| High grapevine planting density, common in many parts of Europe, is a
relatively new concept in California viticulture. Planting costs, equipment
availability, the average price per ton of grapes, and California’s soil
and climate, which allow vines to grow fairly large in most valley floor
locations, were all factors favoring fairly wide vine spacing. In Napa Valley
prior to the late 1980s, it was rare to find vine densities exceeding 600 vines
per acre, and 8x12 foot spacing (vine x row, totaling 454 vines per acre) was
one of the most common plantings.
Emergence of phylloxera in Napa Valley in the 1980s made it clear that a
large percentage of valley floor vineyards would need to be replanted before
the turn of the century. While economically challenging, this created the
opportunity to evaluate not only different rootstock selections, but also new
clones, trellis systems, and vine planting densities. The possible impact of
high-density plantings in Napa Valley vineyards on vine growth, crop yield, and
wine style was largely unknown.
In 1985, one vineyard in the Oakville district of Napa Valley was planted
with 1x1.8 meter vine spacing, a density of 2,178 vines per acre. This was the
first large-scale planting (35 acres) at this density in Napa Valley. When the
vines matured, the vineyard demonstrated that it was possible to produce high
quality wines from closely spaced vines. No alternate spacings were planted,
however, so relative differences could not be compared.
In 1989 Robert Mondavi Winery established a trial on six acres of the
To-Kalon estate in Oakville, CA, to assess the effects of yield and wine style
from different vine spacings. Crop yield and shoot growth characteristics have
been recorded every year since 1992, and wine quality differences have been
evaluated on a production scale (4 to 8 tons) since 1993.
Materials and methods:
There are three vine spacings in the trial: 1x1 meter (3.1 x 3.1 feet),
referred to as close treatment, 1 x 1.8 meter (3.1 x 3.6 feet), moderate
treatment, and 1.5 x 2.7 meters (5 x 9 feet), wide treatment. A vertical
trellis with shoot-positioning wires (VSP) is used in the close and moderate
spacings. In the 1.5 x 2.7 meter spacing, two different trellis systems were
compared: VSP trellis (wide treatment) and a divided-canopy Lyre trellis (Lyre
treatment). These four treatments are described in
Table I. Each treatment has
two replicates, totaling 1.5 acres per treatment.
Two rootstocks, 110R and 5C, were planted in each vine spacing to observe
potential differences between rootstocks. Approximately half the rows in each
treatment and block were planted to each of the two rootstocks.
The rootstocks were planted in the spring of 1989 and field-budded with
Cabernet Sauvignon FPMS Clone 8 that fall. The vines were trained out into
bilateral cordons in 1990 and 1991. Quadrilateral cordons were established in
the Lyre trellis treatment in 1991 and 1992.
Soil in the trial area is described as Bale series clay-loam. The soil is
deep, with no apparent restriction zones down to at least three meters.
Drainage is fairly good. Row orientation is northeast-southwest, with a gentle
slope of less than 3%.
All treatments use vertical shoot positioning wires, but the cordon and top
wire heights vary. For the close spacing, the cordon wire was established 51 cm
(20 inches) above ground level, and the top wire positioned at 122 cm (48
inches). For both the moderate and wide treatments, the cordon wire was at 76
cm (30 inches) and the top wire at 173 cm (68 inches). The Lyre trellis had the
highest cordon development, with the cordons positioned at 112 cm (44 inches)
and the top wire at 198 (78 inches).
The vines were balance-pruned each year to 15 or 16 nodes per kilogram of
pruning weight. Because of vine vigor differences, cordon development time
differed between the treatments. Cordons in the moderate and wide spacings
filled out the trellis within three to five years. The close-spacing cordons
took longer to establish, and some vines in the Lyre trellis still do not have
cordons fully developed. Shoot spacing was kept similar in all treatments,
averaging 16 centimeters between two-bud spurs.
There were two replications of each treatment. Data vine sets used six vines
per rootstock per block in the moderate, wide, and Lyre treatments (n=24), and
10 vines per rootstock per block in the close spacing (n=40). Since this report
focuses on vine spacing, the averages for the two rootstocks in each treatment
have been combined and are presented together, though a few comments on major
differences found between the rootstocks are mentioned in the results.
Because the trial involved production-scale lots (4 to 8 tons), with the
goal of maximizing wine quality for each treatment, some cultural practices
differed between treatments. These included irrigation amounts and timing of
tillage.
Parameters:
For most parameters presented, values represent seven-year averages (1992 to
1998). Leaf-area estimation data was not collected in 1996 or 1997, so all
leaf-area parameters represent five-year averages (1992 to 1995 and 1998).
Single-factor statistical analysis of variance was run on the viticultural
data, evaluating treatment by year. Statistical differences reported are all at
.05 significance level.
Light readings were taken in the fruiting zone with a Decagon Corp.
80-sensor ceptometer. Readings were usually taken in August, shortly after
veraison.
Leaf-size determination from 1992 to 1995 was made by a weight/size
calculation. In each treatment, two 20-leaf samples per rootstock and block
were taken from data vines in early August (after hedging). Main shoot leaves
(taken equally from the basal, mid-, and upper portions of the shoot) and
lateral leaves were sampled separately.
After the petiole stem was excised, 20 leaves from each sample were weighed
as a group and an average leaf weight was recorded. A cork borer of known area
was then used to cut out discs from the mid-portion of each leaf, and the 20
discs were weighed. Leaf size was calculated by comparing disc size/disc weight
against the leaf weight.
In 1998, the leaf size was determined by a leaf area measuring device (Delta
T Manufacturing Co.) at the University of California Oakville Experimental
Vineyard. Some leaves were subsequently sized using the earlier size/weight
ratio method for comparison, and differences between the two techniques were
found to be less than 10%.
Node number was determined by node counts of data vine shoots after leaf
fall, usually in December. While some lateral nodes had broken off,
particularly in years of early rain or frost, it was presumed that the lateral
leaf area not accounted for was small.
Grapes were hand-harvested, crushed in an Amos crusher/destemmer, and
fermented in 10-ton stainless steel tanks. Skin contact time was four to five
weeks, depending on the vintage. The wines were pressed in a membrane press,
and 10-gallon samples were taken after draining and pressing. The wine samples
completed malolactic fermentation, were put through a racking and
SO2 addition program, and were bottled without wood ageing for
analytical and sensory evaluation.
Results:
Canopy growth — Vine growth data are listed in
Table II and leaf canopy
data in Table III. Shoot
number increased with wider vine spacing. The wide spacing, with 50% more space
between vines, averaged 91% more shoots per vine than the close spacing and 42%
more shoots per vine than the moderate spacing.
The close and moderate spacing treatments both have one meter separation
within the vine row, but due to stronger shoot vigor and earlier cordon
development, the moderate spacing averaged more shoots per vine every year
until 1998. Shoot number per vine differences between the close and moderate
vine spacing were not significant, however.
At the 1.5 x 2.7 meter spacing, the divided-canopy Lyre trellis averaged 59%
more shoots than the single- canopy wide treatment. Since the Lyre trellis has
twice the available cordon wire length, this indicates that the Lyre cordons
have not completely filled the available trellis wire.
On a per vine basis, as vine spacing increased, the pruning weight increased
by a higher percentage than the number of shoots. Therefore the average shoot
weight increased with vine spacing, with the wide-spacing shoots significantly
heavier than the close-spacing shoots. On a per-square-meter-of-soil basis,
however, the ratio is reversed, with the close spacing having the highest
pruning-weight and shoot-weight values and the wide spacing having the lowest
of these values of the three vine spacings.
Due primarily to a lower hedging height, the close spacing averaged three
fewer nodes per main shoot than the other three treatments. The moderate, wide,
and Lyre treatments had similar average node numbers per shoot after top
hedging. Main shoot leaf size, total leaf area per shoot, and percent lateral
leaf area did not show significant differences.
The close spacing and Lyre treatments represent the extremes in per-vine
values for number of shoots, cluster number, crop yield, pruning weight, and
percent lateral leaf area (though differences in canopy hedging height probably
affected lateral shoot production). For a number of other parameters, the Lyre
trellis values were closer to the close spacing than either the moderate or
wide spacings, including average shoot weight, cluster weight, leaf size, and
total leaf area per shoot.
Crop yield parameters — Table IV shows seven-year averages of crop yield parameters.
Cluster number and crop weight per vine increased as vine spacing increased,
primarily due to increased shoot number per vine. Cluster weight also
increased, with a 49% increase from the close to the wide spacings.
The increase in cluster weight was due primarily to significantly more
berries per cluster with each increase in vine spacing. There was a trend
towards increased berry size with wider vine spacing, but the differences were
not significant. The increase in both cluster weight and berry number per
cluster as vine spacing increased occurred in all seven vintages.
Comparing trellis systems at the 1.5 x 2.7 meter spacing, the Lyre trellis
had significantly more clusters and greater crop weight per vine, due to more
shoots, but significantly lower cluster weight and number of berries per
cluster than the single-canopy, wide-spacing vines.
Total yields over the seven years averaged between 6.8 to 7.0 tons per acre
for the close, moderate, and Lyre treatments. In no individual vintage were the
values that close, however, and the close, moderate, and Lyre treatments each
had years where they had the highest calculated tons per acre. The wide spacing
averaged 5.6 tons per acre, approximately 20% lower than the other three
treatments, but this difference was not significant at the .05 level.
Canopy/crop ratios — Table V lists two common ratios that relate canopy growth to
crop yield. Differences between values were not significant, but the two 1.5 x
2.7 meter treatments averaged approximately 20% higher
crop-weight/pruning-weight ratios and 20% lower leaf-area/crop-weight ratios
than the two closer vine spacing treatments.
Rootstock differences — The vines on 110R rootstock established
themselves earlier and have averaged slightly higher vine-vigor values (shoot
number, shoot weight, pruning weight per vine, cluster number per vine, and
leaf-area/crop-weight ratio) than the vines on 5C rootstock. The 5C vines
averaged slightly higher crop-yield values (berry weight, cluster weight, crop
weight per vine, tons per acre, and crop-weight/pruning-weight ratio).
Differences between rootstocks that exceeded 10% are listed in
Table VI.
Must and wine analysis —Malic acid, tartaric acid, and arginine content
of berries sampled just prior to harvest are presented in
Table VII, and wine analysis
data are in Table VIII. The
goal was to harvest the treatments between 23.0° and 24.5° Brix,
depending on the vintage, and to have no more than 0.5° Brix separation
between treatments in any one year. This was accomplished in most, but not all
vintages, as other berry maturity characteristics were also used as criteria
for harvest. Since the wines were not always picked at similar Brix values,
statistics have not been run on wine analysis.
In most vintages, harvest of the four treatments happened within one week of
each other. The close spacing was usually harvested first, but often had a
lower alcohol content. Averaged across all vintages, the wine alcohol levels
were all between 13.2% and 13.4%.
The wide spacing had the highest average color intensity (sum of absorbance
values from the 420, 520, and 620 wavelengths). The close spacing had the
highest color intensity in several vintages, but over-cropping in 1993 resulted
in much lower color intensity and total phenolic (A280) values that particular
year.
Most other berry and wine parameters showed few consistent trends from
vintage to vintage. These included wine potassium content and malic acid,
tartaric acid, and arginine values from berry samples taken just prior to
harvest.
Sensory evaluation — Duo-trio difference tastings (in dark glasses) and
preference-ranking tastings (blind, in clear glasses) were performed on
pre-aged wine samples in all vintages from 1992 through 1997. Only
preference-ranking tasting was performed on the 1998 wines. In 1992, separate
wines were made from the two rootstocks in all spacings, and tasting
evaluations were made within each rootstock. In 1997, the Lyre trellis was not
evaluated due to a very sluggish alcoholic fermentation.
No significant differences were found between any wines in 1996 or 1997 or
within the 110R rootstock wines in 1992. In other vintages significant
differences at the .05 or higher level were found in several treatment
comparisons.
Difference tastings between the close and moderate spacing wines showed
significant differences in three of the six vintages they were compared.
Comparisons between both close-to-wide and moderate-to-wide spacing wines were
significantly different in two of five vintages in which they were evaluated.
The Lyre-trellis wine was significantly different from the wide-spacing wine in
only one of five vintages.
In the preference-ranking tastings, the tasters were asked to rank the wines
for overall preference in all vintages, rather than being directed towards any
particular attribute. Significant preference between wines was only found in
two vintages, 1995 and 1998. In 1995, both the wide and Lyre wines were
significantly preferred to the moderate-spacing wine. In 1998, the close
spacing was significantly preferred to the moderate- and wide-spacing wines,
and the Lyre trellis was significantly preferred to the moderate-spacing wine.
Discussion
When this trial was being set up, it was apparent that not all cultural
practices could be identical in the growth and development of the vines.
Different trellis and canopy heights, variable harvest maturity, amounts of
irrigation and thinning, and choosing what criteria to normalize for comparison
purposes were factors that needed consideration.
Differences in vine size ruled out establishing any per vine parameters to
normalize. Having similar tons per acre yield was considered, and multi-year
averages of three of the four treatments have very close values. While economic
comparisons on a per acre basis may be useful, optimum wine quality may result
from different tons per acre yields for the different vine spacing.
It was decided that the most valid way to compare the different vine
spacings was to use the same shoot spacing and balanced pruning formula, and
aim for similar canopy-growth/crop-yield ratios (crop weight/pruning weight or
leaf area/crop weight). Accurately predicting these ratios prior to harvest is
difficult, however. While differences between treatment ratios were not
significant in this trial, they did exceed 20%.
Harvest timing varied because these were production-scale wine lots, and the
winemakers were looking at berry flavor development and phenolic changes in
skins and seeds as guides for determining harvest timing. Unfortunately, these
did not always occur at similar Brix levels between treatments. If the Brix
differences were too large, however, phenolic extraction differences from
differential alcohol levels would occur. In some vintages a compromise was
reached in an attempt to keep treatment alcohol levels within 0.5% by volume.
Concerns of excessive fruit shading with closer vine spacing have not been
realized in this trial site. Although the canopy height exceeds the row width
in the meter-by-meter spacing, and lateral leaf area is highest in this
spacing, a desirable light environment still existed (light readings in the
fruiting zone averaged over 15% of ambient in all treatments). The VSP trellis
pulls most of the leaves out of the fruiting zone, allowing adequate light
penetration to the fruit. In addition, at Napa Valley’s latitude (37
North), sunburn damage is a concern during summer heat spikes, and partial
fruit shading is desirable.
Based on our winemaking experience over the seven years of this trial,
keeping crop-weight/pruning-weight ratios in the 3.5 to 4.5 range and keeping
yields under seven tons per acre are desirable for optimal Cabernet Sauvignon
from this site. Use of cover crops in high-rainfall years and veraison crop
thinning in higher-yielding years are tools that can be used to keep canopy and
crop levels from getting too high.
Conclusions:
As vine density increased, there were decreases in per-vine values for both
canopy and crop size. There were also significant decreases in individual shoot
weight, cluster weight, and number of berries per cluster values. The
consistently smaller and looser cluster size associated with closer vine
spacing, particularly in the meter-by-meter, implies that root competition for
water and/or nutrients occurs early enough in the season to affect berry set.
Smaller cluster size can be desirable, depending on the site, variety, clone,
and wine style for which the grapes are intended.
Seven-year averages showed very similar per acre yields from the two closest
vine spacings, 1 x 1 and 1 x 1.8 meters. Both yielded over one ton per acre
more than the 1.5 x 2.7 meter spacing on the same trellis system. The Lyre
trellis treatment has averaged similar yield to the two closer spacing
treatments, but to date has not fully filled out its cordons.
Within the same vine spacing of 1.5 x 2.7 meters, adding shoots and clusters
via a quadrilateral cordon Lyre trellis system resulted in consistently lower
cluster weight and shoot weight compared to a bilateral cordon, single vertical
trellis. The wines were found to be significantly different from each other in
only one vintage, however.
Wine "quality" conclusions are difficult to make, and blind
comparison tastings did not always show significant differences. The preferred
Cabernet Sauvignon wine style at this site has tended to come from a
combination of factors, rather than one particular parameter of measurement.
These include small cluster size, low to moderate shoot vigor and crop weight
per vine, a fairly low crop-weight to pruning-weight ratio, and limiting
maximum per acre yields.
Edited from presentation to ASEV Vine Spacing Symposium, June 29,
1999
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