Everything you need to know about growing walnuts. Learn about:- 1. Description of Walnuts 2. Climate and Soil Suitable for Growing Walnuts 3. Propagation 4. Cultural Practices 5. Harvesting 6. Uses.
Contents:
- Description of Walnuts
- Climate and Soil Suitable for Growing Walnuts
- Propagation of Walnuts
- Cultural Practices for Growing Walnuts
- Harvesting of Walnuts
- Uses of Walnuts
1. Description of Walnuts:
A large, deciduous, monoecious tree with tomentose shoots, found throughout the Himalayas and hills of Assam at altitudes of 3,000—11,000 ft. Bark grey, longitudinally fissured; leaves alternate, imparipinnate, 6—15 in. long- leaflets 5—13, subsessile, elliptic to oblong-lanceolate, 3-8 in. x 1.5-4 in., usually entire; flowers small, yellowish green- male in pendulous slender catkins, 2—5 in. long, female in 1—3 flowered, terminal catkins; fruit a green drupe with leathery exocarp, indehiscent, ellipsoid-globose, c. 2 in. across- endocarp hard, woody, wrinkled, 2-valved, enclosing 4-lobed, corrugated, oily, edible seed. A number of varieties of J. regia, based on geographical distribution and characters of the nut-shell (endocarp), have been recognised.
Common walnut occurs in natural forests either in pure crops or in mixture with other broad-leaved species or conifers and often attains a height of 80—100 ft. and a girth of 10—15 ft. or more. When grown as a fruit tree, it is trained to have a spreading crown and a comparatively short bole.
Under natural conditions, the fruits fall to the ground under and around the tree, the exocarp cracking and rotting off. The nuts, however, are subject to attack by birds, monkeys and rodents, and large quantities of them are destroyed. A covering of earth or debris over the nuts and a fair degree of warmth and soil moisture are necessary for germination.
Natural reproduction takes place on gentle slopes with loose, but deep, fairly moist soil; where boulders and rock fragments are plentiful, they act as a protection to nuts. Good crops of walnut are found on detrital accumulations formed by the weathering of cliffs or erosion of hillsides and on deep loose rubble on landslips in the open. Walnut is a light demander, though it stands slight shade in youth. It coppices well.
2. Climate and Soil Suitable for Growing Walnuts:
Walnut requires a climate free from frost in spring and extreme heat in summer. A temperature of even 2-3°F below freezing point kills young flowers; if the summers are hot (100°F and above in shade and low humidity), the nuts get sun-burnt and become ‘blanks’. Walnut thrives well in areas with an annual rainfall of 30 in or more. Other conditions being favourable, deficiency in rainfall can be made up by artificial irrigation.
The soil should be deep and well-drained; silt loam, 8-10 ft. deep, containing abundant organic matter gives the best results. The soil should not have a fluctuating water level, hard pan, sandy subsoil or alkali. Trees growing on shallow soil with low moisture are more liable to sun-burn than those growing on deep silt soil.
3. Propagation of Walnuts:
The trees are usually raised from seedlings. In certain parts of Himachal Pradesh and Kulu valley, planting of walnut is considered inauspicious; even seedlings of selected trees are not planted. Only seedlings that come up in nature are allowed to grow and this has resulted in a preponderance of inferior or mediocre type of trees. In Kashmir, planting of seedling walnuts is popular; about 10,000 trees are reported to be planted every year, out of which some 6,000 survive.
Nuts for raising seedling plants should be collected from vigorous and high-yielding trees. Desirable characters to look for in the selection of nuts are the following- big size; bright brown shell of good cracking quality; and bright yellow kernel with good taste and flavour. Nuts should be stored in a cool dry place or stratified after harvest till the following December.
If the soil is ready for sowing, beds may be prepared and nuts sown c. 2 in. deep immediately after harvest in rows 1 ft. apart, the distance between sowings in the row being 1 ft. Germination starts in the beginning of March and seedlings are ready for transplanting in the second year.
Walnut is propagated in other countries by vegetative methods; the methods in use are-whip grafting, cleft grafting, inlay bark grafting and patch budding in early spring. In Kashmir, patch budding and crown grafting have been tried and found successful, but these methods have not been adopted on any appreciable scale.
The improvement of walnut cultivation in India is dependent on the introduction of budded and grafted trees of good varieties. No systematic attempt appears to have been made so far to introduce these methods.
4. Cultural Practices for Growing Walnuts:
Walnut is usually grown under rain-fed conditions; only during the first dry season after planting is water supplied to plants by growers. It has been found advantageous to irrigate the trees during dry weather. Irrigation helps growth and trees come into bearing early.
Irrigation should be continued till the maturity of fruits as it reduces nut fall and favours filling of nuts. If the trees are irrigated one or two weeks before harvest, the husks are seen to open and remain on the trees, while clean nuts drop to the ground; harvesting is thereby facilitated.
Walnut trees are seldom manured in India. On account of its deep root system, the trees forage over an extensive area and bear fairly good crops. The yields, however, are less in comparison with those of manured trees; also, unmanured trees tend to become alternate-bearing. It is a good practice to manure the trees every year and also apply nitrogenous and phosphatic fertilisers, the doses depending upon the age, size and bearing quality of trees, and on the fertility of soil.
Practically no pruning is given to walnut trees. The trees should be trained on a single stem up to a height of 3-4 ft. and scaffold branches should be retained at proper places. Any crossing or surplus branches and diseased and dry twigs should be removed every year.
Walnut is self-fertile, but pollination is not satisfactory in some varieties in certain years, because of the failure of pollen to mature at the time female flowers are receptive. The pollen is distributed by wind and may be carried up to a mile; the effective range is 200- 300 ft.
New plantations of walnuts, if located near existing bearing trees, will, therefore, start fruiting at an earlier age and yield greater quantities of nuts than those located far away from bearing trees. The pollen of any one variety is capable of fertilising its own pistils or the pistils of any other variety. Good and poor crops in Kashmir are generally associated with prevailing weather conditions, especially at the time of flowering.
5. Harvesting of Walnuts:
Walnut fruits ripen in September-October. The husks split at the time of maturity and the nuts which fall to the ground are collected. Nut-fall is hastened by shaking the branches with hand or with bamboo poles to the ends of which hooks are attached; shaking is repeated 2-3 times at intervals. After collection, nuts are cleaned, washed and dried by spreading on floor or canvas sheets.
Nuts which fall to the ground with husks are generally of inferior grade; after removing the husks, cleaning and drying, they are marketed separately. Before marketing, the nuts are graded according to size and colour. In some areas, nuts are bleached by dipping in a mixture of aqueous solution of chloride of lime and sodium carbonate; the mixture is allowed to settle and the clear solution used for dipping. Dilute chlorine solution may also be used for bleaching.
The tree starts to bear crop at the age of 8—10 years. The yield of nuts varies according to the age, size and variety of tree. Outer branches produce the best fruit. A grown up tree of big size may yield 4-5 md of nuts. However, considering the on and off years and varying ages of trees, the average yield per tree may be taken as one maund.
6. Uses of Walnuts:
Walnut oil is used for edible purposes; small quantities are used for artists’ oil colours, printing inks, varnishes and for making soap. The oil dries rather slowly; heat treatment improves the drying properties. The varnish prepared from the oil is pale, non-yellowing and less liable to crack than linseed oil varnish.
The supply of oil is limited owing to the demand for kernels. In U.S.A., walnut oil is expressed from waste meats from shelling plants and occasionally from surplus nuts. The oil is sometimes adulterated with poppy seed and linseed oils.
The oilcake is rich in proteins and is used as cattle feed. Its composition and nutritive value are as follows- dry matter, 86.6; protein, 35.0; fatty oil, 12.2; carbohydrates, 27.6; fibre, 6.7; and ash, 5.1%; digestible nutrients—crude protein, 31.5; fatty oil, 11.6; carbohydrates, 23.5; and fibre, 1.7%; nutritive ratio, 1.7; and starch equivalent, 78.5.
Analysis of walnut shells gave the following values: dry matter, 92.3; protein, 1.7; fatty oil, 0.7; carbohydrates, 31.9; fibre, 56.6; and ash, 1.4%.
Walnut shell flour is used as a filler in moulded plastics; it can be used upto 40% as an extender in resin adhesives. Shell flour contains cellulose, lignin (28%), furfural (5%), pentosans (9%), methylhydroxylamine (6%), sugar, and starch (2.5%). Shells may be used as an anti-skid agent for car and tractor, tyres, as blasting grit for loosening coatings and deposits on metals, and in the preparation of activated carbon.
Fresh leaves, like unripe fruit, are rich in ascorbic acid (800- 1,300 mg./100g. green wt.). Leaves may be preserved by exposing them to sulphur dioxide gas and then rapidly drying at 100—110°; treated leaves may be pressed or extracted with water to obtain concentrates of ascorbic acid (recovery, 80—93%). The leaves are also rich in carotene (30 mg./ 100 g. green wt.). Concentrates of carotene may be prepared from fumigated leaves.
On steam-distillation, the leaves yield an olive brown volatile oil, with an odour reminiscent of tea and amber. Oils distilled from fresh leaves, from Germany (yield, 0.012-0.029%), had the following ranges of constants- d30° 0.9037—0.9137; [a]b, nil; acid val., 9.3— 16.8; ester val., 18.4-27.0; soluble in alcohol (90%) with separation of a paraffin (m.p., 61—62°); the paraffin separates out also on cooling the oil. The constants of the oil examined in France (yield, 0.0087%) were as follows- d30°, 0.9185; [a].D, —17.0°; n25° 1.4922; and ester val. after acetylation, 98.5.
Aqueous extracts of fresh leaves, free of juglone, possess strong bactericidal action against Bacillus anthracis and Coryncbacterium diphtheriae; it is weakly active against Vibrio comma, Bacillus sub- tiles, Pneumococci, Streptococci, Micrococcus pyogens var. aureus, Proteus, Escherichia coli, Salmonella typhosa, S. typhimurium and S. dysenteriae. The extracts are not toxic to mice.
Twigs and leaves of walnut are lopped for fodder. The leaves contain (moisture-free basis); N, 3.22; and ash.
Green walnut hulls, shells, bark and leaves are used for dyeing and tanning. They contain tannin (hulls, 12.23; bark, 7.51; mature leaf blades, 9-11%) and juglone. The bark is sold in bazaars under the name Dundasa and is used for cleaning teeth or for chewing to redden the lips. Green walnut shells have been used as hair dye in the form of an oily extract or alcoholic extract mordanted with alum.
Juglone imparts a brownish yellow colour on mordanted wool and a rose tint on mordanted cotton. The colours are inferior to synthetic dyes in fastness, particularly against light.
Walnut leaves are astringent, tonic and anthelmintic. The leaves and bark are alterative detergent; they are used in herpes, eczema, scrofula and syphilis; the fruit is reported to be used as alterative in rheumatism. The vinegar of pickled young fruit is used as gargle for sore throat. Green hull and unripe shell are anti-syphilitic and vermifugal.
The expressed oil of the fruit is considered useful against tapeworm and as a laxative injection. In Malaya, kernels are recommended in colic and dysentery. The rind of the unripe fruit is used as fish poison.