Walnut trees are any species of tree in the plant genusJuglans, thetype genus of thefamilyJuglandaceae, the seeds of which are referred to aswalnuts. All species aredeciduous trees, 10–40metres (33–131 ft) tall, withpinnate leaves 200–900 millimetres (7.9–35.4 in), with 5–25 leaflets; the shoots have chamberedpith, a character shared with thewingnuts (Pterocarya), but not thehickories (Carya) in the same family.
The 21 species in the genus range across the north temperateOld World from southeast Europe east to Japan, and more widely in theNew World from southeast Canada west to California and south to Argentina.
Edible walnuts, which are consumed worldwide, are usually harvested from cultivated varieties of the speciesJuglans regia. China produces half of the world total of walnuts.
The common namewalnut derives fromOld Englishwealhhnutu, literally 'foreign nut' (fromwealh 'foreign' +hnutu 'nut'),[2] because it was introduced fromGaul and Italy.[3] TheLatin name for the walnut wasnux Gallica, "Gallic nut". The nameJuglans was apparently conjured byLinnaeus himself,[4] replacing the priorNux,[disputed –discuss] by combiningJu from Jupiter with the Latinglans meaning 'acorn',[5] referring to the association of the plant with Jupiter by the Romans.
In 2017, world production ofwalnuts (in shell) was 3.8 milliontonnes, led by China with producing half of the world total (table). Other major producers were the United States (15%) and Iran (9%).
The two most commercially important species areJ. regia for timber and nuts, andJ. 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 anitrogen fixing plant, such asElaeagnus × ebbingei orElaeagnus umbellata, and variousAlnus species, results in a 30% increase in tree height and girth (Hemery 2001).
When grown for nuts, care must be taken to selectcultivars that are compatible for pollination purposes; although some cultivars are marketed as "self fertile", they will generally fruit better with a different pollination partner. Many different cultivars are available for growers, and offer different growth habits, flowering and leafing, kernel flavours and shell thicknesses. A key trait for more northerly latitudes of North America and Europe isphenology, 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.
The leaves and blossoms of the walnut tree normally appear in spring. The male cylindrical catkins are developed from leafless shoots from the past year; they are about 10 cm (3.9 in) 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.[8]
The fruits of the walnut are a type ofaccessory fruit known as a pseudodrupe (or drupe-like nut), the outer covering of the fruit is aninvolucre - in a drupe the covering would be derived from the carpel.[9]
The nut kernels of all the species are edible, but the walnuts most commonly traded are from theJ. regia, the only species which has a large nut and thin shell.J. nigra kernels are also produced commercially in the US.
Two-thirds of the world export market[citation needed][how?][10][11] and 99% of US walnuts are grown in California'sCentral Valley and in Coastal Valleys, fromRedding in the north toBakersfield in the south.[12] Of the more than 30 varieties ofJ. regia grown there, Chandler and Hartley account for over half of total production.[11] In California commercial production, the Hinds' black walnut (J. hindsii) and the hybrid betweenJ. hindsii andJ. regia,Juglans x paradox, are widely used asrootstocks forJ. regia cultivars because of their resistance toPhytophthora and to a very limited degree, the oak root fungus. However, trees grafted on these rootstocks often succumb to black line.[13]
In some countries, immature nuts in their husks are preserved invinegar. In the UK, these are calledpickled walnuts and this is one of the major uses for fresh nuts from the small scale plantings. InArmenian cuisine, unripe walnuts, including husks, are preserved in sugar syrup and eaten whole. In Italy,liqueurs calledNocino andNocello are flavoured with walnuts, whileSalsa di Noci (walnut sauce) is a pasta sauce originating fromLiguria. InGeorgia, walnuts are ground with other ingredients to makewalnut sauce.
Green leaves of a walnut tree with budding walnuts, inKashmir Valley.
Walnuts are heavily used in India. InJammu, they are used widely as aprasad (offering) to Mother Goddess VaisnavDevi and, generally, as a dry food in the season of festivals such asDiwali.
The nuts are rich inoil, and are widely eaten both fresh and incookery. Walnut oil is expensive and consequently is used sparingly; most often insalad dressings. Walnut oil has been used inoil paint, as an effective binding medium, known for its clear, glossy consistency and nontoxicity.
Manos and Stone studied the composition of seed oils from several species of theRhoipteleaceae andJuglandaceae and found the nut oils were generally more unsaturated from species which grow in thetemperate zones and more saturated for species which grow in thetropical zones.[14] In the northerly-growing sectionTrachycaryon,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 saturatedpalmitate, and a maximum concentration of 71%linoleate. In the sectionJuglans,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 sectionCardiocaryon, the nut oils ofJ. ailantifolia andJ. mandshurica were reported to contain (respectively) 7% and 5% of linolenate, 2% of palmitate, and maximum concentrations of 74% and 79% linoleate. Within the sectionRhysocaryon, the nut oils of the U.S. native black walnutsJ. microcarpa andJ. 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; andJ. olanchana contained only a trace of linolenate, 9% palmitate, and 73% linoleate;
The 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 breakdown.
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, deburring, descaling, 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
Flour from walnut shells can be used in thermoplastic starch composites to substitute oil derivatives.[15]
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.
Walnut husks are often used to create a rich yellow-brown to dark browndye used for dyeingfabric, yarn orwood and for other purposes. The dye does not require amordant and will readily stain the hand if picked without gloves.
Walnut shoot cut longitudinally to show chamberedpith, 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 color is dark chocolate or similar in the heartwood changing by a sharp boundary to creamy white in the sapwood. 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.
When walnutvascular cambium is involved in a crotch (a branch fork), it behaves unusually, producing characteristic "crotch figure" in the wood which it makes. The grain figure exposed when a crotch in a walnut log is cut in the plane of its one entering branch and two exiting branches is attractive and sought after.
There are some differences between the wood of the European walnut (Juglans regia) and the wood of the black walnut (Juglans nigra). For example,Juglans regia wood sometimes has patches with a wavy texture.[16] Black walnut wood tends to be darker than European walnut wood, and can suffer from paler sapwood that only really comes to light when the wood has been planed.
Walnut wood has been the timber of choice for gun makers for centuries, including theGewehr 98 andLee–Enfield rifles of the First World War. It remains one of 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 inlutherie and for the body ofpipe organs.
Walnutburls (or "burrs" in the rest of the world) are commonly used to create bowls and other turned pieces. Walnut burlveneer is one of the most valuable and highly prized by cabinet makers and prestige car manufacturers.
The wood of thebutternut and related Asian species is of much lower value, softer, coarser, less strong and heavy, and paler in colour.
Freshly sawn walnut heartwood may be greenish in color, but with exposure to air this color quickly changes to brown due to oxidation of the pigment.
In North America, forestry research has been undertaken, mostly onJ. 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.[17] The Walnut Council[18] is the key body linking growers with scientists. In Europe, various EU-led scientific programmes have studied walnut growing for timber.[19]
TheCherokee Indians would produce a black dye from walnut bark, which they used to dye cloth.[20] As early as the 2nd century CE, shells and kernels of the edible walnut were used to make a dye solution in theLevant.[21][22]
Walnuts are very attractive trees in parks and large gardens. Walnut trees are easily propagated from the nuts. Seedlings grow rapidly on good soils.[17]The Japanese walnut in particular is known for its huge leaves, which have a tropical appearance.
Walnut tree in a garden
As garden trees, they have some drawbacks, in particular the falling nuts, and the releasing of theallelopathic compoundjuglone, though a number of gardeners do grow them.[23][24] 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.[25] Juglone istoxic to plants such as tomato, apple, andbirch, 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.
The bride (a moth) (Catocala neogama) –nominate subspecies on butternut and others,C. n. euphemia on Arizona black walnut, perhaps Texas black walnut and others.
The nuts are consumed by other animals, such as mice and squirrels.
In California (US) and Switzerland, crows have been witnessed taking walnuts into their beaks, flying up to 60 feet or so in the air, and dropping them to the ground to crack the shells and eat the nut inside.[26]
Leaves are very large (40–90 cm), with 11–19 broad leaflets, softly downy, margins serrated. The wood is soft, and the fruits borne in racemes of up to 20. The nuts have thick shells. Native to northeast Asia.
J. ailantifolia Carr. (J. cordiformis Maxim.,J. sieboldiana Maxim.)
Leaves are very large (40–90 cm), with 11–19 broad leaflets, softly downy, margins serrated. The wood is soft. Fruits are borne in clusters of two to three. The nuts have a thick, rough shell bearing distinct, sharp ridges. Native to eastern North America.
The best-known member of the genus is thePersian walnut (J. regia, literally "royal walnut"), native from theBalkans in southeast Europe, southwest and central Asia to the Himalaya and southwest China. Walnuts are a traditional feature ofIranian cuisine; the nation has extensive orchards which are an important feature of regional economies. InKyrgyzstan alone, there are 230,700 ha of walnut-fruit forest, whereJ. regia is the dominant overstory tree (Hemery and Popov 1998). In non-European English-speaking nations, the nut of theJ. regia is often called the "English walnut"; in Great Britain, the "common walnut."
Theeastern 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 because it is easier to extract the nutmeat. The wood is particularly valuable.
TheHinds' black walnut (J. hindsii) is native to northern California, where it has been widely used commercially as a rootstock forJ. regia trees. Hinds' black walnut shells do not have the deep grooves characteristic of the eastern black walnut.
Japanese walnut foliage and nuts
TheJapanese walnut (J. ailantifolia) is similar to butternut, distinguished by the larger leaves up to 90 cm long, and round (not oval) nuts. The varietycordiformis, often called theheartnut has heart-shaped nuts; the common name of this variety is the source of the sectional name Cardiocaryon.
Thebutternut (J. cinerea) is also native to eastern North America, where it is currentlyendangered by an introduced disease,butternut canker, caused by thefungusSirococcus clavigignenti-juglandacearum. 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.
A study[29] 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 theLEAFY gene taken from at least one individual of most of the species of Juglans has supported several conclusions:
The genusJuglans 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 twoclades:
one comprises the more northerly speciesJ. californica,J. hindsii,J. hirsuta,J. major,J. microcarpa, andJ. nigra;
the other comprises the more southerly speciesJ. australis,J. boliviana,J. jamaicensis,J. molis,J. neotropica,J. olanchana,J. steyermarkii, andJ. venezuelensis
J. olanchana var.standleyi seems to be more closely related toJ. steyermarkii than toJ. olanchana var.olanchana, suggestingJ. olanchana var.standleyi might be better understood as either a separate species or a variety ofJ. steyermarkii.
The paper presenting these results did not publish any new names for the subdivisions of sect. Rhysocaryon, for any combinations of the other sections, or forJ. olanchana var.standleyi.
Fossils ofJuglans nuts have been described from theTertiary period of North America.[30] The paleontological history ofJuglans regia in Europe shows signs of a post-Ice-Age re-expansíon from refugia in the southeast, much influenced by people carrying walnut nuts about after the numbers of humans had been much increased by the start of agriculture.[citation needed]
^Knight, Oliver (1956–57), "History of the Cherokees, 1830–1846",Chronicles of Oklahoma, Oklahoma City: Oklahoma Historical Society, p. 164,OCLC647927893
^Aradhya, M. K., D. Potter, F. Gao, C. J. Simon: "Molecular phylogeny ofJuglans (Juglandaceae): a biogeographic perspective",Tree Genetics & Genomes(2007)3:363–378
^D. Stone, S. Oh, E. Tripp, Luis. Gios, P. Manos: "Natural history, distribution, phylogenetic relationships, and conservation of Central American black walnuts (Juglans sect. Rhysocaryon)",Journal of the Torrey Botanical Society136(1)1–25. 2009.
.jpg)
.jpg)
.jpg)
