Flat vs Sloping Vineyards:
The degree of slope to a vineyard can be important for several reasons: air flow through the canopy, soil drainage, level of water retention, movement and possible erosion, ease of working in amongst the vines, managing equipment and then the ease of harvesting the grapes. There is no perfect slope as it will depend on what is the key limiting factor of concern.
The further north or south - the greater the latitude from the equator the steeper the slope needs to be for ideal sunshine exposure and heating of the vines. The slope or inclination is measured as a percentage of elevation change over a horizontal distance. For example: a perfectly flat vineyard would have a slope of 0% and a vertical cliff would have a slope of 100%.
A slight to moderate slope of 5% to 10% is desirable for vineyard sites as it encourages the removal of denser cold air from the vineyard. Cold air is denser than warm air and will flow downhill, if there are no barriers to air movement such as trees or berms. Vineyards with a slope greater than 15% become much more difficult to manage, as it is hazardous to operate machinery on steep, fragile slopes and these sites can also erode more easily.
Slope Ranges:
• Flat 0% – 2.5% : Easy to manage and harvest, little soil erosion, may be prone to cold air inversions in places with frost problems.
• 2.5% – 5% : Allows for adequate air drainage; may have some small erosion concerns.
• 5% – 7.5% : Allows for good air drainage; slightly more concerns of soil erosion; decisions on row orientation and equipment on slope.
• 7.5% – 10% : Allows for excellent air drainage; erosion and nutrient loss can be of concern; issues for row orientation and limit for equipment usage.
• 10% – 15% : Allows for excellent canopy air flow and drainage but with increasing erosion and nutrient loss concerns; likely becoming unsafe for heavy/large machinery usage without some form of terracing or self-leveling equipment, diversion ditches to control run off, and rows oriented perpendicular to slope.
• A slope greater than 15% : Vine management becomes more difficult; plus, erosion and equipment rollover can be of a concern. Very steep sites will require careful management and harvest by hand.
Hillside slopes, especially south-west facing slopes in the northern hemisphere and north-west in the southern, have always been the preferred location for growing quality wine grapes. It has been proposed that this was originally done out of necessity - as many of these lower fertile slopes were planted in grapes because other agricultural crops failed to grow well.
Those sites that are stony, rocky and with lower fertile soils produce smaller vines with less vigour than vines grown on deep, rich alluvial soils. Smaller vines result in smaller clusters/berries and crop-weight, and the wines show unique concentration and intensity (juice to skin ratio). Hillside slopes are generally well-drained and have less frost issues in spring (during budburst and flowering), as long as the cold air has somewhere to flow down too and away.
Though with these benefits, there are difficulties to consider in growing vines on slopes. Tractors and other equipment can find it difficult to work and stay upright and stable. With steep, terraced slopes you have limited choices regarding row direction and spacing. The slope aspect, incline, exposure, geology and topography will dictate where you can plant vines.
An example used to good use to show the influence of flat vs sloping vineyards is the wine region of Burgundy in France. Most of the vineyards of Burgundy are planted on gentle slopes, though some can be relatively steep, reaching a 35% grade near the region’s upper heights. The soil on the highest slopes have a thin topsoil and receives the least amount of rain fall during the growing season. The middle to higher part of the slope receives the most exposure to ideal sunlight and have the best drainage so they are designated as 'Grand Cru' vineyards. The Premier Cru are below Grand Cru vineyards on the slope while the Village / Generic wines are produced from the flatter areas, having diluted levels of sun exposure and poorer drainage.
On flat vineyards, activities like spraying, pruning, netting and harvesting a large area of vines is much easier. It is agreed by those in the know, that placing a vineyard on a steep slope will dramatically increase the effort required to maintain the vineyard and will significantly increase the investment needed to set up. Terracing might be necessary, which can add a great deal of additional costs per hectare to the vineyard establishment.
Quality fruit can be grown on flat vineyards; the vines tend to be a little more vigorous and can require more canopy pruning to keep the vines from getting too wild and woolly, which can take away the vines focus on the grapes.
Gentle slopes are preferred over flatter terrain, as vines growing on a slope can receive a greater intensity of the sun's rays, with sunshine falling on an angle perpendicular to the hillside. On flatter sites, the intensity of the sunlight is diluted as it spreads out across a wider surface area. Small slopes that are elevated above surrounding ground are ideal sites for vines, because these small elevations are less prone to frost. Additionally, a slope affords better drainage, reducing the possibility that the vine might sit in overly moist soil. In cooler regions of the northern hemisphere, south-west facing slopes receive afternoon hours of sunlight and much valued. In warmer climes, south-east and even some north-facing slopes are preferred. In the southern hemisphere, these orientations are reverse.
A wine region which has worked well with extremely steep slopes is the Douro Valley in northern Portugal. In terms of vineyard layout, planting in the Douro poses a multitude of problems associated with very steep and rocky terrain which is more suited to breeding goats than growing grapes. Solving these unique issues is an ongoing challenge - they are shaped largely by the availability of funds, resources, labour and access to construction technology and options.
Other well known wine areas with slopes and terraces that produce quality grapes and resulting sort after wines include: Grand Cru vineyards in Chablis, the gentle slopes of Tuscany, the hills around Barolo, the picturesque slopes in Alsace, the dramatic hillside vineyards in the Rhône Valley and the terraced vineyards in Central Otago - just to name a few. If possible, having vines on both slopes and vineyards which are relatively flat, gives options to a winemaker on varietal diversity, ripeness and grape flavours.