184 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



development. The majority of the trees in the northern part of the 

 State are grown without irrigation, and even in the southern part many 

 groves receive little or no water in addition to the natural rainfall. In 

 seasons of abundant precipitation trees under such conditions get along 

 very well, and it may be said that the walnut is as well adapted as almost 

 any of our ordinary fruit trees to grow without irrigation. There are 

 localities, especially in the northern part of the State, where the crop 

 can be produced successfully and regularly without irrigation. Further- 

 more, there is considerable variation in the moisture-gathering ability of 

 the various kinds of root upon which the tree may be grown so that by 

 proper choice in this respect much may be done to make walnut growing 

 possible on non-irrigated lands. In general, however, it is true of wal- 

 nut growing, as with all other fruit growing in California, that no one 

 factor is as desirable or useful as the possibility of an abundant and 

 regular water supply. Even though the trees may be able to live with- 

 out irrigation and may grow quite well and produce good crops during 

 most seasons, it is still true that even as a matter of insurance against 

 dry years a possible water supply is extremely valuable. Furthermore, 

 there is no spot in the State so favored with natural soil moisture but 

 that with irrigation during the first few years the trees would grow much 

 faster, come into bearing earlier, and produce more satisfactory returns 

 to the owner. 



By this, however, we do not intend to discourage the planting of wal- 

 nuts on non-irrigated lands. The tree is, as we have said, as well adapted 

 to such conditions as any other fruit tree, and, indeed, may be grown 

 more successfully without irrigation than most other fruits if grown 

 upon a proper root. 



As to the particular methods of irrigation practiced with the walnut, 

 it may be said that the tree is most likely to suffer from lack of moisture 

 during the fall and winter after the crop of one season is off and before 

 that of the next year has started to develop. Quite frequently it is the 

 case that the fall rains are late in commencing and the walnut tree is 

 especially likely to suffer from such a condition. Although it may not 

 be in active growth and practically dormant, a lack of moisture at this 

 times dries out the branches, twigs and buds, lowers the vitality of the 

 tree, impairs its vigor for the following season, makes it more susceptible 

 to injury by cold, and is in many other ways injurious and disastrous. 

 For this reason it is coming to be more and more the practice among 

 the best walnut growers to irrigate heavily during the fall and winter, 

 supplementary to the winter rains and creating a storehouse of moisture 

 in the soil for the summer. If the subsoil is thoroughly soaked down 

 and not allowed to become dried out during the fall and winter, the 

 tree, on fairly heavy soil at least, is much less dependent upon sum- 

 mer irrigation than is the case with citrus and many other trees. 



