growing walnuts

8/10/2019 Growing Walnuts

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PNW 235 Reprinted September 1993

Growing Waln ut s

in t h e Pac ific North wes t

A Pac i f i c No r t h w est E x t en s i o n Pub l i c a t i o n

Or egon Wa sh i n gt on Id a h o
http://wwwagcomm.ads.orst.edu/AgComWebFile/EdMat/orderform.htmlhttp://wwwagcomm.ads.orst.edu/AgComWebFile/EdMat/orderform.html


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Early in this century, many acres of land in westernOregon and Washington were devoted to walnutproduction. These nuts all came originally from theMiddle East and are known as Persian walnuts.

Because of severe fall and early winter freezes thatoccurred about once in every 20 years, wind storms, theblack-line graft union disorder, walnut husk flyinfestation, low-producing varieties, and strongcompetition from California, the Pacific Northwestswalnut industry has almost disappeared.

A few orchards remain, mostly in western Oregonand Washington, along with many trees in housingdevelopments, along city streets, and elsewhere. Thesetrees still need management, and homeowners are stillplanting new trees. This publication applies to bothcommercial and home plants.

The cool, wet growing season west of the CoastRange favors development of blight, kernel shrivel, andshell perforation. East of the Cascade Mountains,production is limited by low winter temperatures andlate-spring and early-fall frosts.

Trees on river terraces or valley floors havesustained greater losses from late spring frosts thanthose on hillsides. Orchards on the valley floor, wheresoil is deeper, have borne more heavily than those on

hillsides when late spring frosts did not occur.Before planting new trees, either commercially oron a homesite, use an auger, soil tube, or postholedigger to determine if the soil is at least 6 feet deep. Youmay find solid rock a few feet beneath the surface, orcompact subsoils with a mottled color, indicating pooraeration and drainage.

With few or no pores large enough for roots toenter, this kind of subsoil layer often supports a watertable that restricts root growth. Some surface soils areunderlain by loose gravel and coarse sand, which permitlittle or no root growth.

Excessively high temperatures usually are not alimiting factor in any part of western Oregon and

Washington. However, occasional temperatures around100F may cause sunburn on the hulls of the nuts andmay result in dark-colored kernels, kernels with blackspecks on themor even complete failure to developdepending on the time in the season when the hightemperature occurs.

When the growing season has been unusually cool,thin shell perforation and shriveled kernels have been aproblem. Under the mild climatic conditions of thewestern walnut-growing areas, trees are slow in attainingfull dormancy in fall or early winter. As a consequence,early cold periods may severely injure many trees.

Spring frosts damage the walnut crops much morefrequently than winter freezes injure the tree. Varietiesthat leaf out very early may be injured by spring frosts;do not consider them for this area. Even late-leafingvarieties such as Franquette can be injured by latespring frosts if trees are located in a frost pocket.

Most walnut varieties may produce a few nuts when5 to 6 years old, but they are not considered mature orin commercial production until they are 10 years old.Two-thirds of a ton per acre is considered a good yieldfor a healthy, mature walnut orchard in the PacificNorthwest. In a homesite, with controlled watering andfertilization, a good tree might yield about 100 pounds ofnuts annually. Many produce only half that amount; thereasons for this variation may include variety selection,pollenization, fertilizer, water, soil type, and frost.

Va r i e t i es o f wa l n u t s

Satisfactory trees for planting should have l-year-oldtops and 3-year-old roots. They should be 6 to 8 feet high.Trees satisfactory for western Oregon and Washingtonare only available from nurseries in the area. Californiavarieties often come on black walnut roots and are notsatisfactory for this area.

FranquetteFranquette, the principal walnut grown in Oregon

and Washington, has been popular because of its goodshell seal and very light kernel color. Franquette treesbloom much later than most varieties and therebyusually escape spring frosts. Franquette trees have beenvery severely injured or killed by winter freezes andhave suffered less severe damage in mild freezes. Theycome into bearing slowly and produce less than anumber of other varieties.

The nuts are quite variable in size, with a tendencyto be small in heavy cropping years. They are highlysusceptible to walnut blight. Shriveled kernels are morefrequently encountered with Franquette than with other

varieties.

SpurgeonSpurgeon trees are more resistant to early fall and

winter freezes than Franquette and are sufficientlyhardy for this region. They leaf out with Franquette inspring, and the nuts mature at about the same time. Thetree is moderately vigorous and moderately productive.It is less susceptible than Franquette to walnut blight.

Growing Walnut s in the Pac ific NorthwestR.L. Stebbins

Robert L. Stebbins, Extension horticulture specialist emeritus,

Oregon State University. This publication is a revision of an OSU

Extension Circular with the same name.


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The nut is larger than Franquette, elliptical inshape, and has a slightly higher percent of kernel. Sinceit has little tendency to shrivel, the kernel is usuallymore plump than Franquette. Spurgeon has been moreproductive than Franquette. It is especially attractive tothe walnut husk fly.

HartleyHartley, which originated in California, is not

cold-hardy enough for Oregon and Washington, excepton frost-free hillsides. Hartley leafs out 10 to 14 daysbefore Franquette and matures its nuts from 12 to 14days earlier. It is more susceptible to spring frosts than

Hartley walnuts (natural size)

Franquette walnuts (natural size)

Adams walnuts (natural size)

Spurgeon walnuts (natural size)


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Franquette. Catkins (pollen-producing male flowers) ofHartley are often lost to frost, but female flowers areperfectly timed for Franquette catkins.

The tree is moderately vigorous, often with weakcrotches. The limbs tend to be flexible and drooping,and the tree is more difficult to train. Hartley is a heavyproducer. Franquette or Spurgeon are suggested aspollinizers. Hartley is susceptible to blight.

Selec tions of Manregian parent ageNumerous seedling selections from open-pollinated

seed of the Manregian walnut have been made. All of

these selections bloom earlier than Franquette during atime when, if planted on a river bottom or other frostysites, they would suffer frequent crop loss due to frosts.Most are hardy enough for western Oregon andWashington hillside sites.

Chambers No. 9 and Wepster No. 2 are among thebetter Manregian selections. All have large nuts andlight kernels. Chambers No. 9 is an outstandingly heavyproducer. Its nuts are consistently large and round toelongate in shape. They have a moderate tendency toshrivel. Wepster No. 2 has large nuts with a high percentkernel and has little tendency to shrivel.

Adams trees leaf and bloom 10 to 14 days beforeFranquette, and the nuts mature 12 to 14 days earlier. It

is hardy enough for this region, but spring frosts ofteneliminate the crop.

The Adams tree is vigorous, upright, and very slowto come into production. It is comparativelyunproductive and bears on terminal and lateral shoots. Itis slightly tolerant of walnut blight. The nut is largerthan Franquette, with good flavor and moderatetendency to shrivel.

California varieties and selectionsMost California commercial varieties leaf out too

early in the spring to escape frost injury in Oregon andWashington. Frost injury reduces yields. Among thesevarieties are Eureka, Payne, Serr, Lompoc, Gustine,

Trinta, Chico, Amigo, Pedro, Tehama, Nugget, andothers. No California varieties tested have proved cold-hardy enough for this area.

Carpathian variet ies and se lect ion sSome Carpathian varieties and selections are

reputed to be sufficiently cold-hardy to be grown incertain localities east of the Cascades.

Roo t s t o c k s

Because of their vigorous growth, northernCalifornia black walnut (Juglans hindsii Jeps.) seedlingrootstocks were commonly used for Persian walnuts.

A girdling of the wood at the union between theblack walnut rootstock and the top, known as blackline,has killed many treessome after years of production.A thin, black line develops in the graft union and slowlyextends around the entire tree until it eventually girdlesit and causes its death.

Sometimes this girdling is rapid (3 to 4 years), and

sometimes it is slow (8 to 10 years). However, once itstarts, it never disappears, and no treatments tried so farhave corrected it. The use of black walnut rootstock inthe Northwest is not recommended.

Rootstocks grown from seed of the Manregianvarieties grow vigorously, are of the Persian species,and are not susceptible to blackline. They are beingused as rootstocks for Persian varieties in theNorthwest. Carpathian seedlings are also used.

Pol l i n a t i o n a n d set o f n u t s

All walnut varieties will set a full crop of nuts whenself-pollinated, provided pollination takes place when

the female flowers are receptive. Self-pollination meansthat the pollen comes from male flowers (catkins) of thesame variety, but not necessarily the same tree.

Inadequate pollination may occur because the catkinsshed pollen either before or after the female flowers arereceptive. In such cases, it is necessary to plant apollinizer variety that sheds pollen during the peakperiod of female-blossom receptivity of the main variety.

In some unusually warm seasons, the Franquettevariety sheds all of its pollen before most of the femaleblossoms are receptive. This results in low yields insuch years. This tendency is especially evident in youngFranquette trees.

As the tree ages, there is a greater spread in time ofmale and female bloom. The older the tree is, the moreoverlap of male and female bloom there will be in agiven season. The varieties Moyer and Meylan aresatisfactory pollinizers for Franquette, but they haveother undesirable characteristics.

Pl a n t i n g t h e t r ees

Plant walnut trees in early winter, as soon aspossible after receiving them from the nursery. Do notlet the roots dry out before planting. The earlier a tree is


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planted, the more chance it has to develop a workingroot system before it leafs out in the spring.

To be sure roots do not dry out before planting, keepthem in moist sawdust or peatmoss. Dig the holes 18 to24 inches wide and 10 to 12 inches deep. Digging inwet ground with a power auger may cause compactionof the sides of the hole. If this happens, break down theedges of the hole to eliminate the compacted area andpartially fill the hole. Prune off any broken roots, thenplant the trees so that the uppermost root is 2 to 3 inchesbelow the soil surface. Spread the roots out and pressdown into the bottom of the hole. Tamp the soil firmlyaround the roots to exclude air pockets.

Do not put chemical fertilizer or barnyard litter inthe holes. Trees have been injured or killed by fertilizersplaced in tree holes at planting time. Head the trees asdescribed under training.

St a k i n g t h e t r e es

Staking newly planted trees is necessary becausethe new wood is often too soft to withstand the wind,especially if the wind usually blows from the samedirection. Place stakes 7 to 10 feet long on the windwardside, 6 to 8 inches from the tree, and tie with strips of

burlap, unbleached muslin, or similar material.Loop the strips around the tree, crossing betweenthe tree and stake, and tie firmly to the stake with adouble wrap. Use care and recheck occasionally to seethat ties are not too tight, girdling the tree.

Cu l t i v a t i o n

The purposes of cultivation in a commercialorchard are to destroy a cover crop, to control weeds,and to prepare for harvest.

The amount of moisture in that part of the soilwhere active walnut roots are located is usually justenough to supply the trees needs from the last effective

rains in the spring until the fall rains begin. In unusuallydry seasons, this stored moisture may not be enough.The highest concentration of walnut roots is found inthe top 3 feet of soil. The roots of the cover crop willusually penetrate at least half of this depth.

A cover crop turned under too late, weed growth, oran intercrop will seriously reduce the amount ofmoisture remaining for the trees. Unless you provideirrigation, the result will be a stunted tree and a lightcrop of small nuts.

When the trees are in a home lawn, chances arewater will be adequate. Close mowing and a clean areaunder the trees will make harvest easier.

Use pest i c i d es sa f el y !

Wearprotective clothing and safety devices asrecommended on the label. Bathe or showeraftereach use.

Readthe pesticide labeleven if youve used thepesticide before. Follow closelythe instructions onthe label (and any other directions you have).

Be cautiouswhen you apply pesticides. Knowyourlegal responsibility as a pesticide applicator. You maybe liable for injury or damage resulting frompesticide use.

Weed con t r o l

Use of chemical herbicides around the tree trunk orin the tree row eliminates the need to cultivate orflail-mow close to the tree. This prevents damage to thetree trunk during cultivation. If you treat a continuousstrip in the tree row, there is no need forcross-cultivation. Both preemergence and contactherbicides are registered for use in walnut orchards.

Consult your Extension agent for current informationon herbicides, insecticides, and disease control inwalnut orchards.

Non t i l l a g e o r ch a r d ma n a g emen t

Under commercial orchard nontillage management,a tractor-driven flail mower is used to cut weed growthor cover crop close to the ground, starting in earlyspring. By the time effective rains are over, the cover ismowed within 1/2 inch of the ground. Because of thisclose mowing, the cover crop is shallow-rooted and diesearly in summer. Perennial weeds remain alive allsummer and gradually become dominant. About five

flailings per year are needed.This system reduces erosion, soil compaction, and

mechanical damage to tree roots. It reduces the amountof work needed to prepare for harvest. Under wetconditions, harvesting on flailed ground is much easierthan on cultivated ground. Tree roots can growundisturbed in the more fertile upper 6 inches of soil.

Chemical weed control in the tree row is part ofnontillage orchard management. All equipment shouldhave high-flotation tires to avoid creation of wheel ruts


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or cleat marks. In a home setting, good lawnmaintenance will take the place of cultivation andassure adequate water.

I n sect s an d d i seases

The most troublesome insect pest of walnuts in theNorthwest is the walnut husk fly. White maggots, thelarval or immature stage of this insect, up to 3/16-inchlong, feed in the husk. This destroys the husk tissue andstains the nut shell and kernel, reducing nut quality.Sprays used to kill husk flies may also kill aphids andaphid predators. Although aphids do occur in Northwestwalnuts, their damage to the nuts and the trees is notsevere enough to be of concern.

Walnut blight, caused byXanthomonas juglandis, abacterium, is the most serious disease of walnuts inOregon. The bacterium overwinters on infected budsand to a lesser extent in holdover cankers on twigs ofthe previous years growth. During the spring growthperiod, bacteria are spread by raindrops from thesesources to current seasons growth.

Frequent and prolonged rains, just before andduring bloom and for about 2 weeks thereafter, result insevere blight outbreaks. This is the time when the nuts

are most susceptible. If you follow the spray programfor walnut blight, it will not be necessary to apply anysupplementary spray for control of moss and lichens, asthe growth is killed at the same time as blight iscontrolled. When spraying, thoroughly saturate themosses and lichens.

Deer d a m age

Deer are particularly serious pests of young walnuttrees in outlying areas next to wooded areas. So far, nocompletely satisfactory answer to deer damage has beenfound. Deer fencing around the entire orchard orindividual trees is the most reliable solution, but it is

expensive.Special hunting licenses may be obtained for somerural locations. Chemical repellants have been partiallysuccessful in some instances. Bags of dried blood andbone meal hung in the trees are the most commonlyused repellants. When replaced every 2 or 3 months,they have been relatively successful.

T r ee f er t i l i z a t i o n

Youn g t ree sApply nitrogen only after one growing season has

passed. As long as young trees are growing 18 to 30 inchesannually, no applications of nitrogen (N) fertilizer areneeded.

If you want additional growth, apply nitrogen thisway: 13to 12pound per tree to trees 2 to 5 years old (1 to 4

inches trunk diameter);

12to 34pound per tree to trees 6 to 7 years old (4 to 7

inches trunk diameter); and 34pound per tree to trees 8 to 10 years old (7 to 10

inches trunk diameter).

Boron is another element walnuts need, thoughyoung trees generally dont need itand you mayinjure them if you apply it to them (see figure 1).

Figure 1.Injury to a walnut leaf caused by an excess ofboron


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Mature t ree s

Observations of annual shoot growth and size andcolor of leaves will aid in determining the fertilizerrequirements of walnut trees. Fertilizer will not replacepruning, especially in older orchards.

Leaf analysis is the best guide to walnut treefertilization. See your Extension agent for informationon how to take a sample and obtain a laboratoryanalysis. Table 1 provides a leaf analysis guide fornitrogen (N) application.

Apply the nitrogen about the middle of the dormantseason (February or early March). In addition to thepossibility of loss by leaching, application in the fallmay stimulate the cambium, resulting in increasedsusceptibility to winter injury. Broadcast application in

the outer half of the limb spread is most desirable.Excessive boron applications can be toxic to thetree. Symptoms of B toxicity are round, brown deadspots along the margins of the leaflets, and in severecases these spots appear between the veins approachingthe midrib. Nitrogen applications will gradually reduceboron toxicity symptoms, and in about 3 years they maybe completely eliminated.

If a laboratory leaf analysis shows boron levels arebelow 80 ppm, apply 0.25 to 0.5 pound of boron permature tree to the soil. Do not band soil-applied boron;broadcast it. Applications are usually effective for 2 to 3years. If boron deficiency has occurred, a foliar sprayapplication will give more rapid recovery than soil

application, but it is used up during the season in whichyou apply it.

Spray at the rate of 8 pounds sodium pentaborateper acre.

Pr u n i n g a n d t r a i n i n g wa l n u t t r ees

The objective of training is to develop a tree with astrong system of main scaffold branches that can supporta heavy crop of nuts, ice, or other stress. The limbs

should be developed at a sufficient height to allowpassage of equipment under them. When limbs developat narrow angles between the trunk or main scaffoldlimb, they are structurally weak.

Limbs that develop from buds on the end of a veryshort shoot or neck always have weak crotches, whichbreak out easily. Remove these necked buds and allside-branches developing from new wood that grewduring the same season.

At planting, remove about half of the tops of thetrees. Cut a 10-foot tree back to 5 feet and a 6-foot treeto 3 feet. Gradually raise the height of the first limbs tothe desired level by subsequent pruning.

Avoid large pruning cuts later by making cuts whenshoots are small. Removal of a shoot just after growthstarts causes much less loss to a tree than when theshoot is allowed to grow for one or more seasons and isthen removed. The less a tree is pruned, the larger it willbe. However, you must do some pruning in order tobuild a strong, sturdy tree.

Select three to five branches that are to form themain framework of the tree during the first, second, andthird growing seasons, and remove excess branches. Cutoff any buds that start to grow near the base of the tree.The top bud after the tree has been headed back will

form an upright terminal shoot upon which to developdesired scaffold branches.Space scaffold branches a foot or more apart

vertically. When all main branches arise just below thepoint where the tree is headed at planting, a structurallyweak, vase-shaped tree is formed.

Pruning nonbearing treesPruning of nonbearing trees is simply a

continuation of training. If shoot growth seemsexcessively long, head the terminals, especially in lowerscaffold branches. Without this pruning, they may growso long that in later years the end of the first scaffoldbranches will rest on the ground when weighted with aheavy crop; cultivation and harvesting will be difficult.

Prunin g bearing t ree sModerate pruning is needed every 2 or 3 years after

trees come into bearing. Thin out the shoots in the topsof trees to maintain production throughout the trees.Remove drooping limbs that interfere with cultivation.Remove some but not all of the weak wood in the centerof the trees. Often late-developing catkins are producedon weak wood and they shed pollen late, which maypollinate late-pistillate flowers.

Table 1.Fertilizer guide according to leaf analysis

% leaf N Apply this amount (lb)in August per tree

Under 2.0 (severe deficiency)---------- 6 to 8

2.0 to 2.3 (below normal)--------------- 4 to 6

2.3 to 2.8 (optimal) ---------------------- 0 to 4

More than 2.8 (excess)------------------ None


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Old, mature trees will benefit from more severepruning. These trees are often so tall that a mechanicalpruning tower is required to get into the tree tops to dothe kind of limb thinning that is needed. Cut manybranches back 2 to 4 feet to strong side limbs. This willstimulate new growth near the cuts and allow moresunlight into the lower portions of the tree. Repeat thiskind of pruning every 3 to 5 years, depending on thetrees growth and light distribution.

Rejuvenation pruning of winter-injured treesrequires cutting back to good, live wood. Wait a fullyear after the freeze, when it is more nearly possible todetermine the extent of the injury. Retain all live wood.

When you remove a limb, cut just outside thebranch collar. This is the point where the smaller sidebranch tapers larger as it joins the larger branch ortrunk. The cut surface will be smaller than if you makea flush cut.

Wound paints are not helpful and are notrecommended.

Ha r v est i n g a n d d r y i n g p l a n t s

Walnuts are mature as soon as the husk can beseparated from the nut easily, but they are usually not

harvested until the rains have cracked the husk to thepoint of letting the nut drop to the ground. If nuts areblown off by the wind before the hulls crack, the hullswill ripen on the ground, and you can usually removethem after a week or two. Leave them on the grounduntil the hulls are loose.

Harvest husk-free walnuts as soon as possible afterthey drop. Kernels of nuts that are allowed to remain onwet ground rapidly become discolored. Harvested,undried nuts left in the sack for more than a day or somay heat and become moldy.

Start the drying process within 24 hours of harvest.Nuts are usually dried in the shell, but you can save aconsiderable amount of drying time and you will need

less heat if you shell the nuts before drying.Best drying temperatures are 95to 105F. Aircirculation is as important as (or more important than)temperature, so it is desirable to dry the nuts on ascreen-bottomed tray, in an onion sack, or in any othercontainer that will permit free air passage. You can drysmall lots in the warm air stream above a furnace orradiator, as long as the temperature does not exceed105F. This may require 3 to 4 days. You can dry nuts atlower temperatures, but you will need more time. If thetemperature exceeds 110F, nut quality will be impaired.

Pacific Northwest Extension publications are jointly produced by

the three Pacific Northwest statesOregon, Washington, and

Idaho. Similar crops, climate, and topography create a naturalgeographic unit that crosses states lines. Since 1949 the PNW

program has published more than 450 titles. Joint writing,

editing, and production have prevented duplication of effort,

broadened the availability of faculty specialists, and substantially

reduced the costs for participating states.

Published and distributed in furtherance of the Acts of Congress

of May 8 and June 30, 1914, by the Oregon State University

Extension Service, O.E. Smith, director; Washington State

University Cooperative Extension, Harry B. Burcalow, interim

director; the University of Idaho Cooperative Extension System,

LeRoy D. Luft, director; and the U.S. Department of Agriculture

cooperating.

The three participating Extension Services offer educational

programs, activities, and materialswithout regard to race,

color, national origin, sex, age, or disabilityas required by Title

VI of the Civil Rights Act of l964, Title IX of the Education

Amendments of 1972, and Section 504 of the Rehabilitation Act

of 1973. The Oregon State University Extension Service,

Washington State University Cooperative Extension, and the

University of Idaho Cooperative Extension System are Equal

Opportunity Employers.

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growing walnuts

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