Site Selection

The key to an economically viable vineyard lies in the careful choice of the vineyard site. Good site selection increases viability through the growing of premium fruit and the decreased risk of failed crops. Varieties should be chosen carefully to ensure that they will achieve full maturity at harvest at the desired quality level at an economically sustainable crop load. Major considerations should be climate, soil, topography and water.

Climate

In contrast to the coastal area of British Columbia, the climate of the Okanagan and Similkameen valleys is very dry. It characteristically has hot, dry summers and crisp, largely overcast, snow-free winters with air temperatures below freezing for about ten weeks. The region lies in the rain shadow of the Coast Mountains on which eastward moving moist Pacific air masses rise and lose their moisture. Cold arctic air occasionally intrudes into the valleys during the winter allowing temperatures to drop to -25°C. The dry, hot summers lead to soil mois-ture deficits during the growing season and irriga-tion is required for the production of most agricul-tural crops. Frost during the winter can penetrate soils to a depth of about 50 cm.

Minimum Site

  • Requirements for the Planting of Vitis vinifera
  • Frost free season exceeding 150 days
  • Minimum mid-winter temperatures no lower than minus 25°C
  • Minimum temperatures during the shoulder months of November and March no lower than minus 20°C (minus 10 – 15°C may cause damage at this time depending on the dor-mancy stage of the plant)
  • A minimum of 1200 growing degree days >10°C are needed to mature the fruit for Bordeaux red wine grapes
  • Well drained soils
  • Sunshine between April 1 and October 31 to exceed 1250 hours

Solar Radiation

Solar energy is the primary source for all biological processes. Radiation from the sun has an effect on air and soil temperature, transpiration, soil moisture, atmospheric humidity, and all plant processes such as photosynthesis, cell division and flowering. It also affects sugar accumulation, bud fertility, wood maturity, and crop yield and quality. The amount of solar radiation received at a site and the duration of sunshine during the growing and harvesting sea-son has a direct influence on all these factors.

In general, most regions of the Okanagan and Similkameen Valleys receive adequate solar radia-tion for grape production. However, shading from the afternoon and evening sun in mountainous areas caused by topography or nearby trees de-creases the amount of solar radiation intercepted and impacts on grape development. Solar radiation is associated with growing degree days at particular sites. Factors such as the length of growing season, although not directly related with heat accumulation, are associated with solar radiation. Long grow-ing seasons with low heat unit accumulations are found in cool grape-growing areas such as coastal climates where solar radiation is limited. Development of flavour components in grapes suitable for cooler climates is generally enhanced in such climates. Flavour components for the same varieties are often destroyed or nearly non-existent in areas of too much solar radiation. The selection of grape varieties in cool climate areas is therefore limited to early or mid season maturing varieties with economical yields, or later maturing varieties with uneconomic yields.

Growing Degree Days

Growing degree days (GDD) is an expression of heat summation and is a measurement of physi-ological time. Growing degree days is an expres-sion of the amount of heat the plant receives that is above the basal development temperature. The more degree days accumulated, the faster the rate of production. One growing degree day is ac-cumulated for each degree the mean daily tempera-ture is above 10°C. Accumulations are measured throughout the entire growing season. The formula for calculating Growing Degree Days is listed be-low:

Each daily accumulated GDD is added to previous GDD accumulations to give the total GDD accu-mulated in the season. If the daily average temper-ature is below the basal limit, the GDD for that day is 0. There are no negative GDD values. Early ripening varieties require fewer GDD than late ripening varieties and therefore are best suited in the cooler regions of the valley. Late ripening varieties require more GDD which for some varieties lim-its the regions in which they can be produced suc-cessfully. Below are typical Growing Degree Day accumulations in the regions of the Okanagan and Similkameen Valleys.

Table 3.1 GDD > 10°C in the Okanagan and Similkameen Valleys

Region Location Degree Days (*C)
1 Kelowna 950 - 1360
2 Penticton 1140 - 1500
3 Vaseaux Lake 1320 - 1490
4 Golden Mile 1340 - 1630
5 Black Sage 1360 - 1630
6 Similkameen 1180 - 1540

 

Precipitation

The average annual rainfall in the Okanagan and Similkameen valleys range from 315mm to 380mm with nearly half of the precipitation falling out-side of the grape growing season. In general, the amount of precipitation received in the Okanagan and Similkameen is ideal for growing grapes. How-ever, significant amounts of rainfall during the growing season can have adverse effects upon disease development and grape quality. Grapes grow best under mild, dry spring weather conditions, fol-lowed by long, warm dry summers after bloom. Cold temperatures and rainfall during the flower-ing period may interfere with fruit set. Rain and wet weather at any time can create climate condi-tions conducive to the growth of pathogens detri-mental to crop production and vine health. Rain at harvest may also reduce fruit quality. The ad-vantages or disadvantages of rain depend on when, how long and how much it rains.

Slope

The amount of heat accumulated at a site varies depending on the slope of the land and the direc-tion of the slope. In the northern hemisphere, south facing slopes are the best choice to gain increased solar radiation. North facing slopes gain the least while west facing slopes intercept more solar radiation than east facing slopes. Total accu-mulated heat units are generally greatest near the mid-slope, less on the hilltops and lowest near the base of the slope. Exposed hilltops have lower max-imum temperatures and slightly cooler minimum temperatures than mid-slopes. The angle of the slope, in relation to the location of the sun, is very important to maximize the amount of solar ra-diation collected at a site. Cold air flows down slopes and collects at the base creating frost pockets and areas with late spring frost and early fall frost. The most suitable slopes for grape production have a gentle slope that provides good air drainage and maximizes heat accumulation.

Elevation

Grapes are grown over a wide range of elevations in B.C. (9 to 490 meters above sea level). There are limits in elevation above sea level where grapes are
grown economically. Increases in elevation of 100 meters may reduce the average annual temperature by as much as 1°C. Exceptions have been noted on south slopes and in areas where air inversions may form. Vineyards at higher elevations are therefore generally cooler than vineyards at lower elevations in the same region. Higher elevations are gener-ally wetter due to increased precipitation during the growing season and winter months. Cooler tem-peratures at higher elevations delay bud break, flowering and ripening dates. The list of varieties suitable for viticulture at higher elevations becomes shorter and more restrictive. Complete maturity may not occur for some varieties in all years.

Wind

Moderate air flow is beneficial to grapevines as it generally results in reduced disease pressure. How-ever, significant winds can cause serious damage to grapevines. Many studies illustrate the negative effects of high wind on vine growth, production and fruit quality. Vines create a special climate between the rows and in the leaf canopy that is altered or destroyed by winds.

Exposure to moderate and high winds has a desic-cating effect due to the high evapotranspiration rates, which causes physical damage. High winds often result in tattered leaves, smaller leaves, broken shoots, extensive lateral growth, shorter and fewer shoots, and smaller clusters. Winds in excess of 12 km/hr cause stomata to close, resulting in reduced photosynthesis. Stomata are reported to recover slowly if the reason for their closure was high winds (up to 4 days required to recover) and more quickly if the wind speeds were moderate (up to 1 day required to recover). In the winter, wind re-moves snow cover which may increase the risk of soil drying and root desiccation.

In regions with significant wind issues, row direc-tion should run parallel to the prevailing wind where possible in order to reduce shoot damage. Wind-breaks with 50% reduction of wind may be bene-ficial for areas of the Similkameen and South Okanagan Valley where strong daily winds are typical in the spring and summer. Studies in other areas show that sheltered vines protected by artifi-cial or natural windbreaks have higher percent-ages of bud break, more shoots, higher pruning weights, larger clusters and more berries per clus-ter, lower pH, and potassium. Yield increases have been reported when vines were protected from strong winds. The benefits of wind shelters will vary with the frequency and the degree of high winds.

Water

Large bodies of water, such as Okanagan Lake, moderate temperature effects on surrounding areas. Such bodies of water have a large heat storage capability which has a cooling effect in the summer and warms the surrounding area in the winter. In addition to this moderating effect, vineyards locat-ed on slopes close to large lakes or rivers benefit from the reflection of solar radiation from the water surface increasing the length of frost free period. Lakes or large rivers can also increase the surrounding area humidity and cloud cover. All of these factors reduce the risk of late spring or early fall frosts and extend the growing season.