UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION CIRCULAR No. 258 March, 1923 THINNING DECIDUOUS FRUITS By WAEEEN P. TUFTS The deciduous fruits of California are famed for their excellence. To maintain this reputation in view of the increasing tonnage, it is necessary to give careful attention to all the factors that combine to make a perfect product. Size, color, texture and flavor are the chief points sought for and these taken together go to make up the somewhat elusive term known as quality. Thinning the immature fruit is one of the most essential orchard operations because crowded fruit cannot size properly and neither will the color, texture and flavor be of the best. Considerable thinning is done when the trees are pruned. These two operations are there- fore so closely related that one cannot be discussed without a con- sideration of the other. Little experimental data exist for guidance in thinning. Most growers, however, have demonstrated to their satisfaction that the operation pays. This is particularly true with apricots, peaches and shipping plums. It is the aim of this circular to present as briefly as possible certain general considerations as well as practical sug- gestions for the carrying out of the work. SPECIFIC RECOMMENDATIONS Almond. — The almond is not generally thinned. However, cer- tain growers report success in removing a portion of the crop shortly after the seeds begin to harden to sell to the " green almond" market, the remaining nuts being left to attain greater size and weight. The practice is not well enough established to recommend its adoption, especially since the outlet for the green fruit is restricted. Apple. — The fruit-bud of the apple produces a cluster of flowers, several of which usually set fruit, but only one as a rule should be left at thinning time. When there is a heavy set some spurs must be deprived of all their fruit so that when the apples are ripe they will be from four to six inches apart on the branch. Some varieties Z UNIVERSITY OF CALIFORNIA EXPERIMENT STATION such as the Winesap, must be thinned even more severely so that the fruits possibly are not closer than eight or ten inches. When the crop is not so heavy and growing conditions are favorable, then it may be permissable to leave two specimens to the spur. Apples should generally be thinned immediately after the June drop. However, under certain conditions and with some varieties, notably the earlier maturing sorts, by delaying the thinning for a time it has been found possible to profitably dispose of the immature fruit for culinary purposes and at the same time secure proper devel- opment of the main crop. Specially designed ''thinning shears" are on the market for use in apple thinning, but the use of these with most varieties will not prove faster than removal by hand. Furthermore, there is danger of injuring the specimens left with the sharp points of the shears. Apricot. — Apricots should be thinned so that no two specimens will touch when ripe. Ordinarily the young fruits should not be left closer than from three to five inches on strong shoots and only one fruit to each short spur. Two fruits may be left near together on opposite sides of the branch or even on a short spur, if no other fruits are closer than four or five inches. In the coast sections where brown rot is prevalent, great care should be taken to see that fruits do not touch when ripening. In clusters of apricots there is often enough moisture between fruits that touch each other, to germinate spores of the brown rot. This disease spreads very quickly from fruit to fruit at ripening time, by contact, and within two or three days the entire cluster may be infected. Thinning apricots on trees which have been pruned by the so- called ' ' long system ' ' requires special mention. Such trees ordinarily set a much heavier crop than "short pruned" trees of similar age and therefore require heavier thinning. In some sections apricot trees set fruit on the slender one-year shoots which if left is very likely to produce small, scarred and "fog-marked" specimens. These should all be removed and this is most easily and cheaply accom- plished by means of a light pole to the end of which has been fastened twelve or fourteen inches of old rubber hose. The branches may be struck with this hose to knock off the apricots at thinning time with- out material injury to either branch or buds. Considerable experience is necessary to determine how heavy the set must be to make a systematic thinning of the apricot orchard economical. Observations seem to indicate that crops up to a certain tonnage will be brought through to maturity with marketable sizes Circular 258] THINNING DECIDUOUS FRUITS Fig. 1.— Cluster of apples setting from one fruit-bud. For best results only one fruit should remain after thinning. 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION without carefully breaking up every cluster and separating adjacent fruits as above recommended. Nevertheless, conservative apricot growers make thinning one of their annual orchard operations. Table 1 gives the results of a thinning experiment on apricots conducted by V. C. Blanchard of the Agricultural Extension Services in Los Angeles County during the season of 1922. Table 1 Hand Thinning of Apricots Parmentier orchards, San Fernando Valley, 1922 Thinning Size Yield Proportion of fruit Fruits per Pounds per Pruning Treatment removed pound tree Thinned only (long) One-half 9-10 300 Heavy heading (short) Clusters — One-third 14-15 240 None None 17-18 200 Thinned only (long) Terminals only 12 (No data) The lightly pruned trees set very abundantly and at thinning time it was thought desirable to remove approximately one-half of the fruit. That this treatment was advisable is shown by the size of the individual fruits harvested as well as by the total yield. Heavy pruning, resulting in less fruit — all borne in dense clusters — gave less yield and smaller sizes. The smallest fruit and crop was pro- duced by trees receiving neither pruning nor thinning. In the case where a thinning method of pruning was accompanied by the removal of fruit from only the terminal growths, the size of the individual specimens was quite satisfactory. Unfortunately this tree was har- vested without the weight of the crop being secured. This latter method of thinning, however, is not to be generally recommended. Cherry. — The cherry is not thinned. Peach. — The peach responds most readily of all the deciduous fruits to thinning and is the one fruit which under practically all conditions must be thinned. It has been shown that it takes two two-inch peaches to equal in "green" weight one two-and-a-half -inch peach, and also that when the dried fruit is considered it takes three and three-quarter standard grade Muirs to equal one extra fancy dried Muir. These facts emphasize the importance of securing size. The common recommendation has been to thin peaches so that the fruits are separated by four to six inches at ripening. However, if two fruits are on opposite sides of a shoot and are far removed from other specimens both may be left. It is probably impractical in most cases to attempt to separate "doubles." Remove all such if there are enough "singles" to make a crop. Circular 258] THINNING DECIDUOUS FRUITS Fig. 2. — Fruits composing this cluster of apricots would have reached better size had one or two specimens been removed at thinning. b UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Recently Weldon* has submitted figures, collected by the Southern California Canners' Bureau and the California Growers' Association, showing the number of peaches of different sizes necessary to make a ton of fresh fruit. From these facts the author calculates the number of peaches of a given size which must be left on a single tree planted at a certain distance to produce a given tonnage per acre. Number of tons per acre desired Tons 1 2 3 4 5 6 7 8 9 10 Table 2 Thinning Schedule Number of peaches per tree, of 234 in., 2% in. and 2% in., sizes to produce des'red tonnage per acre as shown in left hand column, from trees planted the different distances indicated below. 20 feet x 20 feet 108 trees per acre 24 feet x 24 feet 75 trees per acre 2Min. 2V 2 in. 2H in. 2^in. 2Y 2 in. : 1% in. 104 70 54 149 101 77 208 140 108 298 202 154 312 210 162 447 303 213 416 230 216 596 404 308 520 350 270 745 505 385 624 420 324 894 606 462 728 490 378 1043 707 539 832 550 432 1192 808 616 936 630 483 1341 909 693 1040 700 540 1490 1010 770 The orchardist can only after long years of experience accurately estimate the tonnage trees of a certain age and condition should pro- duce. Reducing the number of peaches on a tree to a given number will not assure the grower that a given size will be reached. Age of trees, soil and moisture conditions, pest control and pruning treat- ment all must be taken into account when utilizing any such method as outlined above. Tear. — Pears are not generally thinned in California. It would seem, however, that where conditions are not entirely ideal that the same considerations should hold with the pear as with the apple. In sections where the fruit which ripens first is shipped, it has been the common experience that the removal of the earlier maturing specimens helps materially in sizing the balance of the crop. This fact is especially true of the Bartlett variety. Very large sizes are undesirable for either shipping or canning and should be dried. * Geo. P. Weldon, A New Idea in Peach Thinning. Bull. No. 5, Feb., 1923, Chaffey Junior College, Ontario, Calif. Circular 258] THINNING DECIDUOUS FRUITS Fig. 3. — Ideal distribution of apricots on small one-year-old fruiting shoot. 8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Plum and Prune. — Plums, especially those intended for "green" shipment, should always be thinned. This is particularly true of the Japanese varieties which normally fruit so abundantly. Plums should ordinarily be separated from one to three inches when fully mature. Varieties belonging to the European species will as a rule size better in dense clusters than will the Japanese sorts. It has not been found economical to do any thinning of plum varieties utilized for prune making other than that accomplished at the annual pruning. There is, however, one possible exception to the statement just made, namely, the Sugar prune. As grown in certain localities, this variety must be thinned to prevent overbearing with resultant breakage of branches — the wood of the Sugar prune being exceedingly brittle — and to offset a decided tendency towards "alter- nate bearing." REASONS FOR THINNING Thinning may be defined as the removal of a certain portion of the fruit crop from the tree in order to : 1. Improve size, color, texture, flavor and individual uniformity. 2. Prevent breakage of trees by better distribution of crop. 3. Reduce disease and insect injury. 4. Maintain vigor of the tree. 5. Secure more regular bearing. 6. Decrease labor of handling crop. It has been the common experience of growers, and many actual tests have shown, that where there is a heavy set of fruit the reduction of the number "of specimens will as a rule result in better average size, color and uniformity and enhance the general attractiveness of the entire crop. Frequently, either on account of improper thinning of fruiting wood at the dormant pruning, or an unusually favorable season for fruit setting trees may be so loaded that much breakage will result uniess the weight of the crop is reduced by thinning. Certain diseases and insects flourish where fruit is closely crowded on the branch. In order to control these pests it is necessary to break up the clusters. The codling moth, which causes wormy apples and pears delights in making its entrance where two fruits touch. Brown rot of the stone fruits is more rapidly spread when the fruit hangs in dense clusters or even touches. Bigelow and Gore* a number of years ago reported the average composition of six varieties of peaches at different stages of growth. The results are given in table 3. * Bigelow, W. D., and Gore, H. C, U. S. Dept. Agr. Bur. Chem., Bull. 97, 1905. Circular 258] THINNING DECIDUOUS FRUITS Fig. 4. — Ideal distribution of apricots when borne on adjacent short spurs. 10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Table 3 Average Composition of Six Varieties of Peaches at Different Stages of Growth Weight of Total Solids in Peach Flesh Stone Kernel Stage of growth Grams Per cent Per cent Per cent June Drop 9.51 64.55 32.50 2.94 Stone Hardened... 16.75 71.54 25.82 2.89 Market-ripe 73.59 92.49 6.86 0.65 Flesh Per cent 14.77 16.97 14.04 Stone Per cent 9.37 27.35 66.94 Kernel Per cent 6.89 7.54 44.78 A study of the foregoing figures discloses several interesting facts. The proportion of total solids in the pits (stone and kernel) of the peach, even at a relatively early stage of development, is compara- tively great. From this, it may probably be safely concluded that the bringing to maturity of a large number of pits is a greater drain upon the tree than the ripening of a smaller number of fruits, which on account of larger size, aggregate the same weight. It is a matter of common knowledge that the size of the pit in the stone fruits varies very little between large and small specimens of the same variety. Inasmuch as the solids in the pits are proportionately much less before the stone hardens it would seem desirable to thin as soon as possible after all natural shedding of superfluous fruits has stopped. These facts would seem to indicate that the vigor of the tree may be maintained by judicious thinning. Many statements have in the past been made to the effect that thinning, by not allowing all spurs to produce any one season, will prevent the alternation of bearing which seems to be the fixed habit of some apple and pear varieties. Carefully checked experiments have shown this assumption to be incorrect, except in so far as a heavy crop one year tends to deplete the resources of the trees for the following season. With the stone fruits a very heavy produc- tion one year is almost always followed by a light crop the next unless utmost care is taken with all orchard operations. Thinning so as to decrease the number of fruits to be handled at harvest time materially reduces picking, grading and packing costs. Lessening the number of fruits does not necessarily mean less tonnage. Thinning is apt to result in considerable increase in size. The can- ners of California have adopted as a slogan, "one two-and-a-half- inch peach weighs as much as two two-inch peaches." Circular 258] THINNING DECIDUOUS FRUITS 11 Fig. 5. — Apricots on opposite sides of spur which should reach good market able size. Note distance from other fruit. 12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION GENERAL CONSIDERATIONS The fruit tree utilizes all its energies in : 1. Wood growth. 2. Fruit and seed production. 3. Fruit- and leaf-bud formation. 4. Manufacture and storage of reserve food materials. Of the different orchard operations which modify these life processes probably no other one has such a profound and immediate influence as pruning. Any pruning system (such as the one which has been so widely recommended by the University of California during the last few years), which results in a greater storage of plant food reserves, the formation of more fruit-spurs and fruit-buds, and consequent modification of new wood growth, is most intimately associated with the question of fruit thinning. In fact, the thinning of the fruit is an integral part of the pruning system. Plant food materials before being assimilated by the tree in carrying on its activities, must be transformed in the leaves into rather complex plant foods and there is some evidence to show that much of this food is utilized near the point of manufacture. Having this fact in mind, it may be argued that more fruit can be brought to a satisfactory maturity on shoots or spurs having a comparatively large leaf area. Field observation offers convincing proof of the correctness of this assumption. Young trees, especially with a vigorous vegetative growth, tend to set fewer fruits than slower growing mature ones, and as a rule need less thinning. There is likewise a distinct varietal difference in the ability of the various sorts to size their crops. The Winesap variety of apple, for example, needs far more thinning than the average. Soil and moisture conditions must be favorable. Thinning cannot be made to atone for poor cultural treatment. Naturally more fruit may be left under ideal conditions than where there is a deficiency in either soil moisture or fertility. Also it should be stated in passing that during the early development of the fruit, should there be a lack of available moisture in the soil, the leaves have the ability of with- drawing water from the fruit. This condition may readily proceed so far that the fruit is unable to recover even with copious irrigations at a later date. Keep the soil moisture at the optimum at all times. Circular 258] THINNING DECIDUOUS FRUITS 13 Much of the thinning work may be accomplished at the annual dormant pruning, at less expense, by the judicious selection and distribution of fruiting wood. With certain fruits, however, even with tha most careful pruning, a portion of the fruit must be removed after the first and second ' ' drops ' ' or the trees will tend to overbear. Attempts at thinning the fruit at pruning time in some localities where there is more or less of a frost hazard, may result, in unfavor- able seasons, in greatly reduced crops. It has already been pointed out that thinning is best done as early in the spring as the habit of natural dropping permits. This will vary for different localities and fruits. The so-called "June drop" in reality in many fruit sections of California takes place in April or May. A satisfactory explanation of this phenomenon has yet to be proved experimentally. The actual operation of thinning is accomplished in various ways in the different fruit districts of the state. The only satisfactory way is removal of the undesirable fruit with the hand, never by shaking or knocking with a pole. By the latter methods fruit-spurs are broken, even spacing of fruit left is impossible, and no discrimination can be made between good and bad specimens. Fruit made unsalable by insect injuries, plant diseases, frost and mechanical malformations can be removed when thinning is done by hand; otherwise many of these specimens will remain on the trees until harvest and the hand- ling of these culls will impose a further burden at a very busy season. STATION PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION BULLETINS No. 253. Irrigation and Soil Conditions in the Sierra Nevada Foothills, California. 261. Melaxuma of the Walnut, "Juglans regia." 262. Citrus Diseases of Florida and Cuba Compared with those of California. 263. Size Grades for Ripe Olives. 268. Growing and Grafting Olive Seedlings. 270. A Comparison of Annual Cropping, Bi- ennial Cropping, and Green Manures on the Yield of Wheat. 273. Preliminary Report on Kearney Vine- yard Experimental Drain. 275. The Cultivation of Belladonna in Cali- fornia. 276. The Pomegranate. 278. Grain Sorghums. 279. Irrigation of Rice in California. 280. Irrigation of Alfalfa in the Sacramento Valley. 283. The Olive Insects of California. 285. The Milk Goat in California. 286. Commercial Fertilizers. 287. Vinegar from Waste Fruits. 294. Bean Culture in California. 297. The Almond in California. 298. Seedless Raisin Grapes. 299. The Use of Lumber on California Farms. 304. A study on the Effects of Freezes on Citrus in California. 308. I. Fumigation with Liquid Hydrocyanic Acid. II. Physical and Chemical Prop- erties of Liquid Hydrocyanic Acid. 310. Plum Pollination. 312. Mariout Barley. 313. Pruning Young Deciduous Fruit Trees. 316. The Kaki or Oriental Persimmon. 317. Selections of Stocks in Citrus Propa- gation. 319. Caprifigs and Caprification. 321. Commercial Production of Grape Syrup. 324. Storage of Perishable Fruit at Freezing Temperatures. No. 325. 328. 331. 332. 334. 335. 336. 337. 339. 340. 341. 342. 343. 344. 345. 346. 347. 348. 349. 350. 351. 352. 353. 354. 355. 356. 357. 358. Rice Irrigation Measurements and Ex- periments in Sacramento Valley, 1914-1919. Prune Growing in California. Phylloxera-Resistant Stocks. Walnut Culture in California. Preliminary Volume Tables for Second- Growth Redwoods. Cocoanut Meal as a Feed for Dairy Cows and Other Livestock. The Preparation of Nicotine Dust as an Insecticide. Some Factors of Dehydrater Efficiency. The Relative Cost of Making Logs from Small and Large Timber. Control of the Pocket Gopher in Cali- fornia. Studies on Irrigation of Citrus Groves. Hog Feeding Experiments. Cheese Pests and Their Control. Cold Storage as an Aid to the Market- ing of Plums. Fertilizer Experiments with Citrus Trees. Almond Pollination. The Control of Red Spiders in Decidu- ous Orchards. Pruning Young Olive Trees. A Study of Sidedraft and Tractor Hitches. Agriculture in Cut-over Redwood Lands. California State Dairy Cow Competition. Further Experiments in Plum Pollina- tion. Bovine Infectious Abortion. Results of Rice Experiments in 1922. The Peach Twig Borer. Observations on Some Rice Weeds in California. A Self-mixing Dusting Machine for Applying Dry Insecticides and Fungicides. Black Measles, Water Berries, and Related Vine Troubles. No. 70. Observations on the Status of Corn Growing in California. 82. The Common Ground Squirrel of Cali- fornia. 87. Alfalfa. 110. Green Manuring in California. 111. The Use of Lime and Gypsum on Cali- fornia Soils. 113. Correspondence Courses in Agriculture. 126. Spraying for the Grape Leaf Hoppei, 136. Melilotus indica as a Green-Manure Crop for California. 127. House Fumigation. 144. Oidium or Powdery Mildew of the Vine. 148. "Lungworms." 151. Feeding and Management of Hogs. 152. Some Observations on the Bulk Hand- ling of Grain in California. 155. Bovine Tuberculosis. 157. Control of the Pear Scab. 159. Agriculture in the Imperial Valley. 160. Lettuce Growing in California. 161. Potatoes in California. 165. Fundamentals of Sugar Beet Culture under California Conditions. 166. The County Farm Bureau. 167. Feeding: Stuffs of Minor Importance. 169. The 1918 Grain Crop. CIRCULARS No. 170. 172. 173. 174. 175. 178. 179. 181. 182. 183. 184. 188. 189. 190. 193. 198. 199. 201. 202. 203. 205. Fertilizing California Soils for the 1918 Crop. Wheat Culture. The Construction of the Wood-Hoop Silo. Farm Drainage Methods. Progress Report on the Marketing and Distribution of Milk. The Packing of Apples in California. Factors of Importance in Producing Milk of Low Bacterial Count. Control of the California Ground Squirrel. Extending the Area of Irrigated Wheat in California for 1918. Infectious Abortion in Cows. A Flock of Sheep on the Farm. Lambing Sheds. Winter Forage Crops. Agriculture Clubs in California. A Study of Farm Labor in California. Syrup from Sweet Sorghum. Onion Growing in California. Helpful Hints to Hog Raisers. County Organizations for Rural Fire Control. Peat as a Manure Substitute. Blackleg. CIRCULARS — Continued No. 206. 208. 209. 210. 212. 214. 215. 217. 218. 219. 224. 225. 228. 230. 232. 233. 234. 235. 236. 237. No. Jack Cheese. 238. Summary of the Annual Reports of the 239. Farm Advisors of California. The Function of the Farm Bureau. 240. Suggestions to the Settler in California. Salvaging Rain-Damaged Prunes. 241. Seed Treatment for the Prevention of Cereal Smuts. 242. Feeding Dairy Cows in California. 244. Methods for Marketing Vegetables in 245. California. 246. Advanced Registry Testing of Dairy Cows. 247. The Present Status of Alkali. 248. Control of the Brown Apricot Scale and the Italian Pear Scale on Decid- 249. uous Fruit Trees. 250. Propagation of Vines. Vineyard Irrigation in Arid Climates. 251. Testing Milk, Cream, and Skim Milk for Butterfat. Harvesting and Handling California 252. Cherries for Eastern Shipment. 253. Artificial Incubation. 254. Winter Injury to Young Walnut Trees during 1921-22. 255. Soil Analysis and Soil and Plant Inter- relations. 256. The Common Hawks and Owls of Cali- 257. fornia from the Standpoint of the 258. Rancher. Directions for the Tanning and Dress- ing of Furs. The Apricot in California. Harvesting and Handling Apricots and Plums for Eastern Shipment. Harvesting and Handling Pears for Eastern Shipment. Harvesting and Handling Peaches for Eastern Shipment. Poultry Feeding. Central Wire Bracing for Fruit Trees. Vine Pruning Systems. Desirable Qualities of California Bar- ley for Export. Colonization and Rural Development. Some Common Errors in Vine Pruning and Their Remedies. Replacing Missing Vines. Measurement of Irrigation Water on the Farm. Recommendations Concerning the Com- mon Diseases and Parasites of Poultry in California. Supports for Vines. Vineyard Plans. The Use of Artificial Light to Increase Winter Egg Production. Leguminous Plants as Organic Fertil- izer in California Agriculture. The Control of Wild Morning Glory. The Small-Seeded Horse Bean. Thinning Deciduous Fruits.