| WALNUT DESCRIPTION
The Persian or English walnut (Juglans reiga L.) is the only
commercially important walnut species as a nut crop and it
is native to areas extending from the Carpathian mountain
of Poland through the mountain chains of Central Asia, Afghanistan,
and Northern Pakistan, Tibet, and Southern China to eastern
Himalayas. Bhutan falls within this zone, with stands of native
walnut.
The native walnut is hard shelled and has little commercial
value as a nut crop but its timber is of high value. Walnut
is locally called tako (in Dzongkha) or khesae (in Sharchop)
and is well adapted and widely grown in Bhutan. Both native
forest trees and introductions of commercial soft shell walnut
in the past are available. Presently, walnut is produced mostly
for subsistence; any surplus is sold locally (about 204.9MT
produced in 2003).
At present, walnut is gaining interest among progressive
farmers and development agencies as a potential cash crop.
Walnut offer opportunities for the landlocked country like
Bhutan where remoteness, quick access to market, poor transport
system and postharvest facilities are limiting. Walnut is
not so perishable and is relatively resistant to transport
damage, with high value, and low weight commodities.
I. PRE-PRODUCTION STAGE
A. Ideal Growing Conditions
Criteria Description
1. Altitude • Between 1,300-2,500 m.
• Beyond 2,500 m (like in Bumthang), commercial walnut
production is not recommended as it result to erratic and
even total crop loss in some year when there is late spring
frost.
2. Temperature • Adapted to cold temperate zone, requiring
cool period in autumn to promote leaf fall and the physiological
process of plant hardening and induction of dormancy.
• Not recommended in areas where late spring or early
fall frost is common as freezing temperature kills the growing
point of walnut trees and severely affects production.
• Temperature of 380C or more results in sun burning
of hulls and shriveled kernels, with severe damage occurring
at temperature over 400C.
3. Irrigation • Requires 800 mm rainfall or irrigation
equivalent. To obtain high yield, avoid planting walnut in
drought-prone areas.
• Spring rainfall is associated with increased problems
from walnut blight.
4. Soil pH • Requires slightly acidic soils between
pH 5.5-6.5.
• Prone to zinc and boron deficiencies, hence select
a site wherein recent soil analysis show reasonable levels
of zinc and a slightly acidic pH.
5. Soil texture • Grows best on deep, friable loamy
soil where roots can develop to a depth of 3-4 m. Besides
good top-soil, the subsoil should be free of impermeable layers
like clay and rock gravel and anaerobic conditions (clayey
soil coupled with high rainfall)
• Walnut can not tolerate wet soils for any extended
period. A few hours of water logging can cause severe damage.
6. Topography • Gentle slopes are more easily managed.
As walnut plantation is likely to be on steep slopes in Bhutan,
there are two important factors to consider:
1. Choose a site with southerly and south-westerly facing
slopes. This warms up faster in spring and cools down more
slowly in the autumn, thus extending the growing season at
higher altitude. Northerly and easterly aspects tend to be
colder and more prone to frost damage at higher altitudes,
but may allow walnut production at lower altitude.
2. Choose a site with good frost drainage. Unimpeded down
slope drainage of cold air is important particularly in spring
where a build-up of cold air in pocket behind a line of trees
or in a fold in the ground can cause crop loss due to late
frosts.
B. Choice of cultivars
The newly-released cultivars by the Ministry of Agriculture
for general cultivation in Bhutan are indicated below:
Kanthel Selection
• Big and long trapezoid nuts, semi-erect, and vigorous
• Regular bearer
• Thin-shelled
• Leafing out in 1st week of April
• Flowering habit: Female flower first
• Disease: Tip die back
• Nut size index: 4.27 mm
• Nut in-shell average weight: 26.11 g
• Kernel average weight: 9.32 g
• Harvesting in September end under Yusipang condition
Yusipang 2
• Big and long ovate nuts, semi-erect, and vigorous
• Regular bearer
• Thin shelled
• Leafing out in 1st week of April
• Flowering habit: Female flower first
• Disease: Some tip die back
• Nut size index: 4.00 mm
• Nut in-shell average weight: 20.75 g
• Kernel average weight: 8.3 g
• Harvesting in September end under Yusipang Condition
II. PRODUCTION STAGE
1. Plant Propagation
a) Preparing the seeds
• Use the local walnut seedlings as rootstocks.
• Collect the walnut in September to October from the
forest. Hull the seeds and air
dry in the open shed.
• Stratify walnut seeds either by early fall planting
(planting in December where natural stratification occurs)
or by moist cold stratification for spring planting (end of
February).
• At lower elevation, stratify the seeds in moist sand
or peat moss for about two months. At higher elevation, there
is no need to stratify as they are naturally stratified by
exposure to outside winter temperatures in a cold room or
shady open area.
• Dip the seeds for one minute in hot water (that is,
almost boiling condition) before fall planting or stratification.
b) Preparing the nursery
• For fall or spring planting, prepare the nursery
carefully. Plant the seed in about 4 10 cm deep and 15cm apart
in rows and 50cm between the rows.
• Undercut the seedlings when it is about 30cm tall.
Irrigate the seedlings thereafter.
• It takes more than two months for the seed to germinate
in the nursery for fall planting without stratification while
for spring planting(after cold stratification), the seeds
germinate within 1-2 weeks after planting.
• Thin the seedlings, if required, a few days after
undercutting.
• Make sure that the nursery is free of weeds. Irrigate
frequently and fertilize the seedlings to correct soil deficiencies.
• If nursery is properly managed, seedlings attend grafting
size within a year time.
c) Grafting operation
The most suitable time to graft walnut under ambient condition
is towards the end of dormancy or early spring season (mid-March)
especially in areas of 700-2,100 meters above sea level. If
the nursery is located at higher altitude, use grafting cable
or hot callusing technique to maintain suitable temperature
for walnut grafting (20º-30ºC).
Whip grafting is the best method to graft walnut if the size
of stock and scion are same. For the large size stock, used
modified whip grafting. Scion is a short piece of shoot containing
dormant buds that is united to the rootstock on the upper
portion of the graft and form the shoot system of the composite
plant. The procedure is as follows:
• The materials require for grafting are sharp knife,
grafting tape, wax and aluminum vessel, burner (to melt wax),
and plant materials (scion and rootstock).
• Use 1-2 year old healthy rootstocks of Juglans regia
and freshly collected scions of known cultivars for grafting.
Use only dormant scion and active rootstock for grafting.
• Make a slanting cut (about 5 cm) with a sharp knife
at the base of the scion and at the top of rootstock. For
the stock, make the cut in the bark portion or lesser cut
on wood portion depending on scion size.
• Fit the cut sections of stock and scion together making
sure that the cambial layers of both stock and scion fit perfectly.
• Tie the fitting (graft union) with a grafting tape.
• Dip the scion portion of the grafted plant into the
hot wax to prevent scion desiccation.
• Grow the grafted walnut in the nursery or in pots
with a growing medium prepared as 1:1:1 ratio of sand, Farm
Yard Manure (FYM), and soil. Add basal fertilisers.
• Remove the suckers or shoots growth from the stock
as and when it appears.
• Irrigate the nursery 1-3 times a week depending upon
the weather condition and soil types.
• Take note if bleeding occurs during grafting, that
is, sap flow due to root pressure, as it kills the grafts.
Control the bleeding by either cutting off the rootstock 5-8
cm above the grafting site about two weeks beforehand or make
few slanting cuts in the rootstock through the bark into the
wood as low as possible below the grafting point, if latter
any bleeding occurs, it occurs through this cuts. You can
also stop irrigation for about two weeks prior to grafting.
d) Layout and tree density
• Layout the orchard in square or Quincunx (diamond)
design in gentle slope and flat valley. Use contour planting
for hilly and steep slope.
• Consider the number of trees to plant per unit area
in terms of both short- and long-term productivity of an orchard.
The short-term objective is to have maximum numbers of trees
to obtain the earliest economic production potential per unit
area. If the initial tree density is too high, remove specific
number of trees 8-10 years after planting.
• Space the plants depending on the tree type and vigour.
For small and compact tree, use initial spacing of 5 m x 5
m and adjust this to 10 m x 10 m at maturity (high density
planting).
• For highly fruitful lateral-bearing but vigorous cultivars
and terminal- bearing vigorous cultivars, maintain an initial
spacing of 6 m x 6 m and finally adjust to 12 m x 12 m.
• As a general guide, use the following spacing:
Seedling trees 12 m x 12 m
Grafted on Juglans regia 10 m x 10 m
Grafted Juglans nigra 8 m x 8 m
• When polleniser are required, place them in permanent
position to avoid their elimination during the temporary tree
removal. Every eight row, a polleniser variety is planted
perpendicular to the usual direction of the wind.
e) Pit digging and filling
• On clayey or not very fertile soil, dig a pit 1 m
deep and 1 m in diameter. For loamy soil, dig a pit 0.3 m
deep and 0.3 m in diameter.
• Break the hard layer on any side of the pit.
• Keep the top surface soil from the subsoil separate.
• Fill the pit with mixture of rotten FYM and top surface
soil at the ratio of 1:2.
f) Planting
• Order for purchase planting materials in advance
before planting.
• If planting is delayed. lay the seedlings in a trench.
Cover the root portion with moist soil. Do not allow the seedlings
to dry.
• Prepare a hole just enough to accommodate the entire
root system. Spread the roots around in all sides and cover
with soil. Make sure that the graft union is above the soil
zone.
• Plant the seedlings in late dormant/early spring season.
Irrigate immediately.
• In the first year prune the young tree by cutting
the top above the graft union but leaving 5-6 buds.
• Paint the trunk with whitewash.
• Stake the young tree in the windy area.
• As the tree leaf out and growth occurs, pinch the
growing tip from the shoots other than the one selected as
a main trunk.
g) Manures and fertilizer
• Use the following guide below as a indicative amount
of NPK for the walnut in the absent of plant and soil nutrients
analysis information:
Year N Dose Location: circle around the tree (m)
1 100 g/tree 0.5
2 200 g/tree 1.0
3 300 g/tree 1.5
4 400 g/tree 2.0
5-7 500 g/tree 2.5
7-9 600 g/tree 3.0
10 to full production 900 g/tree 3.0
• In the first 1-5 years, place small amounts 100g of
P and K per tree. From fifth year up to full production, apply
40-80 kg/ha of P and 60-100 kg/ha of K based on soil fertility
and plant vigour.
h) Training
• Train the walnut after the first year using the Modified
Central leader System (delayed open centre).
• After the 1st years’ growth, head back to 2
m of its height. Remove all the lateral shoots on the leader,
leaving one or two shoots at lower level on the trunk to provide
shade on trunk’s south and west sides.
• To avoid narrow crotches, remove all primary buds
above 1.5 m from the ground to force secondary buds to grow.
• Select main scaffold limb 1.6 m above ground. Choose
primary scaffold limbs in all directions on the trunk. Ensure
that limbs have wider angles, more than 30 cm apart vertically
on the main trunk.
• Remove the rest of the vigorous branches leaving small
branches undisturbed for fruiting.
• Head back all the selected limbs on terminal fruiting
cultivars but not for lateral fruiting ones.
• In the later (after 2-3 years), allow the secondary
scaffold limbs on primary scaffolds to grow by removing the
extra vigorous branches.
• Head back all scaffold limbs every year in terminal
fruiting cultivars.
• For cultivars that are highly fruitful on lateral
buds, head back a large number of new shoots on the periphery
to reduce fruiting and increase vigorous shoot growth throughout
the trees’ periphery.
• On terminal bearing cultivars, head back selected
branches and thin out limbs completely.
i) Pruning
• Carry-out pruning operation in the dormant season
but early spring is preferable. Further delay causes excess
bleeding.
• For young tree (16-17 year-old) and middle aged trees
(34-35 year-old) cut back 3-year old shoots. For mature trees
(40 year-old), cut back 4-5 year-old shoots.
• Initiate selective thinning out of limbs in the top
and sides of the tree before overcrowding becomes serious.
Do not remove more than 25% of the branch structure at any
time.
• Treat all the cut surfaces with a tree wound dressing
such Copper oxychloride or Bordeaux paste.
j) Tree Thinning
• Practice tree thinning in closely-planted orchards
(high density planting). Remove the tree before it begins
to crowd.
• Remove the one-third temporary tree or alternate tree
at one time as the tree gets crowded leaving trees on the
edges of the plantation.
k) Mulching
• Mulch the tree with plastic or organic materials
like straw or grasses
l) Intercropping
• Intercrop walnut trees with legume fodders in areas
traditionally under pasture farming system. For other farming
systems, use low growing and non-competitive vegetables such
as non-climbing beans and other legumes, tomato, and onion
and more competitive vegetables such as chili and asparagus.
• Intercrop during the first 4-6 years after planting
only.
• If sustained intercropping is planned from the establishment
of the orchard, modify the spacing, that is, much wider between
rows of mature trees.
m) Irrigation scheduling
• Do supplemental irrigation in dry spring and early
summer before the start of monsoon period i.e provide irrigation
from April to June
• No not irrigation is needed from June onwards until
harvest since this is the monsoon season.
n) Top-working
• For walnut orchard establishment, plant the walnut
seedlings directly in the production field without nursery
grafting and top-worked two years later. This is the best
method of orchard development in Bhutan.
• To avoid sap bleeding, either head back the stock
before two weeks before actual operation or make few slanting
cuts on the stock through the bark into the wood just bellow
the grafting point so that if any bleeding occur latter, it
occurs though these cuts.
• Select disease-free young trees for top-working.
• Cut the dormant scion wood from the parent tree in
advance. Wax the scion, pack properly, and store in the refrigerator
at 4oC.
• Do bark grafting for top-working in late spring (end
of March to April) or when new growth has taken place.
• Wrap the graft union with thick plastic sheet
• Paint the stem portion with white wash.
• Remove the shoots other than the scion growing on
the plant.
o) Harvesting and post-harvest operation
• Walnut trees from seedling bear nuts in 10-12 years
and full commercial production 18-20 years from planting.
• Grafted walnut starts production from 4-5 years after
planting and it takes 8-10 years for commercial production
• A fully grown, big size tree produces as high as 100-150
kg nuts but the average yield is 40 kg per tree.
• Crop maturity and harvest in Bhutan starts from August-October
depending on the elevation and cultivars. Do not delay harvesting
to avoid deteriorating the nut quality kernel(brown colour
kernel). The lightest kernel colour commands higher price
in the international market.
• To determine maturity, observe the following:
a) The nuts are mature when the nut dehisce and fall. In higher
altitude, kernel matures earlier to hull dehiscence while
in the lower altitude, hull dehiscence take place prior to
kernel maturity.
b) The walnut kernels are mature when the packing tissue between
and around the kernel halves has just turned brown and crop
can be harvested if 80% of the nut can be removed with 95%
of them hullable. Harvest the remaining nuts few days after
the first harvest
• Collect the nuts from the ground. Clean, wash, and
spread the nuts on a sheet or floor to dry them up to 8% moisture
level.
• Grade the nuts according to size, colour, and variety.
• Store the nuts in gunny bags in small well-ventilated
rooms free from excessive humidity. Nuts have long shelf life
and it can be sold in extended marketing season.
Reference:
1. Gyambo Tshering, 2004, Pointer for Successful Walnut Grafting,
Leaflet, RNRRC-Bajo, CoRRB, MOA, Wangdue
2. Gyambo Tshering, 2000, Causes of graft failure in walnut,
M. Phil Thesis, Reading University, England.
3. IHDP, 1994. Walnut-Guidelines for Orchard Management. Extension
Publication No. 36. Ministry of agriculture.
4. Jackson, D.I. 1986. Temperate and Subtropical Fruit Production.
Butterworths of New Zealand Ltd., Wellington, New Zealand.
5. High Value Horticulture Ltd., 98. Draft Walnut Production
Manual for Bhutan
6. David E. Ramos( 1985), Walnut Orchard Management, University
of California, Davis, USA
By
Mr. Gyambo Tshering
Deputy Chief Research Officer, Horticulture Sector
Renewable Natural Resources Research Center Bajo, Wangdue
Phodrang
Tel. No. 02-481209 (O)/17617917(M)
E-mail address: gtshering@druknet.bt
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