Monday, 27 August 2012

Vine Density

It can get pretty crowded in the vineyards of France. Within Champagne and Burgundy vine densities of around 10000 plants per hectare are normal, but more congested planting is not exceptional.   Head south, towards the Midi, and viticulture becomes more spacious: fewer vines, less leaves and shoots, and much more visible earth.  When I first consulted someone about planting densities for Tixover, the advice was “a tractor width plus a metre x 1.5 metre”; which worked out to 2000 plants per hectare for a 70hp machine. From somebody that had been in the UK business for a considerable time, this was sound practical advice; Nature is hard to dominate without mechanical assistance, and bare soil favours a rapid return to the full diversity of the indigenous flora rather than the unchallenged success of an invasive monoculture. Still, hoeing is hoeing, and the move from man to horses and tractors as the means of weed suppression, if anything, lowered vine densities within Burgundy and Champagne. For the monks of the Abbaye, the hoe must have been an instrument of self-flagellation.

There is an economic argument for high vine densities: more vines = more fruit, and a better return per hectare. In 2004, Champagne produced 140hl/ha. A small and early harvest in 2003 led to a proliferation in bud numbers and vine reserves, and this potential was fully realised in the near perfect flowering conditions of 2004. The subsequent harvest yielded both quality and quantity.

Another perspective on vine density is offered by Alain Carbonneau of Bordeaux University. In cool climates – Northern France, UK, Germany – Carbonneau suggests that shoot length needs to be a minimum of 80% of row width, in order that the vines are put under conditions that favour the induction of hydric stress. Carbonneau’s thesis is that the water capacity of a vineyard soil is depleted more rapidly if the total exposed leaf area of the overlying vineyard is maximised. This prescription works itself out into some interesting ratios. Burgundy’s 1m x 1m planting with a 20cm fruiting wire and shoot topping at 80cm perfectly satisfies Carbonneau’s conditions; whilst a row width of 1.6m demands a shoot length of 1.3m. Worryingly, the recommended tractor-friendly row dimension of 3m would require unworkable 2.4m long shoots to achieve similarly advantageous proportions.

What is common to Bordeaux, Burgundy and Champagne is that the peak in seasonal water deficits coincides with veraison. At some point between late July and early August, the vines divert their resources away from vegetative growth and towards fruit ripening. On the Cote de Nuits, crop evapotranspiration for the 4 week period that has veraison at its centre averages 130mm, with precipitation only replacing 60mm of this loss. Assuming that the vines' access to soil moisture is limited (arguably the defining characteristic of Burgundy’s finest appellations) then vine and berry growth will be constrained, and ripening accelerated. Just how these benefits are conferred on the bunches is still open for discussion. Richard Smart sees the advantages of water deficits deriving indirectly from reduced vigour and the concomitant improvement of the light environment in and around the clusters; whilst Mark Matthews and Stefano Poni posit a more direct link between advantageous gene expression and soil drying. Neither view precludes the other, but which side of the argument you favour has consequences as to how you spend your time in the vineyard. Those who follow Smart believe leaf removal, green pruning and vine architecture can substitute for terroir, whereas subscribers to the Matthews' dictum, “It’s the journey and not the destination that matters” will spend their time seeking out pedological conditions that optimise vine stress.

It may well be that in the UK's relatively damp, cool and sunless climate we have to embrace both strategies, and view the theories and prescriptions of Smart and Matthews as being in some sense simultaneous, rather than conflicting. On average, Burgundy picks Pinot Noir at 940GDD after budburst, and Champagne 870GDD. Nowadays, these summations are achieved in mid-September, but under the UKs maritime climatic regime these values are reached (if at all) in October, under lowering skies, downward spiralling temperatures and the constant threat of rain.  The skewed pattern in phenology between France’s northern appellations and the vineyards of Southern England is entrenched by flowering. Mid-floraison in Champagne and Burgundy occurs 2-3 weeks ahead of the most precocious English vineyards, which pushes veraison on this side of the Channel into late August and early September, when crop evapotranspiration and precipitation are often equivalent, and thus, free of hydric stress.

One anxiety voiced against close planting is that it increases the shading of one canopy by another. Logically, more distant rows will intercept more of the early morning and late evening sun than rows of the same height planted closer together. The idea has appeal, but as the illustration below shows (Courtesy of Cornell University) the gains from planting wider are marginal, and almost certainly counterproductive, as vigour and canopy shading will tend to increase at lower vine densities.  



The other multiple used to calculate vine density is within row spacing. As was the case with row width, the higher vineyards push into the eaves of the l’Hexagone, the tighter the spacing between the vines becomes. At Tixover, our widest spacing is 1.2 metres, and our narrowest 0.6 metres. The wider spacing reflects the advice we were given when we first planted the site, but we have since discovered that our vines are only comfortable pushing 5-6 shoots of even vigour.  To avoid shoot congestion and acrotony along the fruiting canes we decided to spur prune the vines. 

The above choices have given us two vine densities: the 0.6mx1.7m= 9700 vines per/ha; and the 1.2mx1.7m= 4800 vines per/ha. All vines have 1.3m, 15 node shoots, which gets us close to the ratio of shoot length to row width suggested by Carbonneau. The canopy density has been excellent in both vineyards, though we do remove two leaves on every shoot at fruit set - those immediately adjacent to and above the second cluster – to improve light exposure during the pre-veraison period when intra-bunch shading is minimal.

As with any field-based trial, the confounding variable is often the weather. Somewhere between May and July of this year, we lost nearly a month’s worth of an already short growing season, and I am doubtful that the crop will ever reach maturity. Interestingly, vigour this year has shown no increase despite the rainfall, and berry size at lag-phase is small. This can be interpreted in several ways, but I suspect root growth was suppressed by soil saturation through the spring, and the subsequent dry and warm weather we have experienced since fruit set has inducted some stress into the vines. Unfortunately, the two AG moisture probes we have in the ground have also become victims of the weather, so we will have to wait until next year to get good data on fluctuations in soil moisture availability and capacity.

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