dyeing fabric and for other purposes. When picking walnuts, the husks should be handled wearing rubber gloves, to avoid dyeing one's fingers, as the dye does not require a mordant.

Wood

Walnut shoot cut longitudinally to show chambered pith. Scale in mm.

The common walnut and the black walnut and its allies, are important for their attractive timber, which is hard, dense, tight-grained and polishes to a very smooth finish. The colour ranges from creamy white in the sapwood to a dark chocolate colour in the heartwood. When kiln-dried, walnut wood tends toward a dull brown colour, but when air-dried can become a rich purplish-brown. Because of its colour, hardness and grain it is a prized furniture and carving wood. Walnut burls (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 Lee Enfield rifle 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. Walnut is also used in lutherie, i.e. making stringed 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 Juglans 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.[6] The Walnut Council is the key body linking growers with scientists. In Europe, various EU-led scientific programs have studied walnut growing for timber.[4]

Parkland and garden trees

Walnuts are very attractive trees in parks and large gardens. Walnut trees are easily propagated from the nuts. Seedlings grow rapidly on good soils.[7] The Japanese Walnut in particular is grown 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.[5] [6] However, different walnut species vary in the amount of juglone they release from the roots and fallen leaves - the black walnut in particular is known for its toxicity, both to plants and horses. [7] 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 plants

Walnuts are used as food plants by the larvae of some Lepidoptera species. These include[citation needed]:

In addition, the nuts are a popular snack among woodland creatures, specifically mice and squirrels.

Notes

  1. ^ a b Online Etymology Dictionary - "Walnut"
  2. ^ http://waynesword.palomar.edu/fruitid1.htm Identification Of Major Fruit Types
  3. ^ http://waynesword.palomar.edu/ecoph8.htm Fruits Called Nuts
  4. ^ BBC Radio 4 - Open Country - Oxfordshire
  5. ^ Ross (1996)
  6. ^ West Virginia University Extension Service - "Black Walnut Toxicity"
  7. ^ Rood (2001); Pomogaybin et al (2002)

References

See also

Wikimedia Commons has media related to Juglans.
Wikispecies has information related to Juglans.
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