TNITEB8ITY OF CALIFORNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA WALNUT CULTURE IN CALIFORNIA BY L. D. BATCHELOR BULLETIN No. 332 June, 1921 UNIVERSITY OF CALIFORNIA PRESS BERKELEY. CALIFORNIA 1921 David P. Barrows, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Dean. Edward J. Wickson, Horticulture (Emeritus). Walter Mulford, Forestry, Director of Resident Instruction. C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station. B. H. Crocheron, Director of Agricultural Extension. Hubert E. Van Norman, Vice-Director; Dairy Management. James T. Barrett, Plant Pathology, Acting Director of Citrus Experiment Station. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Ralph E. Smith, Plant Pathology. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Fruit Products. Warren T. Clarke, Agricultural Extension. John S. Burd, Agricultural Chemistry. Charles B. Lipman, Soil Chemistry and Bacteriology. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. Fritz W. Woll, Animal Nutrition. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. L. D. Batchelor, Orchard Management. J. C. Whitten, Pomology. *Frank Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. R. L. Adams, Farm Management. F. L. Griffin, Agricultural Education. John E. Dougherty, Poultry Husbandry. W. B. Herms, Entomology and Parasitology. L. J. Fletcher, Agricultural Engineering. Edwin C. Voorhies, Assistant to the Dean. Citrus Experiment Station Staff J. T. Barrett, Acting Dean and Director. W. P. Kelley, Professor of Agricultural Chemistry. H. J. Quayle, Professor of Entomology. H. S. Reed, Professor of Plant Physiology. L. D. Batchelor, Professor of Orchard Management. H. S. Fawcett, Professor of Plant Pathology. R. S. Vaile, Assistant Professor of Orchard Management. A. R. C. Haas, Assistant Professor of Plant Pathology. E. T. Bartholomew, Assistant Professor of Plant Pathology. C. O. Smith, Research Associate in Plant Pathology. E. E. Thomas, Research Associate in Agricultural Chemistry. H. B. Frost, Research Associate in Plant Breeding. F. F. Halma, Research Associate in Plant Physiology. J. G. Surr, Assistant in Orchard Management. Hugh Knight, Assistant in Entomology. S. M. Brown, Assistant in Chemistry. A. B. Cummins, Assistant in Agricultural Chemistry. * In cooperation with office of Public Roads and Rural Engineering, U. S. Department of Agriculture. WALNUT CULTURE IN CALIFORNIA 1 By L. D. BATCHELOR CONTENTS PAGE General Review of the Walnut Industry 142 Business prospects of the industry 143 Length of life of walnut trees 143 Profitable bearing of young trees 145 Climatic Requirements 146 Frost injury 146 Heat injury 147 Soil Requirements 148 Depth and character of soil 148 Drainage and alkali injury 149 Water Supply........ 150 Amount of irrigation water necessary 150 Rainfall in walnut-growing sections 151 Irrigation water of good quality essential 152 Varieties 153 Description of varieties 155 Placentia 155 Eureka 156 Ehrhardt 159 Payne ' 162 Concord 162 Franquette 164 Rootstocks and Choice of Nursery Trees 165 Choice of rootstock 165 Method of propagation 166 Requirements for good nursery trees 166 Starting the Young Orchard 166 Laying out the orchard 166 Care of trees before planting 168 Planting the young orchard ' 170 Training young trees 170 Values of orchards and lands 175 Culture : 176 Soil management 176 Covercropping 180 Irrigation 186 Intercropping 196 Fertilization 201 Pruning 202 Insects and Disease Pests 203 Codling moth and aphis 203 Red spider 204 Walnut blight 205 Melaxuma 206 Winter-injury or Die-back 208 Harvesting, Curing, Packing and Cost of Production 210 Harvesting 210 Washing and curing 212 Packing 214 The standard for first-grade nuts 216 Selling the crop 216 Income to be derived from California walnuts 217 i Paper No. 82, University of California, Graduate School of Tropical Agricul- ture and Citrus Experiment Station, Riverside, California. 142 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION PREFACE The present bulletin represents the results of general observations and specific investigations of the writer and his associates. The subject matter and the method of presentation have been chosen in response to the numerous inquiries relating to general walnut culture, which have been received during the past six years. The writers' thanks are extended to Mr. Carlyle Thorpe, Professor F. T. Bioletti, and Dr. H. S. Reed for their careful criticism of the manuscript, and to Mr. D. C. Wylie, of the California Walnut Growers' Association, for his able assistance in gathering much of the general information in the walnut groves and among the walnut growers in various parts of California. Use has been made of the Experiment Station Bulletin 231, by Professor Ralph Smith, and of "The California Walnut," by Mr. W. T. Webber and Mr. W. E. Goodspeed. For a more detailed discussion of the several ramifications of this subject, such as history, nursery propagation, and the description of rarely found varieties and diseases, the reader is referred to Smith, R. E., Walnut Culture in California, Univ. of Calif. Bull. 231, 1912, and also to Lake, E. R., The Persian Walnut Industry of the United States, U. S. Dept. of Agr., Bur. Plant Ind., Bull. 254, 1913. GENERAL REVIEW OF THE WALNUT INDUSTRY The Persian walnut (Juglmis regia) or, as it is more commonly known, the English walnut, may be seen growing in nearly every county in the state of California. The commercial production of nuts, however, is centered mainly in southern California in the following four counties, mentioned in the order of importance : Los Angeles, Orange, Ventura, and Santa Barbara. Riverside and San Bernar- dino counties have a considerable area of young groves; and in the central part of the state walnut culture is gradually becoming im- portant in the following counties : Santa Clara, San Joaquin, Contra Costa, Napa, and Sonoma. Figure 1 shows a map of the state and the relative importance and distribution of the crop in the several counties. At the present writing, there are approximately 85,000 acres of walnut trees of various ages within the state. The annual production for the state during the last ten years may be seen from the following table : Bulletin 332] WALNUT CULTURE IN CALIFORNIA 143 TABLE I Annual Production of Walnuts in California (Expressed in pounds) 1911 25,000,000 1916 29,200,000 1912 22,500,000 1917 33,000,000 1913 22,700,000 1918 40,231,000 1914 17,800,000 1919 56,200,000 1915 29,650,000 1920 43,000,000 BUSINESS PROSPECTS OF THE INDUSTRY The present condition of the industry indicates that walnuts may continue to be one of the most staple and at the same time most profit- able crops which can be grown on land, and in locations, adapted to this crop. The annual consumption of walnuts in the United States has ranged between 60,000,000 and 70,000,000 pounds annually during the past five years, and has increased more than 30 per cent since 1909. Nuts, which were formerly looked upon as a holiday luxury, are becoming more and more a food to be used the year through in the average household. This increase and wider use of the walnut has been accompanied by an increase in importation exceeding our own average production, and a gradual increase in the prices paid to the California grower. It may thus be inferred that the business of wal- nut culture in California is not likely to suffer from the results of over-production in the near future. It is difficult to forecast future economic conditions relating to the production and sale of any par- ticular crop. Nevertheless, many of the older walnut growers, who have known this industry from its early days, believe that the planting of a walnut grove at present is as safe an investment as it has been at any time during the development of the industry. LENGTH OF LIFE OF WALNUT TREES The limit of the profitable length of life of a walnut grove cannot be told from experiences in California. Provided the soil, climatic, and water conditions are well suited to this crop, the trees may con- tinue to thrive and produce satisfactory crops for a long period, which is difficult to limit. Some observers have expressed opinions that walnut groves, under favorable conditions, may continue to be a satis- factory source of revenue until the trees are from 50 to 200 years old. Some of the oldest walnut groves in Ventura County are the most profitable and productive in the state. Notable examples of such old, yet productive groves, are the J. C. Daly grove of 20 acres planted in 1881 ; 18 acres of the J. M. Sharp grove planted in 1887 ; and the 144 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION T. A. Kelsey grove of 26 acres planted in 1888. The last mentioned grove has only 12 trees to the acre, planted 60 feet apart each way, and is considered by many to be one of the best walnut properties in the state of California. The trees, at their present age of 34 years, Fig. 1. (From < ' The California Walnut. ") show no apparent deterioration. This grove has produced an average annual tonnage of 1941 pounds of nuts per acre during the past four years. Groves in which the trees are planted 60 feet each way may be expected to grow longer without deterioration than groves where the trees are planted closer and thus severely compete with each other. BULLETIN 332] WALNUT CULTURE IN CALIFORNIA 145 PROFITABLE BEARING OF YOUNG TREES The young walnut grove may be expected to bear profitable crops by the time the trees are 6 to 10 years old, depending upon the variety, the number of trees planted per acre, and the natural conditions surrounding the grove. A grove of almost 50 acres of the Placentia variety in Orange County produced an average of 44 pounds of ungraded nuts per tree in its ninth year. As the trees are 60 feet apart this represents 528 pounds per acre. During the first 10 or 12 years this might well have been double planted and thus produced approximately 1056 pounds per acre the ninth year. In 1918 a block of 60 trees of the Chase variety, on a heavy clay loam soil in the Puente district, produced a profitable crop of 35 pounds per tree in the sixth year. The records of these groves and of a grove of 38 Ehrhardt trees are shown in Table II. In considering this table one should keep in mind that each of these plantations is growing under the best natural and cultural conditions, and is thus without doubt considerably above the average. In pro- jecting a business enterprise, the yields shown may be looked upon as approaching the maximum possibilities and should therefore be discounted 25 to 50 per cent to reach a figure which may reasonably be expected to prevail under average conditions. Again, the figures here presented are of little value in making comparisons between the three varieties in question, as no two varieties were of the same age the same season. TABLE II Average Yearly Production per Tree of Young "Walnut Groves Variety r Placent A ia Chase Ehrhs irdt Year r Season's growth Yield in lbs. Season's Yield growth in lbs. (Ungraded nuts at harvest) r y Season's Yield growth in lbs. (Ungraded nuts at harvest) 1915 7th 22f 10th 80 1916 8th 27f .... .... 11th 76 1917 9th 44* 5th 18 12th 79 1918 10th 43* 6th 35 13th 81 1919 11th 104* 7th 72 14th 117 1920 12th 82* 8th 43 15th 98 * Pounds of uncured and ungraded nuts at harvest, t Pounds of cured and ungraded nuts at harvest. 146 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Although the cultural and natural conditions in all these groves were favorable, the plantings are several miles apart and conditions were not identical. Certain seasons are particularly favorable to crop production. The year 1919 was characterized by especially abundant crops of walnuts. These were followed in many cases by lighter crops in 1920, although the young trees were one year older and had a greater bearing surface. CLIMATIC REQUIREMENTS The chief climatic limitations of the walnut are frosts in spring and fall, and extreme heat in summer. FROST INJURY Low frosty sites should be avoided for walnuts. A temperature of even 2 or 3 degrees below freezing (32° F.) will usually kill a large percentage of the young walnut flowers, if they are just starting to bloom. During the season of 1920, late spring frosts during the second week of April reduced the walnut crop to a great extent in several of the inland valleys. In one instance at least, the absolute minimum recorded by a reliable thermometer was only 29° F., yet it was esti- mated that 50 per cent of the crop was destroyed. During this period apricots and the earlier blooming cling peaches in adjacent orchards, were about the size of green peas, and experienced little or no injury. During the morning of April 6, 1921, a minimum temperature of 27° F. was experienced by a seedling walnut grove in which the majority of the trees were in full bloom. This temperature prevailed for about an hour and apparently destroyed nearly the whole potential crop, except in the tops and centers of the trees, where an occasional cluster of blooms escaped injury. As a general rule, the late blooming varieties, such as the Eureka and Concord, may be expected to escape injury from the late spring frost more consistently than the earlier blooming soft-shell types. During the spring of 1915, however, walnut twig tips and young nuts were killed during the first week in May on some of the low sites in Los Angeles County. With a frost as late as May the late blooming varieties may suffer more. injury than the earlier ones, as the young nuts (one half -inch or more in diameter) of the early blooming varie- ties, at this time, will stand more cold than the blossoms of the late varieties. The late bloomer does not, therefore, always escape late spring frosts. It is much better to avoid planting walnuts on land which is considered too frosty for peaches during the majority of Bulletin 332] WALNUT CULTURE IN CALIFORNIA 147 seasons. The varieties of the soft-shell type bloom about the same time as the Bartlett pear and are not nearly so resistant to frost injury of the blooms. Early fall frosts also cause injury, especially to young shoot growth ; though this injury may not be apparent to the casual observer until the following spring, when the tree fails to leaf out on the major portion of the past season's growth. This type of injury is more fully treated under the subject of die-back. It may suffice to explain here that early fall frosts prematurely stop the final ripening process of the twigs by killing the leaves. Such injured leaves will usually fall off within a week after the frost injury, whereas they might otherwise have functioned normally a month or six weeks longer. The bare, immature twigs dry out rapidly and sunburn on the south side during the winter months, killing back several inches, or possibly several feet to the more mature wood, which has been able to withstand the abnormal insolation. HEAT INJURY The chief injury to the walnuts during the extreme hot weather of the summer months is the sunburning of nuts on the outsides, tops, and south sides of the trees. Such burned nuts usually become "blanks" if the injury occurs during June or July. If the sunburn- ing takes place when the nuts are more fully developed, in August, they may be salvaged as culls with a portion of the kernels edible. In mild cases of sunburn injury, a small part of the husk may stick to the shell or may only stain it, causing the nut to be graded out as a cull. The kernels of such nuts may be of first quality, although it is likely that there will occur a high percentage of amber and black kernels and partly shriveled and mouldy meats among this class of nuts. Nuts on trees growing on a deep silt soil withstand the extreme heat without burning much better than nuts on trees growing on a shallow soil with less moisture available. Sandy soils must be fre- quently irrigated in the hot sections if the walnuts, are to avoid serious injury by sunburning. At a maximum temperature of 100° F. in the shade, accompanied by low humidity, walnuts which are exposed to the direct rays of the sun may be expected to sunburn. This tempera- ture, or higher, occurs in many of the inland valleys, 15 to 30 days during the summer months. Under such conditions the crop is always injured by sunburn. Regardless of this, the planting of walnuts is extending more and more inland. The planting of oranges is restrain- ing the development of walnut groves on the coastal sections. At the present time walnut groves have not become as productive in the 148 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION inland valleys, which experience frequent summer days with maximum temperatures of 100° F. and over, as they have been nearer the coast, where the weather is characterized by a smaller daily range and a lower maximum temperature. Varieties especially selected for the hotter sections may tend to correct this difficulty in the future. In the past, most of the walnuts planted inland have been of the same varie- ties as those which were originally selected for the coastal regions. Varietal adaptation is more fully treated under the heading of varieties. At present walnut culture is most successful in the sections of the state which are characterized by a long growing season free from frosts, where the daily range in temperature is not great, where the absolute maximum seldom goes above 100° F., and where winter tem- peratures are moderate and the humidity is high, especially during the summer. Sudden changes in temperature, such as occur in the desert regions, seem decidedly detrimental to the growth of the walnut tree. The localities where favorable climatic conditions occur is indi- cated by the fact that the largest walnut shipping centers in the state are Santa Ana, Whittier, Puente, Saticoy, and Santa Barbara. When the tree is fully dormant, the absolute minimum tempera- tures of winter, during January and February, apparently have very little to do with its welfare. This is especially true of the French varieties. Trees of the Mayette and Franquette varieties have grown for some years without severe winter injury along the west side of the Wasatch Mountains north of Salt Lake City, Utah, where the minimum temperature occasionally was 5° below zero (Fahrenheit) during the winters of 1913-14 and 1914-15. SOIL REQUIREMENTS DEPTH AND CHARACTER OF SOIL The success of walnut culture is dependent upon favorable soil conditions to a greater extent than that of many other tree crops. A well-drained deep silt loam, containing an abundance of organic matter, free from a high or fluctuating water table, hardpan, sandy subsoil, and ' ' alkali, ' ' is essential for a first-class walnut grove. Shal- low soils underlaid by a hardpan or an impervious clay stratum within 5 or 6 feet from the surface will produce only a short-lived, second- rate walnut grove. Such groves are frequently stunted in growth, subject to yellowness, and are poor producers. There are a few moderately successful groves growing in fine silt soil underlaid with fine sand within 4 or 5 feet from the surface, but such properties require the greatest skill in planning the cultural Bulletin 332] WALNUT CULTURE IN CALIFORNIA 149 and irrigation practice and should certainly be avoided by the beginner in walnut growing. Very light sandy soils are usually unsatisfactory, producing slow- growing, stunted, poor-yielding trees. On such soils the nuts are very subject to sunburning. One should not judge the soil conditions entirely by the surface soil. The nature of the subsoil may be of great importance. There are first-class walnut groves in the El Monte district growing, apparently, in a very light sandy soil. The soil changes, however, in some instances 3 or 4 feet from the surface, grading into a silt loam which seems to be an ideal soil for walnut roots. Very heavy adobe soils or silts underlaid by adobe are not best suited to walnut groves. The trees on such soils are usually stunted, making very little growth and fruit wood in the tops when mature. These soils are difficult to cultivate, and it is difficult to obtain a deep penetration of the irrigation water which is so essential to good walnut culture. Such soils frequently contain amounts of alkali harmful to walnuts, and their leaching by any system of irrigation is very difficult. DRAINAGE AND ALKALI INJUEY A water table within 9 or 10 feet from the surface of the soil may make it impossible to maintain a profitable walnut grove. Most of the high water tables in arid regions are characterized by carrying more or less "alkali" 2 in solution. The mere presence of water within 9 or 10 feet of the surface is usually only part of the difficulty, the alkali present is the greatest concern, for the deep-rooted trees obtain a portion of their water from this water table and may be injured by the salts therein, even though there is a seemingly sufficient layer of good soil above the water. There occurs an instance in Orange County where an old grove has been practically ruined by a rise in the water table to within only nine feet of the surface. This water contains 700 parts per million of salts. The water rises approxi- mately 6 feet by capillary action in the soil; thus there is only a 3-foot depth, or one-third of the soil, above the standing water, which is not impregnated with salts. Several instances were noted in Ven- tura County during the fall of 1919 where walnut trees were severely injured by the presence of an "alkali" ground water 9 to 13 feet from the surface. When the trees were planted 20 years ago in this district, the water was at least 30 feet from the surface. It is difficult to place a definite safe boundary beyond which injury from a high 2 For a discussion of alkali in the irrigation water see page 152. 150 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION water table is likely to occur. Apparently quite as much depends upon the nature of the water as upon the actual depth from the surface, within the limitation of the root zone. The rainfall, irrigation practice, and nature of the soil, and the irrigation water may also greatly influence the limitations, for walnut culture, of land with a high water table. Rarely there is found an exception to the rule of injury resulting from a high water table. There is one notable example in Santa Barbara County where an orchard is sub-irrirgated by a natural flow of water from the hills. Judging from the appearance of the trees,* this ground water contains no (or an exceedingly small amount of) "alkali," a condition which is exceptional to the usual shallow water tables in irrigated areas of the arid west. WATER SUPPLY In most of the California walnut districts, irrigation is necessary. The amount of water to be applied and the seasons of application naturally depend upon the rainfall, the amount of evaporation, the nature of the soil, and other factors. The walnut tree can not be expected to yield profitable crops unless furnished with an abundant supply of water during the entire year. AMOUNT OF IRRIGATION WATER NECESSARY From 12 to 24 acre-inches per acre 3 of irrigation water is usually applied to bearing walnut groves, varying according to some of the conditions related in the preceding and the following paragraphs. 3 For those not familiar with the various methods of measuring water, the fol- lowing explanation and table is offered converting miners' inches to acre inches. An acre inch of water is the quantity of water required to cover one acre to a depth of one inch. 50 California "miners' inches" = 1 cubic foot per second. 1 cubic foot per second = 7.48 gallons per second. Table Miners' inches to acre inches. Miners ' Inches Hrs. 1 2 3 4 5 6 7 8 9 10 1 .0198 .0396 .0594 .0792 .0990 .1188 .1386 .1584 .1782 .1980 2 .0396 .0792 .1188 .1584 .1980 .2376 .2772 .3168 .3564 .3960 3 .0594 .1188 .1782 .2376 .2970 .3564 .4158 .4752 .5346 .5940 4 .0792 .1584 .2376 •.3168 .3960 .4752 .5544 .6336 .7128 .7920 5 .0990 .1980 .2970 .3960 .4950 .5940 .6930 .7920 .8910 .9900 •s PI 6 .1188 .2376 .3564 .4752 .5940 .7128 .8316 .9504 1.0892 1.1880 7 .1385 .2772 .4158 .5544 .6930 .8316 .9702 1.1088 1.2474 1.3860 ; '*"'"4V« • w : * : '*k F5#i-.d®" Fig. 9.— Result of heading low, starting all the branches from one point. This could have been prevented when the tree was young by spacing the branches through a greater distance on the trunk of the tree. With the vase type this perpendicular spacing throughout 18 to 24 inches will give greater strength to the framework than grouping the 174 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION branches at more nearly one point of origin. The lateral branches to be left for the framework of the tree may be 3 to 5 in number, as evenly spaced as possible as they radiate outward from the trunk. Fig. 10. — Method of bracing open vase type of tree to prevent splitting. The upper or second bud from the top of the original tree will usually produce a branch which grows in a nearly upright position ; this, of course, should be "pinched back" during the early summer or cut BULLETIN 332] WALNUT CULTURE IN CALIFORNIA 175 out during the dormant season, in forming the framework of an open vase-shaped tree. Figure 12 illustrates an example of this type of tree. In the case of the leader type it is essential to space the laterals far apart and not leave too many; otherwise the leader, or central shaft, will be "choked" out, as illustrated by Fig. 13. Only two or three of the laterals should be left in addition to the central leader at the end of the first year, as other lateral branches will grow out during the following years from the central leader; thus the shape of the trees is formed during the first two or three years' growth. The branches which are left on the tree are not cut back in common practice and by all means the leader should not be cut back. Figure 8 shows a desirable shape for a tree of this type. Some of the most desirable types of trees judging from the view- point of strength of framework and area of bearing surface, are a sort of modification of both the leader and the vase-shaped type. Figure 14 illustrates such a tree; there is a pronounced leader through 4 or 5 feet of the main frame of the tree which finally divides up into a strong, compact, spreading type of tree. This type can be trained by maintaining the leader type through the first four years and then during the dormant season, pruning back the leader to a strong lateral branch 9 or 10 feet from the ground. The tree shown in the illus- tration might well serve as a model, or ideal, in training young walnut trees. VALUE OF ORCHARDS AND LANDS The valuation of walnut orchards and lands suitable for walnut production varies considerably in the different localities and even in the same districts. Such factors as suitability of the soil, water rights, frost conditions, and proximity to transportation and civic centers are reflected in the appraised valuation of walnut properties. The value of established groves may be greatly influenced by the variety of nut, rootstock, planting arrangement, etc. The average or the most general valuations are all that can be presented here. Replies to questionnaires sent out by the California Walnut Growers' Association in 1919 and again in 1921, indicated that mature bearing orchards are valued at about $1400 per acre, while the young non-bearing properties are valued at approximately $900 per acre. The following table shows the exact average valuation and the number of properties considered. 176 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION TABLE IV Average Valuation of Walnut Groves (Including water rights) Year observations were made Valuation A Number of properties represented Bearing Non-bearing 1919* $1299.02 $995.00 922 1921f $1477.05 $828.65 231 Land suitable to walnut culture, plus adequate water rights, may be purchased at from $600 to $1000 per acre in the proved walnut- growing sections of southern California. Such land may have a rather fixed value for the production of beans, beets, or even citrus fruits; thus its potential value for walnut production is only one factor which determines its appraisement. In the central or northern portion of the State, where the walnut industry is not so extensively developed as in the south, land which is likely to prove suitable for walnut culture may be purchased for $200 to $300 per acre. This may not include a water right in any estab- lished irrigation system, but such lands occur in areas where good wells are known to exist. CULTURE SOIL MANAGEMENT There is no one best way to cultivate all walnut groves. The methods of performing the various operations of stirring the soil, such as plowing, discing, and harrowing, vary widely in different walnut groves. These different methods may give equally good results in their respective groves, when measured by profitable crop produc- tion. A practice which leaves one soil in first-class condition may be very poor practice on another soil of a different type. Most walnut trees are deep-rooted and it seems quite apparent in many cases that the character of surface soil cultivation is of only minor importance to the welfare of the tree, except as this cultivation affects the con- servation and use of soil moisture by the tree. There are many door- yard and roadside trees which produce heavy crops and yet the soil around them may not be stirred from one year's end to another. It does not follow from this, however, that a walnut grove should not * ' ' The California Walnut, ' » p. 15, 1919. Published by The California Walnut Growers' Association. f Brief of Facts Eclating to the American Walnut Industry, p. 23, 1921. Pub- lished by the Walnut Protective League. Bulletin 332 WALNUT CULTURE IN CALIFORNIA 177 be plowed or cultivated. There are frequently many conditions sur- rounding roadside and dooryard trees which are favorable to the pres- ence of an abundance of soil moisture, which cannot be duplicated under orchard conditions. Isolated trees have the advantages of better exposure to sunlight and a far greater range for root develop- ment and available soil moisture than trees planted in orchard form. Fig. 11. — Walnut trees when planted are usually cut back to within 5 or 6 feet of the ground. (a) If they are not trained they will frequently send out shoots along their entire length during the early summer. (&) Frequently if the lower buds within two feet of the ground are allowed to grow, the upper 24 inches of the trees may remain dormant. (c) If the lower buds are suppressed the upper ones are forced into growth and the framework of the tree is started in the upper 24 inches of the trunk. 178 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The majority of walnut groves are plowed once a year, usually in the very early spring. A disc, springtooth or spiketooth harrow is then used to put the land into good physical condition. The clean cultivation during the summer months is done in relation to the irri- gation practice and the conservation of soil moisture. The practice of plowing the groves once a year seems essential to most of the soil types found in the walnut districts of California. The relation of such plowing to the penetration of the rains into the subsoils and the application of irrigation water alone justifies the oper- ation. Putting 6 to 10 inches of the surface soil into a friable, mellow condition every year makes it practical to use deep irrigation furrows and thus to insure deep penetration and a more complete use of the irrigation water applied the following season. An annual plowing prevents the tree roots from becoming permanently established in the surface six inches of soil, and thus becoming subject to the great variations of soil moisture content which occur there during the summer months. If the land is plowed in the fall and left rough, the rains may be expected to penetrate deeper into the subsoil because of the lessened run-off. This object is accomplished best, in some instances, if the plowing is done at right angles to the grade of the land. Where a winter cover crop is grown, fall plowing may be impractical because of the necessity of planting the cover crop early, either just before the harvest, or immediately after. When winter cover crops are not grown, the practice of fall plowing has much to commend it. A medium to light irrigation may well precede the fall plowing, and the land left rough without further cultivation until the early spring. Late spring plowing is objected to by some walnut growers as they believe the cutting of the feeder roots by such plowing to be especially harmful when the tops are just starting to grow, during late March and the month of April. The instances which are frequently cited to illustrate the poor crop conditions following late spring plowing, are not readily diagnosed. It usually seems that there may be other more important factors conducive to poor crops, accompanying the late plowing. The dry soil condition which is so frequently correlated with late plowing, may in itself be more truly responsible for the poor crops than the mere season of plowing. Many orchards which are plowed late are cover-cropped or heavily covered with volunteer growth of barley, oats, native annual plants, etc. In most seasons, and in most walnut districts, the presence in late March or early April of a sown cover crop or a volunteer crop, may be taken as a safe indication that the soil moisture is largely depleted, unless the Bulletin 332] WALNUT CULTURE IN CALIFORNIA 179 land has been thoroughly irrigated once or twice during the growth of the crop. The orchard in which a volunteer crop is allowed to grow is most likely to be neglected in regard to winter irrigation. Many such orchards are plowed in the spring when the annual plants have actually wilted for lack of moisture, or after recent rains have revived them by moistening the soil only to the depth of the plow Fig. 12. — A typical open-vase type of tree. furrow. The subsoil from the second to the eighth foot, in groves thus managed, may be nearly dust dry at this season when the trees are commencing to leaf out. The amount of water required to produce a cover crop is often underestimated by the walnut grower. This is especially true when the crop is produced during the rainy season and makes a satisfactory growth without irrigation. This matter is more fully treated under the heading "Cover Crops/' Many such instances of the above have 180 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION been noted, when an examination of the subsoils has shown a dryness 16 hardly credible. The fact that the orchard was being plowed at the time the observation was made, and a few feeder roots were being cut in the late spring, has seemed to some observers as unimportant, com- pared with the extremely unfavorable soil moisture conditions existing at the commencement of the growing season for the trees. It seems reasonably safe to assert that the chief factor contributing to poor crops in many late plowed orchards is the dry soil condition during the winter and early spring, and the late plowing which may be asso- ciated with this dryness is in no great degree responsible. Late plow- ing may be accompanied by ideal soil moisture conditions as noted later, in which case the crops may not suffer. Judging the question of late spring plowing from another angle, there are many first-class, heavy producing orchards which are annually spring-plowed in April, when the leaves are coming out on the trees. Some of these orchards are located on the lowlands subject to winter floods, where it is necessary to cover crop the land to prevent erosion and where this threat of flood damage is not past until April. Orchards in these districts are seldom or never plowed until late, yet many of them produce approximately 2000 pounds per acre. Such plantings are usually well irrigated during the spring or late winter months, however, either by regular irrigation methods or by the passage of flood waters over the lands. This discussion should not be taken as an encouragement to late spring plowing, as there are points in favor of plowing early and it seems to be the practice most usually recommended by successful walnut growers. This discussion is offered mainly to encourage growers to feel that they may maintain a first-class walnut grove, even though for any reason they are obliged to plow late, providing other factors, chiefly soil moisture conditions, are favorable to the early spring growth of the tree and nuts during March and April when such plowing is frequently done. • COVER CROPPING The practice of cover cropping walnut groves prevails in probably somewhat less than 50 per cent of the total area of bearing groves. is Walnut groves have been examined by the writer in which the soil below the penetration of winter rains contained only hygroscopic moisture. Walnut trees commonly reduce the soil moisture to this point during the fall months. A winter rainfall of 8 to 12 inches may not percolate below the second or third foot of soil if there is a cover crop or weed crop growing on the land. If the rains come in small amounts distributed over 5 or 6 months, as frequently occurs, their shallow percolation is more pronounced than when large volumes fall at a time. BULLETIN 332] WALNUT CULTURE IN CALIFORNIA 181 Many groves which are not sown to cover crops produce a volunteer growth of various annual plants, such as bur clover, lupins, wild oats, and alfilaria, during the rainy season. There is very little or no definite information concerning the subject of the effect of cover crops on walnut trees. 17 Though we have no definite field trials to inform us concerning the effects of cover crops in walnut groves, a discussion of some of the factors to be considered may be useful. Possibly the primary factor is the problem of supplying sufficient irrigation water for the walnut trees and the cover crop combined. While it is true that walnut trees do not draw very heavily upon the soil moisture during the winter months, yet it is highly important to have the water content of the soil somewhere near optimum during the entire winter. It is a mistake to allow a winter cover crop to deplete the soil of its neces- sary moisture. There should be no misconception about the saving of soil moisture because of the shade of the cover crop. This shade and thus possible conservation of moisture in the few surface inches of soil, is hardly worthy of mention, compared with the soil moisture necessary to produce the cover crop itself. The actual water required to produce a fair crop of the several plants commonly used as cover crops in California, has never been carefully studied, so far as known to the writer. A good general idea, however, of the water requirements of such crops can be obtained from both the controlled experiments with related plants, and the experi- ences of farmers. In studying the water requirements of certain plants, Briggs and Shantz 18 found that different plants require different amounts of water to produce equal quantities of dry matter. 19 The average requirement of alfalfa, field peas, and melilotus alba was 803 pounds, which is more than is needed by common graius. i? The beneficial effects of plowing under green manure crops, as measured by the crops of annual plants which are later grown on the same ground, cannot be used as a safe guide in theorizing on the effects of this practice on walnut trees. In the case of annuals, there is no competition between the cover crop and the primary crop. Even with annuals, much of the benefits ascribed to the cover crop may have been due to crop rotation, as has been shown by various experiments which were so laid out as to measure this latter factor. Again, the root zone of many of the annual crops which have been used to indicate the effect of cover crops upon the productivity of the soil, corresponds more closely to the root zone of the cover crops than to that of the very deep-rooted walnut trees. is The Water Eequirement of Plants, U. S. Dept. of Agric, Bur. of Plant Indus., Bull. No. 284, 1913, p. 47. 19 Dry matter is that portion of the plant which remains after all the water has been evaporated off by a temperature slightly above the boiling point of water. 182 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION TABLE V Water Requirements, Measurements of Crops at Akron, Colorado Crop Variety- Water requirement (lbs.) Alfalfa Grimm 1068 Field peas Canada 800 Sweet clover (M. alba) 709 Average of three legumes 803 In the absence of any more information, we can use these results to estimate the approximate water requirements of the cover crops used in walnut groves. Using this water requirement of 803 pounds of water as a guide, and a yield of 13 tons per acre as an average yield, 20 with 20 per cent dry matter, the example works out as follows : 5200 (lbs. of dry matter) X 803 (lbs. of water required per lb. of dry matter) =4,175,600 (lbs. of water per acre) = 18.47 acre inches per acre of cover crop. This figure of 18.47 inches is based upon control experiments where the crops were grown in cans and the surface evaporation from the soil was practically eliminated, whereas under orchard conditions there is considerable loss from surface evaporation while the young cover crop is becoming established during the early fall months. This figure is, therefore, probably too low for orchard conditions. Turning now to a practical example : A cover crop of yellow sweet clover (M. indim) was grown in a one-year-old walnut grove on the Citrus Experiment Station grounds during the winter of 1920-21. The land was cropped to barley during the spring of 1920, and there- fore contained practically no soil moisture available for the growth of a cover crop in the fall of the same year. The cover crop was planted in September, 1920, and plowed under in April, 1921, when it was just past full bloom. The clover growth contained some volun- teer barley, so that it closely corresponded to the cover crops in the flood districts, where barley and clover are sown together. From the date of planting until the middle of March, 15.8 acre inches of irriga- tion water per acre had been applied to this crop. At this time the clover was just starting into bloom and the land was so dry that it could not be plowed with a mould-board plow until after it was irri- gated. It was then irrigated with 7 acre inches per acre. By April 11 20 W. M. Mertz, Green Manure Crops in Southern California, Univ. of Calif., Agric. Exp. Sta., Bull. No. 292, 1918, p. 10. Bulletin 332] WALNUT CULTURE IN CALIFORNIA 183 the land was again too dry to plow, and in addition to the irrigation water previously given, 7.2 inches of rain had fallen during the growth of the cover crop, making a total of 30.0 acre inches per acre which had been used by the small walnut trees and by this mixed cover crop of sweet clover and barley of maximum tonnage growing to maturity. 21 Fig. 13. — The central leader in this tree has been starved out by the lateral branches, all starting out at nearly the same place and cutting off the food supply from the central trunk. 2i At this time the land received an additional 7 acre inches per acre and was then plowed as soon as practicable ; the last 7 inches should not be charged to the cover crop, as it was largely conserved in the soil by prompt plowing, leaving the soil in a good condition for the young trees and the planting of a summer intercrop of beans. 184 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION If the crop had been plowed just as the clover came into bloom following promptly the March irrigation 22 there would have been 15.8 acre inches per acre irrigation water and 7.2 inches of rain, making 23 inches as a total used by the crop up to this period in its growth, and leaving the soil quite as dry as it was the fall before. 23 The stage of development of the crop at this time, just beginning to bloom, would have been the ideal condition for plowing under. The tonnage was nearly at its maximum, and the green succulent condition of the stalks of both the clover and the barley would have probably assured a more rapid disintegration after plowing than turning it under in April, when the flowering season was practically past. From the two examples cited, it seems probable that the growth of a legume cover crop in a walnut grove should be expected to require from 20 to 24 acre inches per acre of irrigation water, aside from the needs of the trees. The winter rains alone are frequently no more than sufficient to meet the requirements of the trees under clean cultural conditions. In practice this will mean from 3 to 4 heavy irrigations during the winter and early spring, in localities receiving only 10 to 15 inches of winter rainfall. (See "Irrigation.") It is sufficient here to remind the reader again that there should be ample water available to grow the winter cover crop, and that following the plowing in of the crop, the walnut trees should be given a thorough irrigation at the commencement of their early spring growth. If the requirements of the cover crop alone are considered, at the time of plowing the subsoil may be entirely too dry to promote an early vigorous growth of the trees and nuts, even though the soil is moist enough to plow with a mould-board plow. One of the benefits derived from the use of cover crops in walnut groves has been the improvement of the physical condition of the heavy soils as a result of the incorporation of large amounts of organic matter. The experiences of many observing farmers seem to agree that the clay loams are made more friable and the irrigation water penetrates them much more readily when cover cropping is practiced. The latter point, with reference to the deeper penetration of the irri- gation water, may alone justify the use of cover crops on certain soils. The legume cover crops most commonly grown in walnut groves during the winter months are bur clover, yellow sweet clover (Melilotus indioa), purple vetch, and horse beans. 22 It was too dry to plow before this irrigation, as noted. 2 3 The crop wilted badly before the March irrigation and the soil was judged to be between the wilting point and the hygroscopic point. Bulletin 332] WALNUT CULTURE IN CALIFORNIA 185 The seed may be drilled in, or sown broadcast, and then " brushed in" or harrowed lightly with a spike-tooth harrow. The amount of seed required per acre is as follows : Melilotus indica and bur clover 20- 30 lbs. per acre Vetch 70- 80 lbs. per acre Horse beans 100-200 lbs. per acre Fig. 14. — One of the most desirable types of walnut tree, combining the good points of both the leader and vase shape types. It is essential to plant the cover crop as early in the fall as possible after harvesting the nuts, in order that it may make a good start before the cold weather commences, and thus make sufficient tonnage by the middle of March to be worth plowing under. The cover crop grows successfully, sometimes, if sown at the time the land is leveled off, preceding harvest, and the furrowing out and irrigating of the land postponed until the harvesting operations are over. Figure 15 shows a good crop of yellow sweet clover (M. indica) ready to turn under in late March, while Fig. 16 shows a similar cover crop being plowed deeply after first laying the clover down with a drag. Cover crops are occasionally disced under, as shown by Fig. 17. This is not so desirable as deep plowing, because it does not incorpo- rate the organic matter in the soil mass so closely to the tree roots. A cover crop disced under improves the physical condition of the surface soil, but it does not supplement the nlant food of the soil so much as though it were plowed under. 186 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION IRRIGATION Before discussing the irrigation of any crop, it is well to understand the extent and distribution of the root system of the crop in question. It is a commonly held opinion that walnut trees are very deep-rooted when grown on the majority of the orchard soils of southern Cali- fornia. There are exceptions to this general rule, but in the deep friable soils of the typical walnut sections the problem usually is not how deep do the roots go but where do they stop? In treating the matter of irrigation, the following discussion may be taken to apply to the volume of soil included in the first eight feet from the r v ■ mm I >*.%%"• • ** - m 'M^n^titum *** j£T **■* .»> **