Influence of air and soil temperature on vineyard productivity

Heat supply: how the sum of active temperatures determines the harvest

Temperature regime of air and soil directly affects vine development, berry ripening, and the overall productivity of a vineyard. When assessing the heat supply of a site, agronomists distinguish between positive and negative temperature indicators. Among positive temperatures, the sum of active temperatures during the growing season, the temperature of the warmest month, and the daily amplitude of fluctuations are of key importance.

• SAT for an annual harvest — at least 3100 °C
• Temperature of the warmest month for sugar accumulation — no lower than 16 °C
• Frost-free period for commercial plantings — at least 150 days
• Threshold for green shoot death in spring — from −1 to −2 °C
• Area of covered viticulture — about 500 thousand hectares

The sum of active temperatures (SAT) above 10 °C determines the fundamental feasibility of industrial grape cultivation in a specific zone and the farm's specialization. The lower threshold for cost-effective production is considered to be an SAT of 2500 °C, at which only ultra-early and early cultivars can be cultivated. In the hottest southern zones, this indicator reaches maximum values of 4500–5000 °C.

For stable harvest yields, higher heat supply indicators are required. To ensure 90% berry ripening, the sum of active temperatures must be at least 2800 °C, and for annual ripening — no less than 3100 °C. These critical marks form the basis for planning the varietal composition and vineyard layout.

In addition to SAT, heat intensity and daily temperature amplitude during the ripening phase have a strong influence on sugar accumulation and juice quality. The average monthly air temperature during this period should be no lower than 16 °C, especially in the European viticulture zone, where heat is often insufficient. For the production of dried grapes, vacuum must, grape syrup, as well as dessert and fortified vintage wines, the temperature requirements during the ripening phase are even higher.

Negative temperatures: choice of cover technology and protection from frost

Winter frosts and spring/autumn frosts are the main risk factors capable of completely destroying the harvest and damaging perennial wood. To protect the plantings, an agronomist needs to analyze the average absolute annual minimum air and soil temperatures, their frequency, frost dates, and the total duration of the frost-free period. Based on this data, the optimal method of crop management for a specific site is chosen.

The average absolute minimum temperature directly dictates the technology for protecting the vine from freezing. Since grapes are sensitive to severe frosts, in many regions, bushes are covered with soil for the winter. The total area of such covered vineyards is about 500 thousand hectares.

In regions with mild winters, where critical frosts occur only 2–3 times every 10 years, it is advisable to use a conditionally covered crop method. This same method is used in microzones with unstable weather, when strong cold sets in abruptly after winter thaws. With this technology, one part of the bush is covered with soil, while the other is left to winter uncovered.

Frosts during the growing season pose a serious threat to the plant's green organs. For grapes to fully complete their development cycle, the duration of the frost-free period in an industrial cultivation zone must be at least 150 days.

Soil temperature directly affects the vineyard: it determines when sap flow begins, how shoots grow, and whether the roots will survive the winter. Root system of different grape species wakes up at different temperatures. If the soil has not warmed up to the required level, the plant will simply not begin the growing season.

For active development of the root system, the soil must be warm. In cold soil, the roots of European cultivars stop growing, which threatens the development of the bush. At the same time, rootstocks of American origin are capable of withstanding even negative temperatures in the root zone.

• Optimal soil temperature for root growth — 28–32 °C
• Critical temperature for the roots of European cultivars — 6 °C
• Limit temperature for the roots of American cultivars — -10 °C

Soil moisture and air humidity: balance between drought and disease

The second most important productivity factor is water supply. In areas with insufficient rainfall, it is regulated by means of irrigation. The main reference point for monitoring the water regime is the field capacity (FC) of the soil.

The grape's need for moisture changes throughout the season. It is necessary to maintain a high level of soil moisture during periods of active vegetative growth and fruit formation. At this time, a moisture deficit is unacceptable.

The FC index should be higher during the following phases:

• bud burst and shoot growth;
• berry growth.

During the berry ripening and shoot maturation period, the soil moisture level should be lower. This is necessary for the timely cessation of vine growth and the accumulation of sugars.

Relative humidity of the air determines the intensity of transpiration and the overall condition of the bush. The optimal index for a vineyard lies within 60–80%. Any sharp deviations from this norm during the growing season create serious problems for the harvest.