Everything you need to know about growing and harvesting walnut! Learn about: 1. Botany of Walnut 2. Climate and Soil Required for Growing Walnut 3. Orchard Cultural Practices 4. Propagation 5. Pollination 6. Planting Operation 7. Flowering and Fruiting 8. Training and Pruning 9. Harvesting and Handling.
Botany of Walnut:
Walnut belongs to the family Juglandaceae and genus Juglans. The genus Juglans includes about 21 species among which Juglans regia (English or Persian walnut) is the most important being commercially cultivated in many countries. The other species are grown locally for timber and nuts. The somatic chromosome number (2 n) of Juglans is 32.
The important species of genus Juglans are as follows:
(i) Juglans Califonica- Southern California black walnut
(ii) Juglans Hindsii- Northern California black walnut
(iii) Juglans Jamaicensis- West Indian walnut
(iv) Juglans Major- Arizona black walnut
(v) Juglans Microcarpa- Texas black walnut
(vi) Juglans Nigra- Eastern black walnut
(vii) Juglans Ailantifolia- Japanese walnut
(viii) Juglans Cathayensis- Chinese walnut
(ix) Juglans Mandschurica- Manchurian walnut
(x) Juglans Cinerea- The buttera nut
Walnut is a deciduous tree. Leaves alternate, odd pinnate, large, aromatic, estipulate, leaflets opposite, serrate or entire. Flowers monoecious, the staminate flowers on last year’s branchlets in lateral peduncles catkins, each consisting of a bract bearing 8-40 stamens, 2 bractlets and 1-4 calyx lobes.
Pistillate flowers in few to many flowered terminal racemes with 4 calyx lobes and a 3-lobbed involucre consisting of a bract and 2 bractlets. Fruit a large indehiscent drupe. Nut thick walled, incompletely 2-4 celled, indehiscent or finally separating into 2 valves. Seed 2-4 lobed, remaining within the shell in germination. Flowers appear before or after the leaves. Edible portion is cotyledons.
Climate and Soil Required for Growing Walnut:
Walnut is found growing in all parts of the Himalayan region between the elevations of 1200 to 2100 metres above sea level. It requires a climate which is free from frost in spring and from extreme heat in summer. A temperature of even 2-3°C below freezing point (0°C) results in the killing of a large percentage of young walnut flowers if they are just starting to bloom.
If the summers are very hot, the nuts get sunburnt and such nuts usually become Blanks particularly if the temperature goes to about 37.8°C in the shade and humidity is low.
Sufficient chilling is needed for bud opening and blooming. The chilling requirement of different varieties ranges from 700 to 1500 hours. But, most cultivars needed 800 hours of winter temp, below 7°C for normal growth. Temperature of 29-32°C near harvest time results in well filled kernels. In cool summers, walnuts become excessively shriveled. The walnut is mostly grown in the mid-hills of Kashmir, Kulu Valley in Himachal Pradesh and Uttarakhand and Arunachal Pradesh. They grow well in areas with rainfall of about 76 cm or more.
In Kashmir, walnut cultivation is practiced at an altitude of about 2000 m or more. A site, where frequent hailstorms are experienced or cold air settles down in low land making a frost pocket, is unsuitable for walnut. Higher northern sloping land is preferable to southern slopes and low lands as the plants growing there are less subjected to frost injury. The growth on northern slopes starts later because these are cooler than southern slopes. Evaporation on northern slope is considerably less as compared to southern slope.
The soil most suitable for walnuts is a well-drained, deep, silt loam containing an abundance of organic matter. Soil pH influences the availability of mineral nutrients and maturity of which are available to the plant in the pH range of 6-7. Soil should not have a fluctuating water level, hardpan, and sandy sub-soil.
A soil 2-3 metres deep gives the best results because walnut roots penetrate upto a depth of about 3 metres. The sub soil should be free from solid rock, impervious clay or gravel layers which restrict root growth. The trees growing on shallow soil with less moisture content are liable to suffer more from sunburn than the trees growing on a deep silt soil. Alkaline soils and sandy sub-soils should be avoided for walnut growing.
Orchard Cultural Practices for Growing Walnut:
i. Irrigation:
Walnut trees are grown under rainfed conditions. However, whenever the rain is not sufficient and well distributed, irrigation is essential for walnut cultivation. Inadequate water supply to the trees results in slow growth, reduced yield and low nut quality, but excess of it retards orchard development and causes root and crown diseases. Where irrigation is available, it will be an advantage to apply casual irrigations to the trees.
This will help them to put on better growth and start bearing early. To the bearing trees, irrigation from the time of fruit set till its maturity proves helpful in reducing the fruit drop and better filling of nuts. The water requirement decreases with the development of roots in adult trees. If irrigation facility is available it should be applied 2-4 weeks before expected harvest for obtaining plump kernels.
ii. Manuring and Fertilization:
Generally, no fertilizers are given to walnut trees in India. To avoid alternate bearing, it will be a good practice to manure the trees every year with nitrogenous and phosphatic fertilizers.
Nitrogen fertilization is essential for successful walnut cultivation. An application of 175-225 kg N per ha is sufficient to prevent the occurrence of nitrogen deficiency. Walnut does not respond to phosphorus application. To cope the need of potassium, a single application of 350 kg potassium sulphate per ha should be given.
For good growth of walnut trees 180 kg N, 180 kg P2O5 and 60 kg K2O per hectare is considered optimum. For getting the highest yield and net profit, a general application of 60 kg/ha each of N, P and K is suggested. The bearing tree of walnut should be supplied with 50-100 kg of farmyard manure every year in the month of December. The fertilizer mixture should be applied during early spring before the start of new growth.
In Kashmir, 2 kg nitrogen, 500 g phosphorus and 1.5 kg potash is suggested to full grown tree of 20 years and above. Phosphorus, potash and half nitrogen is applied two weeks before expected bloom. Rest of the nitrogen should be split into two equal doses, one to be applied three weeks after fruit set and second during early July.
Zinc deficiency occurs in walnut trees. It can be corrected with the foliar spray or 0.4 per cent zinc sulphate.
Propagation of Walnut:
Walnut can be propagated either by seed or by vegetative methods. The seedlings are highly variable producing the nuts of inferior quality with an average shelling percentage of 30-35 per cent as compared to 50-60 per cent of good commercial cultivars.
Seed Propagation:
The majority of trees in India are of seedling origin and as such they bear nuts of variable quality. The seedlings are usually raised for rootstock purpose but in this case hard-shelled nuts of inferior quality are sown. Although walnut germinates without stratification under some conditions, yet proper stratification is desirable for good germination and growth.
The walnuts should be collected from vigorous and high yielding trees. The desirable characters to look for in the selection of nuts are big size, bright brown colour and good cracking quality of the shell, bright yellow colour, good taste and flavour of the kernel.
The moist nuts are placed in alternate layers with a suitable medium such as vermiculite, peat moss or sand in container and then subjected to cold treatment (3-5°C) for a period of 2-3 months. The nuts can also be sown directly in the nursery or field in fall, but the soil is to be kept just moist, neither dry not wet.
The walnuts are stratified till December. If the soil is ready for sowing, the nuts are sown immediately in well prepared beds at a distance of 60 cm in row to row and 30 cm in seed to seed. The walnuts are sown 8-10 cm deep. The germination starts in the beginning of March.
The seedlings do not attain good height in the first year but their root system is well developed. In the second year, the seedlings develop quickly and are ready for transplanting by the time the growth stops in the autumn. Seeds can also be sown in situ in December.
A large number of factors such as nut type, size, position of seed, planting depth, soil moisture and temperature and seed treatment influence the germination of seeds. The planting of large nuts at 7 cm depth in soil in vertical position gives better germination and seedling growth. The walnut of medium size (3-4 cm long) gave a germination of 90 per cent compared with only 71 per cent for smaller seeds and 85 per cent for larger ones.
Higher germination of seeds sown in a vertical position was noticed than those placed flat in the soil. Treatments of nut with gibberellic acid at 750-1000 ppm increase the germination percentage and height of seedlings. Treating the seeds with ethrel at 100-1000 ppm was found to improve germination.
When the seedlings are to be left in place for about 2 years or more, a spacing of one metre between rows and 30 cm between seeds is a common practice. The mulch can be placed to conserve moisture and control the weed which helps in better germination and growth of the seedlings. In order to ensure the establishment of grafts after transplanting, their taproot is cut off 20-25 cm below the ground level which should be done about a year before actual transplanting.
Vegetative Propagation:
Walnut can be propagated vegetatively by grafting, budding and stooling. The per cent success in grafting and budding in walnut is lower than in pome and stone fruits. Better success in grafting by cleft, tongue and splice methods was obtained when grafting was either done in a greenhouse have temperature 28°C and relative humidity 80-100% or the grafts after preparation were placed in the incubator at 25 ± 1°C temperature with 75 ± 2% humidity.
The most common method of grafting practiced in the case of this nut is cleft grafting. It gives higher success when incubated at 26°C and 90% relative humidity for 3 weeks before planting in the nursery. Better survival of grafts is obtained when grafting operation is carried out in the middle of January rather than in the last week of December or in the middle of February. Veneer grafting carried out in July gave more than 80% success followed by tongue grafting done in the late January with 50% success.
Patch budding is common among the budding methods which give 30-35% success. Mid June is found an optimum time of budding. Removal of leaf blades 15 days before budding operation enhanced bud take. In Kashmir, the methods of patch budding and crown grafting have been found fairly successful.
Walnut can also be propagated through annular budding and chip budding. Annular budding gives a very high percentage of bud take in walnut when performed during the month of July. Chip budding during May give good performance in walnut.
Stooling is another method of vegetative propagation which can be performed in walnut. To obtain good rooting and better establishment of stool layers, the growth regulators like IB A, IAA and NAA can be used. The mother stools of the commercial cultivars will have to be raised by cuttings or layers in the first instance.
Pollination in Walnut:
The walnut is monoecious. The pollen is distributed by wind. The wind may carry the pollen upto 2 km but the effective range of distribution is 60 metres to 90 metres. The walnut is self-fertile but pollination is not satisfactory in certain varieties mostly because these varieties fail to mature their pollen at time when the female flowers are receptive. The pollen of any variety is capable of fertilizing its own pistils or those of any variety. Snowfall or very cold weather at the time of flowering causes poor setting of the crop.
In certain regions, the varieties Payne, Eureka and Franquette shed their pollen before their pistillate flowers are receptive. In other places the varieties Placentia and others of Santa Barbara soft shell seedling group tend to shed their pollen after the pistillate flowers have ceased to be receptive. Hot spring weather somewhat hastens the development of catkins and makes them shed their pollen quickly.
The pollination difficulties in established orchards can be solved temporarily by bringing catkins from the adjoining plantations and hanging them in the trees and permanently by top-working certain limbs with the desirable varieties. From pollination and marketing point of view, Waterloo and Eureka and Hartley and Payne are examples of desirable combinations.
Young trees just coming into bearing often do not produce enough catkins to supply pollen for all the pistillate flowers. The catkins on such trees have also a greater tendency to shed their pollen and drop before the pistillate flowers are receptive.
More catkins are produced as the trees grow older and the pollen is shed over a long period thus making pollination more satisfactory. The new plantation of walnut if located near the existing bearing trees will come into bearing at an earlier age and yield more than if they are planted in isolation away from the bearing trees.
Planting Operation for Walnut:
Walnut when planted on flat land, square, quincunx and rectangular system are followed but when it is to be planted on sloping land, contour and terrace system of layout are practised. Square system is commonly adopted in walnut. In India, walnut is usually grown on hill slopes.
It is better to dig pits of one square metre dimension and then fill them with mixture of well rotten farmyard manure and soil 1: 2 proportion in the month of November. The plants should be purchased well in advance of actual planting and be healed immediately on arrival after laying them in a trench and covering the root portion with moist soil which should never be allowed to go dry in the process.
The planting is done in late dormant season or early spring season. A walnut tree takes about 15-20 years to occupy the entire space allotted to it in conventional planting system leaving sufficient space unutilized in early stages. Therefore, a spacing of 6.6 x 6.6 m should be kept initially and should be adjusted 6.6 x 9.3 m finally by pruning and removal of temporary trees.
In certain areas, the distance from row to row and plant to plant is kept 12 metres, which accommodates 70 plants per hectare. The planting can be kept 7.5 to 9.0 m depending upon the soil and climatic conditions.
The plant should be topped 40-50 cm above the graft union. Frequent watering is required in the first summer season. The trunks should be painted white to prevent sunburn.
Flowering and Fruiting in Walnut Trees:
The walnuts seedlings take about 8-10 years to come into bearing but grafted begin fruiting early in about 4 or 5 years. Walnut is monoecious plant i.e. staminate and pistillate flowers are born separately on the same tree. The staminate flowers emerge laterally on the previous season’s growth and hang in catkins.
The pistillate flowers appear in pairs in the form of nut-lets at the tip of current season’s growth. Though the fruit bud formation of a pistillate flower is initiated during summer, yet its final development occurs shortly before anthesis in the following spring. Under Shimla conditions, peak flowering period of walnut is mid-April which is prolonged till the first week of May in late blooming cultivars.
The fruit growth begins and continues with the stimulation of pollination and fertilization. The non-pollinated pistillate flowers make some initial growth but ultimately drop down after about three weeks in absence of fertilization. The mesocarp started to form on 22-25 May, the testa on 3-8 June, the endocarp began to develop on 20-26 June and to harden on 5-10 August and the nuts matured on 15-25 September.
Two rapid periods of walnut fruit growth are interspersed by a period of relatively slow growth. During the first growth period, the fruit gains size and weight. The fruit attains form and becomes richer in chemical constituents during the second growth period. In the developing walnut fruit, glucose, sucrose and fructose are the pre-dominant sugars. Ethylene application hastens fruit abscission.
Training and Pruning of Walnut:
Walnut bear fruits on one year old wood which is produced either terminally or both terminally and laterally. In this way walnut cultivars are of two types, viz. Terminal bearer and Lateral bearer. The cultivars which bear terminally are thinned but not headed back. The lateral bearer cultivars are both thinned and headed back heavily in order to encourage shoot growth which get suppressed due to early fruit production.
Walnut trees are trained according to central leader or modified leader system. After the first year growth, the plant is headed back at the height of 2 m from ground level. All lateral are removed leaving one or two shoots at lower level on trunk. To avoid narrow crotches, all primary buds 1.3 m above the ground should be removed forcing the secondary buds to grow.
Select 5-6 scaffold branches spirally spaced about 50 cm apart. After selecting the main scaffold limb 1.6m above the ground, the other primary scaffold limbs are selected to the various directions on the trunk. The distance between these should be kept 30 cm.
All other vigorous branches should be removed whereas small branches are left undisturbed for fruiting. All the selected limbs should be headed back on early fruiting cultivars but not on late fruiting ones. In later years, secondary scaffold limbs on primary scaffold ones are also allowed to grow by removing the extra vigorous branches. In early fruiting cultivars all scaffold limbs are headed back every year.
Pruning of Young Bearing Trees:
In the cultivars highly fruitful on lateral buds, the shoot growth is suppressed due to heavy crop load. Therefore, a large number of new shoots should be headed back on the periphery to reduce fruiting and increase vigorous shoot growth throughout the tree’s periphery. The pruning in terminal bearing cultivars consists of heading back of selected framework branches and thinning out of competing limbs.
Pruning of Mature Trees:
The main objective of pruning in mature trees is to obtain regular and high production of quality nuts. The overcrowding of branches, insufficient production of renewal wood and inadequate availability of sunlight reduced the yield and quality of nuts.
The pruning of mature tree should be directed to control tree size and maintain vigour and fruitfulness of spurs. It should be done in such a way to develop renewal wood and to replace older and less productive branches. The weak, dead or interfering branches should be removed.
In mature trees shading is a problem as it reduces photosynthesis thereby weakens spur growth. Hence selective thinning out of limbs in the top and sides of the tree should be initiated well before overcrowding becomes serious. The pruning should be carried out in the dormant season or early spring.
Harvesting and Handling of Walnut:
Harvesting at proper maturity and subsequent handling of the nuts are of prime importance. Any delay in picking after maturity of kernels deteriorates the quality and increases the incidence of mould and pests. For determining the harvesting date of the walnut, two maturity indices are normally followed. The nuts are covered over with hulls which split at the time of maturity causing the fall of nut on the ground.
When about 80% of the hulls have cracked exposing the nuts, it is the time for walnut picking. The harvesting is usually done after about two weeks of browning. If the harvesting is further delayed, the kernel turns darker. Ethephon at the rate of 200-500 ppm on kernel maturity advances harvesting by about 10 days.
The best time of harvesting the walnut is September-October. The nuts falls down on the ground are collected or they are made to drop on the ground by shaking the branches with hand or striking them with bamboo poles. The total harvest is done in two or three times at few days interval. The nuts are collected from the ground, cleaned, washed and dried by spreading them on a sheet or floor. Dipping of nuts in 1500 ppm ethephon while the hulls are still on loosens the hull in 3-4 days.
Sometimes in order to improve the appearance of nuts, these are bleached with either alkali or acid solution. The nuts which fall down with their husks intact are generally of second grade and after removal of the husks, cleaning and drying they should be stored and marketed separately to fetch higher rate of superior lot.
The nuts should be graded suitably according to the size, colour and variety. This practice can bring higher returns than marketing the whole lot mixed. The Indian walnuts are categorised into four categories viz. Paper shelled, Thin shelled, Medium shelled and hard shelled.
Yield varies according to the age, size and variety of trees. Full commercial bearing commences after 18-20 years in seedling trees and after 8-10 years in grafted trees. A full grown big size tree may bear 150-175 kg nuts but an average yield is only around 40 kg.
Delay in drying causes rapid loss in nut quality and makes it susceptible to mould. Walnuts are dried to remove excess moisture from the shell and kernel. Drying of nuts stabilizes the product weight and prolongs storage life. The nuts are dried up to 8 per cent moisture level.
Walnuts are stored in gunny bags in small ventilated rooms free from excessive humidity. For export purpose, they are packed in double gunny bags. The boxes are lined with paper on all sides to avoid shaking and breaking of walnuts.