PNW 235 • Reprinted September 1993

Growing Walnutsin the Pacific Northwest

A Pacific Northwest Extension PublicationOregon • Washington • Idaho

Growing Walnuts in the Pacific NorthwestR.L. Stebbins

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Early in this century, many acres of land in westeOregon 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 thoccurred about once in every 20 years, wind storms, black-line graft union disorder, walnut husk flyinfestation, low-producing varieties, and strongcompetition from California, the Pacific Northwest’swalnut industry has almost disappeared.

A few orchards remain, mostly in western Oregonand Washington, along with many trees in housingdevelopments, along city streets, and elsewhere. Thetrees still need management, and homeowners are splanting 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, anshell 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 onhillsides 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. Ymay find solid rock a few feet beneath the surface, orcompact subsoils with a mottled color, indicating pooaeration 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 arunderlain by loose gravel and coarse sand, which pelittle or no root growth.

Excessively high temperatures usually are not alimiting factor in any part of western Oregon andWashington. However, occasional temperatures arou100°F may cause sunburn on the hulls of the nuts andmay result in dark-colored kernels, kernels with blackspecks on them—or even complete failure to developdepending on the time in the season when the hightemperature occurs.

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

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Spring frosts damage the walnut crops much morfrequently than winter freezes injure the tree. Varietiesthat leaf out very early may be injured by spring frostsdo 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 wh5 to 6 years old, but they are not considered mature oin commercial production until they are 10 years old.Two-thirds of a ton per acre is considered a good yielfor a healthy, mature walnut orchard in the PacificNorthwest. In a homesite, with controlled watering anfertilization, a good tree might yield about 100 pounds nuts annually. Many produce only half that amount; thereasons for this variation may include variety selectionpollenization, fertilizer, water, soil type, and frost.

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Varieties of walnutsSatisfactory trees for planting should have l-year-o

tops and 3-year-old roots. They should be 6 to 8 feet higTrees satisfactory for western Oregon and Washingtoare only available from nurseries in the area. Californvarieties often come on black walnut roots and are nosatisfactory for this area.

FranquetteFranquette, the principal walnut grown in Oregon

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

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

SpurgeonSpurgeon trees are more resistant to early fall an

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. Ttree is moderately vigorous and moderately productivIt is less susceptible than Franquette to walnut blight.

Robert L. Stebbins, Extension horticulture specialist emeritus,Oregon State University. This publication is a revision of an OSExtension 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. Siit has little tendency to shrivel, the kernel is usuallymore plump than Franquette. Spurgeon has been moproductive than Franquette. It is especially attractive the walnut husk fly.

Hartley walnuts (natural size)

Franquette walnuts (natural size)

Adams walnuts (natural size)

Spurgeon walnuts (natural size)

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HartleyHartley, which originated in California, is not

cold-hardy enough for Oregon and Washington, excon frost-free hillsides. Hartley leafs out 10 to 14 daybefore Franquette and matures its nuts from 12 to 1days earlier. It is more susceptible to spring frosts th

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Franquette. Catkins (pollen-producing male flowers) oHartley 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 heavproducer. Franquette or Spurgeon are suggested aspollinizers. Hartley is susceptible to blight.

Selections of Manregian parentageNumerous seedling selections from open-pollinat

seed of the Manregian walnut have been made. All ofthese selections bloom earlier than Franquette duringtime when, if planted on a river bottom or other frostysites, they would suffer frequent crop loss due to frosMost are hardy enough for western Oregon andWashington hillside sites.

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

Adams trees leaf and bloom 10 to 14 days beforeFranquette, and the nuts mature 12 to 14 days earlieis hardy enough for this region, but spring frosts ofteneliminate the crop.

The Adams tree is vigorous, upright, and very sloto come into production. It is comparativelyunproductive and bears on terminal and lateral shootis 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 anWashington. 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 colhardy enough for this area.

Carpathian varieties and selectionsSome Carpathian varieties and selections are

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

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RootstocksBecause of their vigorous growth, northern

California 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 trees—some after years of productioA thin, black line develops in the graft union and slowextends around the entire tree until it eventually girdleit and causes its death.

Sometimes this girdling is rapid (3 to 4 years), ansometimes it is slow (8 to 10 years). However, once itstarts, it never disappears, and no treatments tried sohave 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.

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Pollination and set of nutsAll walnut varieties will set a full crop of nuts when

self-pollinated, provided pollination takes place whenthe female flowers are receptive. Self-pollination meathat the pollen comes from male flowers (catkins) of tsame variety, but not necessarily the same tree.

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

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

As the tree ages, there is a greater spread in timemale and female bloom. The older the tree is, the mooverlap 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.

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Planting the treesPlant walnut trees in early winter, as soon as

possible after receiving them from the nursery. Do notlet the roots dry out before planting. The earlier a tree

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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, keethem in moist sawdust or peatmoss. Dig the holes 18 24 inches wide and 10 to 12 inches deep. Digging inwet ground with a power auger may cause compactioof the sides of the hole. If this happens, break down thedges of the hole to eliminate the compacted area anpartially fill the hole. Prune off any broken roots, thenplant the trees so that the uppermost root is 2 to 3 incbelow 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 fertilizeplaced in tree holes at planting time. Head the trees adescribed under training.

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Staking the treesStaking newly planted trees is necessary because

the 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 windwaside, 6 to 8 inches from the tree, and tie with strips ofburlap, 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 sethat ties are not too tight, girdling the tree.

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CultivationThe purposes of cultivation in a commercial

orchard are to destroy a cover crop, to control weedsand 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 tree’s needs from the last effectrains in the spring until the fall rains begin. In unusuadry seasons, this stored moisture may not be enoughThe 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,an 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 arunder the trees will make harvest easier.

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Use pesticides safely!• Wear protective clothing and safety devices as

recommended on the label. Bathe or shower aftereach use.

• Read the pesticide label—even if you’ve used thepesticide before. Follow closely the instructions onthe label (and any other directions you have).

• Be cautious when you apply pesticides. Know yourlegal responsibility as a pesticide applicator. You mbe liable for injury or damage resulting frompesticide use.

Weed controlUse of chemical herbicides around the tree trunk

in the tree row eliminates the need to cultivate orflail-mow close to the tree. This prevents damage to tree 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.

Nontillage orchard managementUnder commercial orchard nontillage manageme

a tractor-driven flail mower is used to cut weed growtor cover crop close to the ground, starting in earlyspring. By the time effective rains are over, the covermowed within 1/2 inch of the ground. Because of thisclose mowing, the cover crop is shallow-rooted and dearly in summer. Perennial weeds remain alive allsummer and gradually become dominant. About fiveflailings per year are needed.

This system reduces erosion, soil compaction, anmechanical damage to tree roots. It reduces the amoof work needed to prepare for harvest. Under wetconditions, harvesting on flailed ground is much easithan 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 shoulhave high-flotation tires to avoid creation of wheel rut

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

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Insects and diseasesThe most troublesome insect pest of walnuts in th

Northwest 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 astains 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 Northwewalnuts, their damage to the nuts and the trees is notsevere enough to be of concern.

Walnut blight, caused by Xanthomonas 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 year’s growth. During the spring growthperiod, bacteria are spread by raindrops from thesesources to current season’s growth.

Frequent and prolonged rains, just before andduring bloom and for about 2 weeks thereafter, resultsevere blight outbreaks. This is the time when the nutare 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,the growth is killed at the same time as blight iscontrolled. When spraying, thoroughly saturate themosses and lichens.

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Deer damageDeer are particularly serious pests of young waln

trees in outlying areas next to wooded areas. So far, completely satisfactory answer to deer damage has bfound. Deer fencing around the entire orchard orindividual trees is the most reliable solution, but it isexpensive.

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

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Tree fertilizationYoung trees

Apply nitrogen only after one growing season haspassed. As long as young trees are growing 18 to 30 incannually, no applications of nitrogen (N) fertilizer areneeded.

If you want additional growth, apply nitrogen thisway:• 1⁄3 to 1⁄2 pound per tree to trees 2 to 5 years old (1 to

inches trunk diameter);• 1⁄2 to 3⁄4 pound per tree to trees 6 to 7 years old (4 to

inches trunk diameter); and• 3⁄4 pound per tree to trees 8 to 10 years old (7 to 10

inches trunk diameter).

Boron is another element walnuts need, thoughyoung trees generally don’t need it—and you mayinjure them if you apply it to them (see figure 1).

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

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Mature treesObservations of annual shoot growth and size an

color of leaves will aid in determining the fertilizerrequirements of walnut trees. Fertilizer will not replacpruning, 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 dormaseason (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 inthe 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 approacthe midrib. Nitrogen applications will gradually reduceboron toxicity symptoms, and in about 3 years they mbe completely eliminated.

If a laboratory leaf analysis shows boron levels arbelow 80 ppm, apply 0.25 to 0.5 pound of boron permature tree to the soil. Do not band soil-applied borobroadcast it. Applications are usually effective for 2 toyears. If boron deficiency has occurred, a foliar sprayapplication will give more rapid recovery than soilapplication, but it is used up during the season in whiyou apply it.

Spray at the rate of 8 pounds sodium pentaborateper acre.

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Pruning and training walnut treesThe objective of training is to develop a tree with

strong system of main scaffold branches that can suppa heavy crop of nuts, ice, or other stress. The limbs

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should be developed at a sufficient height to allowpassage of equipment under them. When limbs develat 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, whibreak 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 treto 3 feet. Gradually raise the height of the first limbs tothe desired level by subsequent pruning.

Avoid large pruning cuts later by making cuts wheshoots 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 andthen removed. The less a tree is pruned, the larger it wbe. 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, anthird growing seasons, and remove excess branches.off any buds that start to grow near the base of the treThe top bud after the tree has been headed back willform an upright terminal shoot upon which to developdesired scaffold branches.

Space scaffold branches a foot or more apartvertically. When all main branches arise just below thepoint where the tree is headed at planting, a structuraweak, 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 lowscaffold 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.

Pruning bearing treesModerate pruning is needed every 2 or 3 years aft

trees come into bearing. Thin out the shoots in the topof 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 cenof the trees. Often late-developing catkins are produceon 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 82.0 to 2.3 (below normal) --------------- 4 to 62.3 to 2.8 (optimal) ---------------------- 0 to 4More 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 mechanipruning tower is required to get into the tree tops to dthe kind of limb thinning that is needed. Cut manybranches back 2 to 4 feet to strong side limbs. This wstimulate new growth near the cuts and allow moresunlight into the lower portions of the tree. Repeat thkind of pruning every 3 to 5 years, depending on thetree’s 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 possibledetermine the extent of the injury. Retain all live woo

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

Wound paints are not helpful and are notrecommended.

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Harvesting and drying plantsWalnuts are mature as soon as the husk can be

separated from the nut easily, but they are usually noharvested 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 afthey drop. Kernels of nuts that are allowed to remain wet 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 harvesNuts are usually dried in the shell, but you can save aconsiderable amount of drying time and you will needless heat if you shell the nuts before drying.

Best drying temperatures are 95° to 105°F. Aircirculation is as important as (or more important thantemperature, so it is desirable to dry the nuts on ascreen-bottomed tray, in an onion sack, or in any othecontainer that will permit free air passage. You can drsmall lots in the warm air stream above a furnace orradiator, as long as the temperature does not exceed105°F. This may require 3 to 4 days. You can dry nutslower temperatures, but you will need more time. If thtemperature exceeds 110°F, nut quality will be impaired.

Pacific Northwest Extension publications are jointly producedthe three Pacific Northwest states—Oregon, Washington, andIdaho. Similar crops, climate, and topography create a naturageographic unit that crosses states lines. Since 1949 the PNWprogram has published more than 450 titles. Joint writing,editing, and production have prevented duplication of effort,broadened the availability of faculty specialists, and substantireduced the costs for participating states.

Published and distributed in furtherance of the Acts of Congreof May 8 and June 30, 1914, by the Oregon State UniversityExtension Service, O.E. Smith, director; Washington StateUniversity Cooperative Extension, Harry B. Burcalow, interimdirector; the University of Idaho Cooperative Extension SystemLeRoy D. Luft, director; and the U.S. Department of Agriculturcooperating.

The three participating Extension Services offer educationalprograms, activities, and materials—without regard to race,color, national origin, sex, age, or disability—as required by TitleVI of the Civil Rights Act of l964, Title IX of the EducationAmendments of 1972, and Section 504 of the Rehabilitation Aof 1973. The Oregon State University Extension Service,Washington State University Cooperative Extension, and theUniversity of Idaho Cooperative Extension System are EqualOpportunity Employers.$1.00/$1.00/$1.00