John R. Behrs
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
"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
Table 2. Relative effects of compass direction of site (aspect)
on various climatological and vine developmental (phenological)
Climatological or phenological parameter
Time of bud-break
Daily maximum vine temperature
Speed of foliage drying in morning
Radiant heating of fruit in summer
Radiant heating of vines in winter
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:
||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
|Slopes directly facing the midday sun
|Slopes directly facing away from the midday sun
|Slopes facing east or west
|Markedly stony, rock, limestone or chalk-based soils
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
[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
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.
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.