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Walnut

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Walnut
Shoot and nut of Juglans regia
Shoot and nut of Juglans regia
Plant Info
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Fagales
Family: Juglandaceae
Genus: Juglans
L.

Species
See text

Walnuts (genus Juglans) are plants in the walnut family Juglandaceae. They are deciduous trees, 10 - 40 metres tall, with pinnate leaves 200 - 900 millimetres long, 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.

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

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 nuts", the nut fit for a god.

Walnuts are used as food plants by the larvae of some Lepidoptera species including Brown-tail, Common Emerald, Emperor Moth, The Engrailed and the Coleophora case-bearers C. laticornella (recorded on J. nigra) and C. pruniella.

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

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) - Persian 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 cinerea L. - Butternut
    • Jugl

ans mandschurica Maxim. (J. cathayensis Dode, J. formosana Hayata, J. hopeiensis Dode, J. stenocarpa Maxim.) - Chinese Walnut or Manchurian Walnut

 
Japanese Walnut foliage and nuts

The best-known member of the genus is the Persian Walnut (Juglans regia), native from the Balkans in southeast Europe, southwest & central Asia to the Himalaya and southwest China. In Kyrgyzstan alone there are 230,700 ha of walnut-fruit forest, where J. regia is the dominant overstorey (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 merchant marines once controlled its world commerce.

The Black Walnut (Juglans 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 Butternut (Juglans 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, and the nuts oval.

The Japanese Walnut (Juglans ailantifolia) is similar to Butternut, distinguished by the larger leaves up to 90 cm long, and round (not oval) nuts.

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

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

Walnuts are light-demanding species but benefit from protection from wind. Research has shown that interplanting walnut plantations with a nitrogen fixing plant such as Elaeagnus × ebbingei or E. umbellata, and various Alnus species can have a dramatic impact on tree height and girth, resulting in a 30% increase according to some studies (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 N. 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.

Nuts
 
Persian Walnut nuts
 
Inside of a Persian Walnut nut with green outer layer visible in the top left corner
 
Persian Walnuts

The nuts of all the species are edible, but the walnuts commonly available in stores are from the Persian 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. Oil paint often employs walnut oil as an effective binding medium, known for its clear, glossy consistency and non-toxicity . 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.

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.

Walnut nut 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.

Wood

The Persian Walnut, and the Black Walnut and its allies, are important for their attractive timber, which (except in young trees) is hard, dense, tight-grained and polishes to a very smooth finish. The color ranges from creamy white in the sapwood to a dark chocolate color in the heartwood. When kiln-dried, walnut wood tends toward a dull brown color, but when air-dried can become a rich purplish-brown. Because of its color, 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 presitige car manufacturers. Walnut wood has been the timber of choice for gunmakers for centuries, including the Lee Enfield rifle of the First World War. Today it is used for exclusive sporting guns, by makers such as Purdy of London. The wood of the Butternut and related Asian species is of much lower value, softer, coarser, less strong and heavy, and paler in color.

In North America research has been undertaken mostly on Juglans nigra aiming to improve the quality of planting stock and markets. The Walnut Council is the key body linking growers with scientists. In Europe, various EU-led scientific programs have studied walnut growing for timber (e.g. in the UK).

Parkland and garden trees

Walnuts are very attractive trees in parks and large gardens. The Japanese Walnut in particular is grown for its huge leaves, which have a 'tropical' appearance.

Walnuts are not particularly well suited to smaller urban gardens. They drop numerous small twigs, leaves, branches or nuts, so are considered "messy" by some people; the falling nuts in late summer and early autumn can be quite dangerous. Both the fallen leaves and the roots secrete a substance called juglone which kills many popular garden plants, such as tomato, apple and birch; all walnuts produce juglone, but Black Walnut produces larger amounts than other species. Juglone appears to be one of the walnut's primary defense 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 many years even after a walnut is removed as its roots slowly decompose and release juglone into the soil.

An old English rhyme states:

A spaniel, a woman, and a walnut tree;
The more they're beaten,
The better still they be.

The meaning of this is the source of some debate, with three different takes on it given in the BBC's Gardeners' Question Time.

Health benefits of walnuts

 
Juglans regia walnuts.

A new study published in the Journal of the American College of Cardiology found that eating walnuts after a meal high in unhealthy fats can reduce the damaging effects of such fats on blood vessels. Researchers from Barcelona's Ho spital Clinic conducted a study on 24 adult participants, half of whom had normal cholesterol levels, and half of whom had moderately high levels of cholesterol. Each group was fed two high-fat meals of salami and cheese, eaten one week apart. During one meal, the researchers supplemented the food with five teaspoons of olive oil. The researcher added eight shelled walnuts to the other meal, the following week.

Tests after each meal showed that both the olive oil and the walnuts helped reduce the onset of dangerous inflammation and oxidation in the arteries after the meals, which were high in saturated fat. However, unlike the olive oil, the walnuts also helped the arteries maintain their elasticity and flexibility, even in the participants with higher cholesterol.

Lead researcher Dr. Emilio Ros said walnuts' protective effects could be because the nuts are high in antioxidants and ALA, a plant-based omega-3 fatty acid. Walnuts also contain arginine, which is an amino acid that the body uses to produce nitric oxide, necessary for keeping blood vessels flexible.

Source: Journal of the American College of Cardiology - Oct. 17, 2006 edition

Walnuts in the future

Walnut species are deep rooted and already play an important role in slope stabilization, e.g. in the Himalaya. Some species may become more suited to northerly climes under a changing clmate, for example becoming more productive (timber and nuts)in the UK, and northerly US states. Their dark quality hardwood is potentially a valuable domestic timber for temperate areas and therefore an altenative source of tropical timbers such as Mahogany. Fast-grown hybrid walnut may produce a viable wood product for wood energy systems under Short Rotation Forestry.

References

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