Frosts in Spring and Winter.
—One of the frequent effects of spring showers is spring frosts. They are always injurious to the tender Muscat vine, and if occurring more than once during the same spring may entirely ruin the crop. In California such frosts occur sometimes in April, and observations inform us that they are most to be feared between the tenth and fifteenth of that month, while sometimes they come even later. The young buds are then either opening or fully developed into shoots, which the lightest frost will blacken and cause to dry up. Where the vines are irrigated and strong, one such frost may not ruin the crop, as new buds will start out in place of the old ones destroyed, and new shoots and new blossoms will come out. I have seen as many as three such crops of shoots develop from the winter buds, but each succeeding crop of such shoots is weaker than the preceding one, and bears less and later grapes. Happily, these frosts occur but very seldom in the Central and Southern California raisin districts, and during fifteen years of observation I have seen only three such frost years in which the branches were partially injured. In neither of these seasons was the crop materially injured as to quantity, the principal effect of the frost being a retarding of the crop for a week or more. Smoking of vineyards can only be successfully carried out in small valleys sheltered from heavy winds, but on the open plains such smoking is accompanied with difficulties, and its effect is uncertain. The growing of a limited number of windbreaks has in the Fresno district no doubt modified the climate, and made spring frosts rarer and less to be feared.
While the spring frosts are injurious to the grapes, winter frosts are on the other hand most beneficial, if not necessary to a continued raisin cult. The raisin grape must have a season of recuperation, and winter frost is the only climatic phenomenon which, without injury to the vine, can procure it that rest which is so necessary for all deciduous trees, by nature destined to enjoy alternate periods of growth and sleep. The absence of frost causes the sap of the vine to circulate more or less in the wood, and the vine never ceases to grow. This is one of the reasons why our deciduous vines do not succeed well in the tropics, where there are no cold seasons to cause the leaves to fall and the sap to become dormant. In the tropics, therefore, our deciduous vines keep on growing, set little or no fruit, and prove unprofitable. This phenomenon is shared there with other trees, and peaches, pears and apples act in the same manner. They all appear to need the rest afforded them by the winter frosts. It is also a question of very great importance, whether the continued and unnatural activity of the vine, at a time when it should be dormant, does not invite diseases of various kinds, which find the exhausted vines unfit to withstand their ravages. It may be possible that mal nero, the vine plague and other similar and as yet insufficiently understood diseases, are especially destructive to vines growing in frost-free climates, while in colder climates they make but little headway, the vines as it were being protected by the heavy frosts, which either kill the enemies of the vine or enable the latter to gather the necessary strength to battle with them through the growing season. There can be little doubt that at present the healthiest vineyards are those growing in countries where winter frosts are severe, but on the other hand we know that grapevines have been growing for ages in temperate climates, where the frosts, even if not entirely unknown, are still of very rare occurrence.
Summer Temperature.
—The temperature in summer time must be sufficient to properly ripen the grapes, but must not be so great as to injure them either while they hang on the vines, or while they are exposed to dry on the trays. The average heat required to do the work of maturing is not exactly known, but it is certain that a very high degree is not absolutely needed to produce sweet grapes. As far as our experience goes, it seems that cool weather, with the average temperature of ninety degrees Fahrenheit, would be most beneficial in the fore part of the season, while when the grapes begin to ripen a greater heat is desirable. It is not the warmest countries nor the warmest seasons which produce the earliest grapes. Thus while the season of 1888 was in California unusually cool, with the thermometer seldom reaching one hundred degrees in June and July, the raisin season was nevertheless a very early one, and much earlier than seasons remarkable for their early high temperature. In Malaga and Smyrna, the heat seldom reaches one hundred degrees, and the grapes ripen several weeks earlier than in Fresno, where the summer temperature averages ten degrees higher. In Fresno, on the contrary, the season is earlier than in Southern California, where probably the seabreezes retard the ripening of the grapes. A temperature of over one hundred and five degrees proves injurious to unprotected or exposed grapes in the central region of California, but in San Diego county several degrees less is sufficient to scald the grapes or give them a cooked flavor if they are already exposed on the trays to dry. It is certain that with an average highest temperature of ninety degrees, the grapes develop better and become larger and sweeter than where the heat is excessive and reaches one hundred and ten degrees or more.[6]
[6] Whenever the temperature is referred to it means the degree of heat (Fahrenheit) in the shade, and not in the sun except when so expressly stated.
The time when the greatest temperature occurs is of practical importance. Excessive heat can be tolerated by grapes protected by leaves and branches or otherwise sheltered, but if it happens at a time when the bunches are exposed to dry on the trays, the injury to the berries will be great or even ruinous. Early localities are especially apt to suffer in this way, and it is well to experiment before too much confidence is placed in very early regions. To such places, however, there remains the possibility of curing the raisins in partial shade, as is done in Chile, thus producing raisins of an entirely different type from the Malaga or California product.
Winds, Injurious or Beneficial.
—In the California raisin districts hot, electrical winds are much feared in the months of June and July, or before the grapes have begun to ripen. In the San Joaquin valley these winds come from the northwest and sweep down over the vines, often scorching the leaves and frequently drying the berries on the exposed side. In the course of a few days the berries dry up entirely, and the whole bunch is lost. These drying winds are not peculiar to any certain part of California, but occur from time to time in every raisin district on the coast, in the south as well as in the center, on the coast as well as inland. The remedy is to grow the vines low and to keep the berries well covered. The planting of windbreaks will also modify these winds, and in places where they formerly were common they have now entirely ceased or become so modified that they cause no injury to the grapes, but prove even beneficial on account of the quicker and better maturing of the fruit. In certain districts, especially in San Bernardino and in Orange, some very drying winds are experienced late in the season,—in September and October. For the raisin-growers these winds are a blessing. They quickly dry the exposed grapes, which have been retarded in drying, or perhaps even injured by a previous shower of rain or by continued heavy dews. These winds are undoubtedly desert winds, and similar to the Terral of Malaga, which, sweeping over the inland plains of La Mancha, reach the coast vineyards and quickly mature the grapes. In Malaga there is also a moist southern wind, the Levante, which retards the ripening and the curing of the grapes, and which must be considered as our southwesterly fall winds, which, saturated with moisture, swell the overdried raisins as well as prevent the yet green grapes from drying. They are precursors of the rainy season, and warn the grower to prepare his dryer if he possess one. In Greece and Smyrna such hot or moist winds are also known and feared, and cause at times much damage in one way or another. We might here also mention the cold “northers” which are common in the California raisin districts in springtime, and which sometimes both cause the young shoots to break off from the old wood and make it easy for the mildew to attack the flowers or the young berries of the vines. Against these northwest winds we have two remedies, summer pruning and sulphuring, which, if applied in time, are both quite effective.