Manual of American grape-growing

by U. P. Hedrick

Under the assumption, then, that climate, of all factors, is chief in playing providence to the grape, let us examine somewhat critically the relations of climate to grape-growing. When analyzed, the essentials of climate, as it governs grape-growing, are found to be six: first, length of season; second, seasonal sum of heat; third, amount of humidity in summer weather; fourth, dates of spring and autumn frosts; fifth, winter temperature; sixth, air currents.

Length of season.

To reach true perfection, each grape variety has a length of season of its own. With each, if it is grown in too low a latitude, the vine is uninterrupted in growth; its leaves tend to become evergreen; and not infrequently it produces at the same time blossoms, green fruits and ripe fruits. This is, of course, the extreme to which grapes pass in the far South. Again, many northern varieties fail where southern grapes succeed because the fruits pass too rapidly from maturity to decay. On the other hand, very often southern grapes are hardy in vine in the North, but the season is not sufficiently long for the fruit to mature and to acquire sufficient sugar to give them good keeping quality, properly to pass through vinous fermentation, or even to make a good unfermented grape-juice. In the uneven topography of this continent, it is not possible to state the range in latitude in which grapes can be cultivated to advantage, for latitude is often set aside by altitude. Thus, isothermal lines, or lines of equal temperature, are much curved in America and do not at all coincide with the parallels of latitude.

Other factors, of course, than length of season enter into the ripening of grapes. The daily range in temperature, not always dependent on latitude, affects ripening. Cool nights may offset warm days and delay ripening. Certainly rains, fogs and humid air delay maturity. The bottom heat of loose, warm, dry gravelly or stony soils hastens maturity. Sunshine secured by a sunny aspect or shelter hastens maturity.

The seasonal sum of heat.

Successful cultivation of the grape depends on a sufficient amount of heat during the summer season. The theory is that buds of the grape commence to start when the mean daily temperature reaches a certain height, and that the sum of the mean daily temperature must reach a certain amount before grapes ripen. Manifestly, this sum must vary much with different varieties, low for the earliest sorts, high for the latest. There have been many observations as to the temperatures at which buds of the grape start growth, so that it is now known that the temperature varies in accordance with locality and degree of maturity. Roughly speaking, grape buds start at temperatures from 50° to 60° F. The seasonal sum of heat for ripening is probably 1600 to 2400 units. A variety ought not to be planted, therefore, in a region in which the average seasonal sum of heat is not sufficiently high. The seasonal sum of heat can be determined for a locality from data published by the United States Weather Bureau; and by comparing with the sum of heat units in localities where a variety is known to thrive, the grape-grower can determine whether there is sufficient heat for any particular variety.

The grape seldom suffers from hot weather in a grape region. The fruit is sometimes scalded in the full blaze of a hot sun, but the ample foliage of the vine usually furnishes protection against a burning sun. At maturing time, the heat of an unclouded sun, if the air circulates freely, insures a finely finished product. Deep planting helps to offset the harmful influences of warm climates.

Humidity of summer weather.

The grape is very sensitive to moisture conditions, and grows best in regions where the summer rainfall is comparatively light. A damp and cloudy summer brings disaster to the vineyard in several ways; as small growth of vine, small set of fruit, a crop of poor quality, and the development of the several fungous diseases. Although the grape stands drought, a superfluity of moisture in the soil may do little harm, as is shown in irrigated vineyards, but a humid air is fatal to success especially if the air is both warm and wet. Moist weather during the time of maturity is particularly disastrous to the grape, as are frequent fogs. Cold wet weather in blooming time is the grape-grower's vernal bane, since it most effectually prevents the setting of fruit. It may be laid down as a rule that the grape lives by sunlight, warmth and air—it often thrives on the desert's edge. These considerations make it manifest that the monthly and seasonal means of precipitation must be considered in selecting a locality to grow grapes.