BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 177 



much fear of spring frosts in relation to walnuts. About almost any 

 of the towns in this portion of the State there may be found early trees, 

 which to all appearances produce as regularly and abundantly as those 

 in the south. While walnuts are sometimes damaged by frost even in 

 the best of the southern California districts, the tree is not unusually 

 sensitive in this respect and even the earliest of our walnuts come out 

 later than many of our ordinary fruit trees. We very much doubt 

 whether the walnut in many northern California districts would be 

 injured by frost any more than the almond, apricot, peach or grape, 

 and the prospective grower need not absolutely discard all early vari- 

 eties for this one reason alone. It may further be said that during the 

 spring of 1911 when there was much late frost in the State, some of 

 the latest varieties of walnut were badly injured, when earlier varieties 

 in the same locality had reached sufficient development so that they 

 withstood frost with no injury whatever. 



Trees which suffer for moisture during late fall and winter are likely 

 to be severely killed back at temperatures which otherwise would not 

 affect them. See Die Back, page 372. 



Heat . The walnut tree is one which does not take kindly to extreme 

 summer heat, and its commercial culture is limited to a considerable 

 extent by this factor. In the extremely hot desert valleys of the State 

 the tree is not at all well adapted, becoming injured and repeatedly 

 killed back by drying and burning of the leaves and tender shoots. 

 The idea which once prevailed, however, that walnut culture is possi- 

 ble only in the immediate vicinity of the coast has become considerably 

 modified in recent years. Sufficient plantings have been made to indi- 

 cate that the tree flourishes practically all over the State, except in 

 the very hottest and driest portions, and it has been demonstrated that 

 nuts can be produced over a similar territory. A serious drawback, 

 however, is the burning or discoloration of the nuts by the sun even 

 in localities where the tree flourishes well. Here again the matter of 

 choice of varieties becomes important since there is much variation in 

 its susceptibility to sunburn. The most serious form of sunburn takes 

 the form of an actual shriveling and blackening on one side of the 

 green hulls of the immature nuts. This is worst on the nuts which are 

 most exposed to the sun, also on trees which are unthrifty or not in a 

 good vigorous condition. Walnut trees or varieties best adapted to 

 the hottest localities are, therefore, those which have the most abundant 

 foliage, in which the nuts are borne well down among and under the 

 leaves, and which are naturally of thrifty, vigorous growth. Burning 

 by the sun is also less severe upon trees standing in good soil with 

 plenty of moisture than upon those in light dry soil. A less prominent 

 effect of sunburn consists in a burning or discoloration of the thin pel- 



5231 



