John R. Behrs
July 2001


Much has been written about the importance of site selection of vineyards in areas with extreme continental climates (i.e. prone to cold injury), such as we have here in Colorado.  The former Colorado state viticulturist, Rick Hamman, emphasized this topic in his book, "Colorado Grape Grower's Guide". An on-line description of site selection in Virginia can be found in in Site Selection for Commercial Vineyards  by Tony K. Wolf, Viticulture Extension Specialist, Virginia Agricultural Experiment Station, Winchester, Virginia.  (See also,  The Mid-Atlantic Winegrape Grower's Guide. p .35)  Much of this information is applicable in other areas of the country with continental climates as well.  A interesting mapping of the Virginia for vineyard suitability was mentioned in  Viticulture Notes Volume 15, Number 1 -- May-June 1998.

An article in the Colorado Fruit Grower's newsletter (Vol 20. No 1, Jan 2000) titled "Dormancy and Cold Hardiness in Fruit Trees" got me thinking again about vineyard site selection in Colorado.  Particularly the statement, "... temperatures several degrees above freezing can cause significant tree and crop damage if they occur early or late.  In addition, these are the also the ones that go unnoticed or are forgotten about."  This statement is a good reminder of how temperature variability can be as important as absolute temperates, but so often the grape grower focuses only on absolutes, primarily because absolutes are readily available in the form of temperature readings.  Ballpark cold tolerances of vinifera are known, so it is easy to deduce when cold injury has definitely occurred due to an abnormally cold day.  But an absolute temperature below the cold tolerance of vinifera is a relatively infrequent event at the better sites in Colorado.  Indeed, statistics from the Colorado Climate Center for the town of Palisade for 45 years ending in 1995 show only three years (1963 (-20 degF), 1989 (-12 degF), 1990 (-10degF)) where the temperature dipped to -10 degF or below.  Damage at these temperatures is usually routine.  However, another particularly damaging cold injury event that occurred in Colorado in 1998 was the trunk splitting disorder.  Some growers reported as much as 69% trunk death. The yield of Chardonnay and other wine grape cultivars was reduced by more than half. The culprit was determined to be temperatures of 4 degF to 9 degF above average in January and February followed by a low temperature of 13 degF on March 8.  This problem caught nearly everyone by surprise because it was temperature variability, not an absolute, that caused the problem.

What has this to do with vineyard site selection in Colorado?  It is generally known that the aspect of the vineyard slope can be an important variable in growing vinifera.  However, as so often is the case with continental viticulture, conventional viticultural wisdom does not apply. According to Rick Hamman [emphasis added]:

"The exposure of a slope relative to the sun is a more complicated factor.  During the day, a south or west facing slope will be somewhat warmer throughout the year; however, this feature is not necessary to ripen grapes in Colorado even though it is important in other locations such as northern Germany.  In fact, warming of the soil on a south slope during the winter in Colorado can reduce winter survival."

 According to Tony K. Wolf [emphasis added]:

The aspect of a slope refers to the compass direction which the slope faces (e.g., north, south, etc.). Eastern, northern, and northeastern slopes are probably superior to other aspects. Often, however, other factors, such as the presence of woods, steep slopes, and exposed rocks will dictate that another aspect must be used. The preference for eastern and northern aspects relates to the heat load differences between various slopes. Southern and western exposures are hotter than eastern and northern exposures. Southern exposures will warm earlier in the spring and will advance bud-break relative to vines on a northern slope. The consequence of advanced bud-break is the increased potential for frost damage. Southern aspects can also be expected to cause vine warming on sunny winter days to a greater extent than on northern slopes. The consequences of that warming could be reduced cold resistance and subsequent cold injury. Bark splitting and trunk injury to the southwest sides of fruit trees is commonly observed in Virginia and is related to trunk warming on sunny winter days with subsequent, rapid cooling. Southern and western aspects can also be expected to be hotter during the summer than northern and eastern aspects. As discussed earlier, hot fruit is generally of lower quality than cool fruit. Eastern aspects also have an advantage over western aspects because the eastern slopes are exposed to the sun first. Vines on an eastern slope will dry (dew or rain) sooner than those on a western slope, which should potentially reduce disease problems. The basic effects of slope orientation on vine performance are summarized in Table 2.

Table 2. Relative effects of compass direction of site (aspect) on various climatological and vine developmental (phenological) parameters.

 

 

Aspect 

Climatological or phenological parameter 

North 

South 

East 

West 

Time of bud-break 

Retarded 

Advanced 

Retarded 

Advanced 

Daily maximum vine temperature 

Less 

Greater 

Less 

Greater 

Speed of foliage drying in morning 

--- 

--- 

Rapid 

Slow 

Radiant heating of fruit in summer 

Less 

Greater 

Less 

Greater 

Radiant heating of vines in winter 

Less 

Greater 

Less 

Greater 

Gladstones has suggested effective temperature adjustments (for predicting vine development and ripening) as for vineyard topographies in mid latitutes, as compared with nearby flat recording locations with non-stony and non-calcareous soils:
 

Terrain Adjustment to max temperature Adjustment to min temperature
Moderate slopes in undulating terrain -0.5 degC (0.9 degF) +1.0 degC (1.8 degF)
Steep slopes, or moderate and steeper slopes of free-standing hills -0.5 degC +1.5 degC
Slopes directly facing the midday sun +0.25 degC +0.75 degC
Slopes directly facing away from the midday sun -0.25 degC -0.75 degC
Slopes facing east or west no change no change
Markedly stony, rock, limestone or chalk-based soils -0.5 degC +1.0 degC

Reduced adjustments apply to lesser topographical or soil contrasts.  All individual adjustments are additive.

In my opinion, the primary importance of aspect in Colorado is due to its effect on temperature variability and how it can retard or advance the vines entry into and exit out of dormancy.  The theory is that the cooler the vine can be kept through the period leading up to a cold injury event, the less damage it would ultimately suffer.

According to the above table, moderate slopes with a northerly aspect (i.e. a slope that faces towards the north, away from the sun) in the northern hemisphere have a -0.75 degC maximum and a +0.25 degC minimum adjustment (adding the adjustments for moderate slopes and slopes facing away from the midday sun) as compared to a flat valley floor.  A moderate slope with an easterly aspect has a -0.5 degC maximum and a +1.0 degC minimum adjustment. A moderate slope with a southerly aspect has a -0.25 degC maximum and a +1.75 degC minimum adjustment.  The northerly or easterly aspects have less heat accumulation than either a valley floor or a southerly aspect.  For this reason, a slope with a northerly or easterly aspect is most desirable in a Colorado.

[Note: The Gladstones table assigns "no change" to adjustments with a westerly slope.  This conflicts with the Tony Wolf table "greater" maximum vine temperature.  This conflict could be due to climatic differences such as late afternoon cloud cover.  In a climate like Colorado that continually warms during the day (maximum summer temperatures are usually recorded around 6pm), a westerly slope would obviously accentuate the maximum temperature.]

A secondary benefit of any aspect of sloping terrain is less variable minimum and maximum temperatures, potentially increasing the quality of the crop.

I believe that even slight changes in elevation and aspect are important and I have seen this effect in my own vineyard.  On May 6, 1999, we had a late frost which affected the part of my Chardonnay vineyard that had already broke bud.  This part of the vineyard has a slight south-eastern facing slope of about 1%.  The lower part of the vineyard, essentially flat, had not broken bud and was unaffected, and produced the bulk of my 1999 Chardonnay crop.

I am continually amazed by the noticeable temperature changes caused by elevation differences of only a few feet.  At dusk as I drive my tractor between my two vineyards located two miles apart, I notice how the low spots on the undulating road are chilling as compared to the higher spots, not more than ten feet different in elevation.  I would conjecture that the vines notice this difference as well.  My two vineyards, located at the highest elevation on both ends of the road are always significantly warmer than any spot in between.
 

References


Hamman, Richard A., et. al, Colorado Grape Grower's Guide, Cooperative Extension Resource Center, Colorado State University, Fort Collins, CO 80523, 1996.

Hamman, Rick, A Summary of the 1998 Trunk Splitting Disorder, Vineyard News Vol No. 7, Colorado State University, OMRC, 3168 B 1/2 Road, Grand Junction, CO 81503, April 5, 1999.

Gladstones, J., Viticulture and Environment (Adelaide, 1992). Cited from Robinson, Jancis, The Oxford Companion to Wine, Oxford University Press, p 618. 1997.

Colorado Fruit Grower's newsletter (Vol 20. No 1, Jan 2000) titled "Dormancy and Cold Hardiness in Fruit Trees", p 4-5.
 
 

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