Part 1
Part 2
Part 3
Part 4
Part 5
Grape Index
Chapter 5. Site Selection and Weather Modification
The two most important characteristics of a grapevine site are the climate and the soil. Temperature is usually the most important climatic factor.


American Varieties. The winter minimum is especially important in areas such as the middle western and eastern parts of the United States. In New York State length of growing season (from bud burst to harvest) is important, and a growing season of at least 170 days or lenger is optimum for the fruit to ripen properly (Shaulis and Dethier, 1970). In New York, temperatures of -180F (-280C) or below usually result in commercially important damage to Concord buds or trunks (Shaulis et al., 1964, 1973).

Concord grapes withstand humid summers and cold winters better than pure vinifera. They grow better in regions of moderate summer humidity than in the very dry climate of California's interior valleys. Grapes usually grow poorly in a hot tropical climate with high humidity.

Vinifera Varieties. Most vinifera grapes require long, warm to hot dry summers and cool winters for best development. They are not adapted to humid summers because they are susceptible to certain fungus diseases that flourish under such conditions. They cannot withstand winter cold below -80F (-220C) to -150F (-260C) without protection. Frosts that occur after vine growth starts in the spring can kill most of the fruitful shoots and reduce the crop.

Rain is desirable in winter although irrigation can make up for deficiencies. When rains occur early in the growing season it is difficult to control disease, although vine growth is not otherwise hindered. Poor berry set may occur as a result of rains, cold, or cloudy

weather during the blooming period. During the ripening and harvesting, rains can result in severe rotting of fruit. Grapes can tolerate a high humidity in cool regions than in warmer regions. For the sundrying of grapes for raisins a month of clear, warm, rainless weather after the grapes ripen is necessary.

Vinifera grapes usually require a winter rest period of about 2 months, with an average daily mean temperature below 500F (100C), and some freezing but no temperatures below 100F (-120C). Shoot growth in spring begins soon after the daily mean temperature reaches 500F (100C).
Heat Summation. For proper vine development and maturation, most varieties require a daily mean temperature at least 650F (180C); some require temperatures of 700F (240C) to 850F (290C). the time required for grapes to reach maturity is determined mainly by the total amount of heat received, which can be expressed in terms of temperature time values called degree days or heat units. The effective heat summation for a given location largely determines the length of time from bloom to ripening for a given variety. Heat units are usually determined for the months from April through October, but sometimes are calculated from full bloom.

The number of heat units required for a growing area can be estimated as follows (Jacob, 1950):

Determine the average daily temperature by averaging the lowest and highest temperature of each day. Subtract 500F form the mean daily temperature and add up the mean daily temperature (-500F per day) for the months from April through October to determine the degree days for the season. It is easier to use the mean monthly weather data. Multiply this figure (-500F) by the number of days in the month, which gives the degree days for that month. For example, if 59,40F is the mean temperature at Fresno of April, (59.40-500F) x 30 days = 282 degree days for the month of April.

Early grapes require about 1600 degree days to mature, and late ones at least 3500 degree days (Jacob, 1950). If the temperature measurements are begun at full bloom, Thompson Seedless will be mature for table use (180 Brix) when the heat summation above 500F reaches about 2000 degree days. This variety will be ripe for drying of raisins when the summation reaches 3000. Similarly Tokay will be ripe for table use at about 2000, and Emperor at about 3300 degree days. Temperature, especially during the ripening period, greatly influences the sugar and / or acid content of grapes and thus affects their quality for various uses. Climatic Regions for Grapes. Based on the summation of heat as degree days above 500F for the period from April 1 to October 31, grape producing areas in California fit into one of five temperature groups or regions (Jacob, 1950). These regions with representative locations are listed below :

1.Cool regions less than 2500 degree days of heat. Napa, Hollister, Mission San Jose, Saratoga, Bonny Doon, Guerneville, Santa Rosa, Sonoma, Lompoc, Watsonville, Campbell, Aptos, Santa Cruz, Gonzales, Hayward, Peachland, and Santa Maria.

2.Moderately cool regions-2501 to 3000 degree days of heat. Rutherford, St. Helena, Glen Ellen, Healdsburg, San Jose, Los Gatos, Santa Barbara, Gilroy, Sebastapol, San Luis Obispo, Soledad, San Jose, Grass Valley, Napa, Sonoma, and Placerville.

3.Warm regions 3001 to 3500 degree days of heat. Calistoga, Hopland, Cloverdale, Livermore, Alpine (in San Diego County), Ukiah, Paso Robles, Pinnacles, Cuyama, Santa Ana, King City, St. Helena, Healdsburg, Clear Lake Park, Jamestwon, Camino, Mokelumne Hill, Potter Valley, Ramona, Mandeville Island, and Lodi.

4.Moderately hot regions 3501 to 4000 degree days of heat. Davis, Manteca, Modesto, Ontario, Maritnez, Escondido, Upland, Suisun, Colfax, Turlock, Linden, Vacaville, Sacramento, Clarksburg, Sonora, San Miguel, Fontana, Pomona, Stockton, and Auburn.

5.Hot regions more than 4000 degree days of heat. Madera, Fresno, Delano, Visalia, Bakersfield, Chico, Red Bluff, Redding, Ojai, Oakdale, Brentwood, Antioch, Woodlland, Reedley.

The best table wines are produced in the cool and moderately cool regions, the best natural sweet wines in the warm regions, and the best dessert wines and the commercial table and raisin grapes come from the moderately hot and hot regions.

The five climatic grape growing regions in California are shown in figure. 5-1.

The heat summation method described above is the most widely used technique for forecasting the maturation data for various varieties. In Michigan, a modified heat unit method has been successful for predicting the harvest date of the Concord grape (Brink, 1974). Heat units were counted from April 1 at a base temperature of 500F (100C). Optimum maturity was reached 85 days after the date when 1000 units were accumulated for the season. Maturation was largely determined by temperatures during the first 100 days of the season.

Soils for Grapes

Grapes grow fairly well in many different types of soil. They are commercially grown throughout the world in almost every type of soil from gravely sand to clay loam, shallow to very deep soil, and in soils ranging from high to low in fertility. It is preferable, however, to avoid very heavy clays, very shallow soil, poorly drained soil, and soil with relatively high concentrations of alkali salts, boron, and other toxic materials.

The deeper and more fertile soils usually produce the heaviest crops and are therefore preferred for raisins, standard wine grapes, and table grapes such as Tokay and Thompson Seedless. Certain varieties, such as Malaga and Emperor, attain quality on soils of limited depth.

Aids for Vineyard Site Selection

In selecting a site for grape growing one should consult the local County Farm Advisor and successful growers in the area. Information on climatic conditions, temperature, frost, wind, topography, rainfall, soil depth and fertility, availability of water, labor sources, and distance to market is valuable. It is safest to plant grapes in a new area on a trial basis only, and also to plant a variety that has already been grown successfully in an area. Although site selection refers mainly to choosing new vineyard locations, site selection and rejection is a continuous problem for the life of the vineyard. A vineyard site should be constantly on trial (Shaulis and Dether, 1970).

Techniques Used to Modify The Weather

Frost protection during the spring months after shoot growth begins is desirable in many areas where air temperatures in grape vineyards drop down to 310F(-0.50C).

Frost Protection with Overhead Sprinklers

Mechanism. Water releases heat as it freezes. When it is sprinkled on vines, heat is produced in the brief time that droplets are freezing on the green shoot and leaf surfaces. The release of heat maintains the temperature of vine parts covered by the ice water mixture to near 320F(00C) even when the air temperature and plant parts not in reach of sprinklers is as low as 150F(-90C). To prevent freezing damage the vines must be kept wet constantly. Sprinkler installations provide sufficient water application rates to maintain plants at a minimum of 310F(0.50C), a safe temperature for any developmental stage of the vine.

The ice film itself provides almost no protection because it is a good heat conductor. This explains why heat produced at the surface is readily conducted to the plant.

Precipitation rates required for protection under frost conditions have been fairly well determined (Marsh and Meyer, 1973). Water must be sprinkled on all parts of the vineyard throughout the frost period. However, rates vary with the variety, stage of cluster growth at time of frost, wind conditions and relative humidity during the frost period, and the type of frost.

The precipitation rate should be kept at the minimum required to provide frost protection to avoid waterlogging of the soil. Sprinklers must be operated continuously during frost periods to maintain a water ice interface. A precipitation rate of 0.11 to 0.12 in (2.8-3 mm) per hour or more is usually sufficient for over vine sprinklers. The amounts of water required in gallons per minute per acre (gpm/acre) for various precipitation rates are presented in Table 5-1 (Meyer and Marsh, 1972).

When to Begin Sprinkling. A drop temperature due to evaporative cooling usually occurs when sprinklers are first turned on. When a frost is expected, the usual practice is to start the sprinklers when temperature drops to 340F(10C), thus providing a margin of safety (Marsh and Meyer, 1973). Since grapevines at the bud break stage of development are more sensitive to frost when wet than when dry, a starting temperature for sprinklers lower than 340F(10C) should not be risked.

When very dry atmospheric conditions prevail (low dewpoint temperature), it may be necessary to start the sprinklers at a higher temperature than 340F(10C) to increase the relative humidity before the frost occurs. The dewpoint is the temperature at which the relative humidity reaches 100 percent. Table 5-2 shows the proper temperatures to begin sprinkling at various predicted dewpoints to prevent evaporative cooling below 310F(-0.50C).

A larger amount of water must be immediately available for frost protection as compared to that required for irrigation, since during a freeze all parts of the vineyard must be sprinkled at the same time. For example, a precipitation rate of 0.12 in (3.0 mm) per hour to protect 40 acres (16.2 hectares) of vines requires a sprinkler system with a total capacity of 2160 gpm (8176 liters).
An over the vine continuous sprinkling at a rate of 5 gpm (18.9 liters pm) provides protection of 60-80F (30-40C).

Sprinklers can be turned off when the air temperature outside the treated area has risen to 320F (00C), if there is no wind. If it is windy, it is best to wait until the air temperature has risen to 340F (10C), although it is not necessary to wait until all the ice has melted.

Cultural Practice to Prevent Frost Damage

Cultivation and Training. Table 5-3 shows data collected during radiant frosts in the central San Joaquin Valley concerning expected protection for vines under different soil surface conditions (Swanson et al., 1974).

For warmest temperature the soil must be bare. Trash, weeds, and cover crops are detrimental because they insulate the soil surface so it cannot absorb or release heat readily. Any ground cover that is kept should be mowed as low as possible.

Proper training may provide some protection against frost. The air is cooled at the ground surface and gradually builds up a cold layer, thus the lower shoots are usually frozen first. This advantage is utilized in the Orange River Valley of South Africa, where Thompson Seedless vines are trained to a high trellis with a horizontal cross arm at the top.

A compact, bare soil can be made by turning under cover crops or heavy weed growth. However, a loosened soil is as hazardous in frosts as soil with vegetation.

Later or Double Pruning. Late pruning after the buds on the apical parts of the canes have started to grow will delay the leafing out of the buds on the retained spurs. This delay may vary from 7 to 10 days depending on the temperature (Schultz, 1962): when it is very warm the delay will be short, in cold weather the delay will be longer. Shoots on the higher parts of the vines should not be permitted to grow more than 3 or 4 in. (7.6 or 10.1 cm) before pruning, lest the vines be weakened (Fig. 5-2).

To delay pruning in large vineyards until the apical buds have grown 3 or 4 in. (7.6 or 10.1 cm), however, may result in difficulties when labor is scarce, especially in seasons when the shoots grow rapidly. Thus if one desires to delay the growth of the buds on the spurs, double pruning may be used. During the winter, all canes except those to be used for spurs should be removed and the retained canes should be pruned to rods or half long canes 15 to 20 in. (38.1-50.8 cm) long. In spring, after the apical buds on these half long canes have produced shoots 3 to 4 in. long , the canes should be cut back to spurs one to four buds long depending on their diameter. Double pruning will delay leafing out of the lower buds just as effectively as late pruning and it facilitates work in the dormant season, such as brush disposal and cultivation. This method also avoids the possibility of a labor crisis or of working under adverse weather conditions at the critical stage in growth, since cutting back of half long canes can be done very rapidly. Late pruning or double pruning is only practical for small acreages.

Enhancement of Air Flow

Elimination of cold air pockets is another technique to prevent local spring frosts (Schultz et al., 1962). On clear and calm nights, the coldest air settles near the ground. The influence of the terrain often brings this cold air into motion down the slopes and out of the valleys. Cold air drainage flow is very beneficial because it always draws warm air down from overhead. Wherever the cold air flow is hindered by embankments, a row of brush, low branched trees, or barns, frost danger areas will form. Vegetation and buildings that stop the air flow and dam up the cold air should be removed.

Wind Machines

During the day the sun warms the earth?s surface soil and vines which in turn warms the air in contact with it. A layer of warm air is formed over the vineyard as the air at ground level warms and rises.

At night the process is reversed and the soil surface loses heat through outward radiation to the sky. The air in contact with the earth's surface is cooled and, since cold air is heavier than warm air, it remains at ground level or flows into low areas. The nighttime condition is called inversion.

Frosts that develop under these conditions are called radiation frosts, and are characterized by colder air below and a warm air layer ranging from 20 to 100 and more feet (6.1-30.5 m) above the ground. Both wind machines and heaters work better when warm air or ceiling is near the ground, a condition called strong inversion by meteorologists.

The main purpose of wind machines is to mix the upper warm air layer with the cold air layer near the vines. The difference between the air temperature at 5 ft and 40 or 50 ft (12.2 or 15.2 m) above the ground is a common method used measure the strength of an inversion. Well designed wind machines require about 8 to 10 hp per acre. This is almost equivalent to two tower mounted dual machines or four movable or ground level machines for 30 to 40 acres (12.1-16.2 hectares). The two kinds of wind machines on the market are the permanently installed tower mounted type and the movable type mounted on a short tower.


Orchard heaters provide heat by direct radiation and by convection. Hot stack heaters, like the return stack heater, give out 25 to 30 percent radiant heat (Fig. 5-3), which travels directly from the heater to the vine or any other object in the vineyard.

The hot air and gases created by the heater tend to rise out of the vineyard. If there is a strong inversion creating low ceiling of warm air above the vineyard, the heated air rises less. In this case, all types of heaters burning at equivalent rates work equally well. When there is little effective ceiling, the hot stack heaters provide greater protection because their radiant heat output is not affected by the small inversion.

An installation of 40 to 50 heaters per acre (99-124 per hectare) is recommended for coastal grape vine districts, although they would seldom have to burn all at the same time. This provides 50F(30C) protection or more depending on the burning rate and type of inversion. Twenty five heaters would give 30F (20C) protection or more, depending on the same variables. At the starting time which for economic reasons might not be above 310F (minus 10C) air temperature, it is necessary to light only one third of the total number of heaters although additional heaters should be lit if the temperature drops. Distribution of the burning heaters should be even except along the edges, where a greater concentration is needed for border heating. This is especially important at borders where the nightly drainage flow of cold air occurs or at those exposed to adjacent grassland or alfalfa fields (Schultz et al., 1962).

Growers who use heaters should carefully abide by the regulations of the Air Pollution control District and do everything possible to hold smoke nuisance to an absolute minimum.

Wind machines and heaters working together provide more frost protection than either used alone. This increased efficiency results from mixing the hot air and gases generated by the heaters directly with the air in the vineyard. The most beneficial effect from the heater and wind machine combination occurs on nights when there is only small inversion (little warm air above the vineyard for the wind machine to pull down, and no ceiling to stop the upward progress of the warm air generated by the heaters). For example, in the North Coastal region of California 20 to 25 heaters per acre (49-62 per hectare) with a wind machine provide up to 40-50F (20-30C) protection (Burlingame et al., 1971).


An inversion layer of upper warm air is needed by helicopters for beneficial effects similar to that for wind machines; with helicopters, however, the operator has the advantage of selecting the flight pattern that will give maximum heat at the vine level. Depending on the inversion, one helicopter can protect from 40 to 100 acres (16-40 hectares) (Swanson et al., 1974). The helicopter should carry a rapid response thermometer to determine temperature in the upper warm layers. It is also a good idea to monitor the helicopter from the ground so the pilot can be guided to colder spots in the vineyard.

Crop duster planes can also be useful for mixing upper warm air.

Fog Machines

Fog machines can produce a blanket of fog over the vines that reduces heat loss by radiation. Fog can keep air temperatures 1-20F (0.6 to 1.10C) higher and leaf temperatures 2-30F (1.1 to 1.70C) higher (Swanson et al., 1974). The main problem with using fog is to keep it over the vineyard and not let it drift away.

Heat Blocks

Blocks such as Tree Heat can be used; at least 100 or 200 blocks per acre (247 or 494 per hectare) should be used. They can also be used to supplement other heating devices in more severe frosts.

Heat blocks are also used in New York State. These petroleum blocks will burn for approximately 5 hours, and 100 blocks per acre (247 per hectare) will raise the temperature about 20F (10C).


In Japan, Holland, and other northern European countries where vines cannot be grown out of doors, high quality table grapes are grown in greenhouses. Both glass and plastic covers are used in these greenhouses, which are usually steam heated in winter.

Covers of polyethylene strips or other material over the vines can give 30F (20C) protection against frost (Schultz, 1961), and this amount can be increased by extending the plastic to the ground at night.

In some countries such as Israel, Italy, and South Africa, there has been experimental and small scale use of plastic to hasten ripening of grapes. When plastic is placed over the vines before bud break, shoot growth is much more rapid and fruit ripens a couple of weeks earlier than on the control. However, the plastic must be removed before the weather gets too hot, usually soon after flowering, to prevent burning of the shoots.

Labrusca Vines in The Eastern United States

In New York the winter minimum temperature often falls low enough (-120F to –180F) (-24 to – 280C) to kill grape tissues or vines. High position of canes can decrease injury from spring freeze, and there is usually less injury to shoots above 4 ft (1.2 m) from the soil than to those at height of less than 3 ft. (0.91 m). In May 1963 the soot kill was about 30 percent and 60 percent, respectively (Shaulis et al. 1964).

A major portion of injury from winter freeze in New York vineyards is to the trunk at heights of 1 to 3 ft (0.30-0.91 m) above the soil. Use of double trunks from the soil level can reduce loss of vines from freezing injury in winter. Loss may be reduced by 80 percent (Shaulis et al., 1964).

A vigorous vine with good foliage cover until the crop is harvested is essential to reduce damage from a preharvest freeze. Exterior foliage protects the partially exposed foliage beneath. Foliage with potassium deficiency symptoms is more susceptible to freezes than is healthy foliage.

Treatment of Frost Damaged Vines

After a freeze, damage to the grapevines becomes apparent after a few hours although the full extent of cluster damage is often not clear until after fruit set (Kasimatis and Kissler, 1974; Swanson et al., 1974). In most vineyards the best treatment may be to do nothing, but some recovery may obtained by shoot removal treatments.

After vines have been frozen some recovery of crop might be obtained from secondary or tertiary buds or from dormant and latent buds. If the shoots produced from the primary bud freeze, partial recovery can sometimes be obtained by breaking out shoots at the base, if the secondary and / or tertiary buds are fruitful. Usually however, shoot break out treatments fail to enhance growth from secondary growing points out treatments fail to enhance growth from secondary growing points (Kasimatis, 1974). Secondary and tertiary buds usually develop in about two weeks in cases where growth does occur. Shoot removal should be done immediately after a freeze, and best results occur when shoots are less than 6 in. (15.2 cm) long when frozen. When longer shoots are broken out the secondary and tertiary growing points are also removed. After a frost one should break off only those shoots on which clusters have frozen, omitting those shoots that have been killed back to the base. No benefit can be obtained by breaking off completely frozen shoots.

Since Thompson Seedless has no fruitful secondary or tertiary buds, no treatment is recommended. If the crop is lost by freezing, canes may be cut back to spurs to promote shoot growth and better canes for the following year. No treatment is beneficial on spur pruned vines with nonfruitful secondary or tertiary growing points such as in the Emperor variety. However, in spur pruned varieties with fruitful secondary and tertiary buds, such as Cardinal, Riber, and most wine varieties, shoot removal may sometimes result in greater yields.

The growth of dormant and latent buds present at the bases of spurs and canes is of little benefit in cane pruned Thompson Seedless but can sometimes increase yields in spur pruned vineyards. Suckers or water sprouts from other parts of the vine can produce some fruit especially in wine varieties. If shoots are pruned off, only lateral buds develop near the base of the shoots that are not fruitful.

Heat Suppression With Overhead Sprinklers

From June to September the San Joaquin Valley of California has high daytime temperatures. Maximum daily air temperatures often consistently exceed 900F (320C) to 950F (350C) for extended periods, and the relative humidity drops below 25 percent during the day. Temperature in excess of 1120F (440C) have frequently been recorded in the San Joaquin Valley.

Air temperatures ranging from 770F (250C) to 860F (300C) are considered optimum for growth of most temperate zone crops. Leaf temperatures higher than 860F (300C) can cause internal water deficiencies, sunburn damage, dehydration of fruit, and reduced growth rates. High temperatures can also decrease the acid content and increase the pH levels of wine grapes, inhibit color formation and increase raisining (sunburning) of table grapes. Low atmospheric relative humidity places additional water stress on the vines (Aljbury et al., 1973).

Preventing injury by evaporative cooling can be obtained by intermittent operation of sprinklers (Fig. 5-4), which can be turned on for 3 minutes, followed by an off period of 15 minutes or more. Decreases in leaf temperature can be as much as 250F (140F) during sprinkling at 15-20 percent relative humidity. Little rise in air temperature occurs between water applications.

Increases in relative humidity accompany cooling. Under conditions of 15-20 percent relative humidity, the humidity can be increased by approximately 10 percent. Precipitation rates during sprinkler operation need be only 0.08-0.10 in (2.0-2.5 mm) per hour or less with sprinkler spacings similar to those used in irrigation.

Simple on off switch panels for pumps with manual lateral line valves are commonly used.

Water Quality

Water quality may limit the usefulness of this technique for cooling vines. ?Burning? of foliage may occur with waters that contain in excess of 7.5 meq/liter of total dissolved salts or 3 meq/liter or sodium. Waters containing 1.5 mg/liter of bicarbonates can cause deposits to form on the leaves and fruit (Aljibury et al., 1973).

Time of Day

Air and plant temperatures were reduced no matter what time of day sprinkling was done. Air temperatures were reduced 7-100F (4-80C) during the peak heat period of the day, and humidity was increased 10 to 20 percent. Temperatures of leaves and exposed berries were usually decreased about 7-200F (4-110C), and that of shaded berries were decreased about 7-120F (4-6.70C) (Aljibury, 1973). Fresh weight of berries of cooled fruit is increased over that of uncooled fruit (Kliewer and Schultz, 1973; Aljibury et al., 1975).

Bunch rot was increased by sprinkler cooling but avoidance of night time spraying and fungicide treatments might lessen these effects. Vine growth was stimulated by cooling, and vines that are cooled are greener, more lush, and more vigorous. This result may be partially due to an increase in soil moisture.

In a vineyard near Reedley, California, cooling of Chardonnay, Semillon, and Chenin blanc fruit by sprinkling sequentially from véraison to harvest reduced the levels of soluble solids and delayed fruit maturation by two or three weeks (Aljibury et al., 1975). However, sprinkling from bloom to shortly after véraison had little effect on soluble solids of Cardinal, Carignane, and White Riesling (Kliewer and Schultz, 1973).

Sprinkling increased coloration of Cardinal fruit (Kliewer and Schultz, 1973). The value of overhead sprinklers to cool commercial vineyards for improvement of quality is not yet known.

Wine Quality

Wine produced from sprinkler grapes are rather unsatisfactory and no better than wine made from the same varieties unsprinkled. Whether heat suppression can result in improved wine quality is still unknown (Aljibury et al., 1973).

Continue to Chapter 6