Thursday, 29 August 2013

Early Season Leaf Removal - Poni and Kemp

Over the last decade viticulturists have rowed back from Richard Smart’s original hypothesis that improvements to the light environment of vine canopies unanimously benefit the quality and quantity of grape harvests. In particular, it has been shown that the combination of high ambient temperatures and strong UV light can negatively impact grape colouration. In the warmer regions of the World it is possible to get too much of a good thing, and Smart’s original prescriptions are now customised to site and variety.

In separate studies, Dr Belinda Kemp and Professor Stefano Poni have researched the consequences of early season leaf removal. Belinda Kemp found that for Canterbury-grown (NZ) pinot noir, mechanical leaf removal around the clusters 7 days after flowering improved the perception of fruitiness for tasters, whilst simultaneously lowering the negative ascription of “green” characters. to the wine. Similarly,  Poni, working with grenache, sangiovese and graciano, attributes accelerated ripening and a rise in the ratio of desirable tannins and anthocyanins within the must to early season leaf removal.  Moreover, for both Kemp and Poni early season removal conditions the clusters to ultra violet light, and consequently the berries don’t scorch or sunburn. If complete fruit zone leaf removal at veraison improves a wine’s quality, then the chances are that leaf removal at flowering will bring an even greater gain, as long as you are prepared to tolerate some drop in productivity. Removal of basal leaves diminishes the vine’s photosynthetic potential at a crucial stage in the development of both this year’s inflorescences and next year’s buds

Poni has speculated that early leaf removal might bring about physiological changes in the vine that favour the production of phenols, but as yet no mechanism as to how this might work has been forthcoming.  Returning to Smart’s work, it seems logical that at fruit set intra-bunch shading – berry-on-berry light interception - is minimal, whereas closed or veraised clusters only present a fraction of their cumulative berry surface area to direct sunlight.  Strip leaves off early and the clusters expose every bit of their green-skinned-selves to mid-summer’s forceful sun.


Pinot at fruit set
UK Conditions

Dr Steve Smith estimates that at the equivalent latitude, New Zealand’s vineyards receive 40% more UV than those of France; whilst the regions of Champagne and Burgundy intercept between 15%-25% more light through the growing season than does Southern England. Moreover, differences in seasonal temperature accumulation between England and Continental Europe mean that even in our warmest vineyards, flowering and veraison occur later in the year than they do in Central and Northern France. On the Cote de Nuits, pinot veraises around the beginning of August, when UV levels are high and the sun tall, whilst in England veraison takes place three weeks later, when days are shortening and the sun’s trajectory is much lower. If you want to make a still pinot noir in England then the autumnal sun might just not be the strong radiating source you need.

With this in mind we have adapted the principles of Kemp and Poni to our vineyard. Removing leaves at flowering maximises cluster exposure to sunlight at a time of the year when light levels are at their highest and intra-bunch shading is at its lowest. It gives the grapes every opportunity to build their defences against the light, and we hope these adaptions will ultimately benefit the quality of our wine.


Pinot Noir

We have a few different clones - 777, 828, Abel - and some rows of massales.  Between flowering and veraison the clusters are held above the horizontal which means that it’s only necessary to remove the primary leaves (by hand) from the node adjacent and immediately above the second cluster (assuming two clusters per shoot). This has the advantage that the basal nodes retain their leaves which seems prudent given that we spur prune. We also remove the laterals around the clusters, which at flowering are just starting to elongate. In this way we create a window in the canopy just above the clusters. Poni and Kemp experimented with stripping primaries and laterals through nodes 1-6, but in our situation this wouldn’t necessarily increase the exposure of the clusters to direct sunlight. 

Pinot Noir with reflective mulch between the rows
Over the last three years, cluster exposure has been good all the way through to harvest, though this year we have only trimmed the vines once. Ripening grapes in a cool climate requires an active canopy late into the season, and we have taken the decision to retain more laterals this year. Sunlight exposure on the clusters may be slightly less through September than it was previously, but more leaves should potentially mean higher sugars and the prospect of increased hydric stress on our shallow soil. I am happy to eulogise the advantages of a long, cool ripening period, but vines slide inexorably towards entropy and rot under October’s lowering skies.  I am hopeful that the intermittent periods of hydric stress and photon flux experienced over the previous 3 months can cumulatively get us across this year’s finish line.


Acolon at veraison with two leaf excision
Acolon’s (blaufrankischxdornfelder) phenology is more precocious than of pinot. Veraison begins in the middle of August, when one hopes the energy in the atmosphere is dedicated to drawing water from the soil rather than depositing it at its surface. Acolon has a different growth habit to pinot noir, it grows straighter with fewer laterals, and berry and cluster weight is twice that of pinot. Retaining leaf area invariably means a tall canopy, so we’ve adopted a policy of green harvesting to one cluster, with 12-13 primary leaves retained per shoot.

We planted the acolon at a high density – 0.6mx1.5m – which is very crowded by UK standards, but rocky soil, spur pruning to four buds and the use of 161-49 throughout seems to have balanced the vines’ vigour. Close planting also keeps our yield per hectare high, so losing the 2nd cluster doesn’t seem such a hardship.

The latter arrangement reminds me of UC Davis’s Mark Matthews comment about leaf plucking and green harvesting, that “It is the journey and not the destination that matters”. Viticulture is a way of gilding nature’s lily, not creating it. On a vigorous site you can hack at vegetation like the Prince in Sleeping Beauty, but you may never find your Princess.  If you are always fighting the impulses of the vine, the chances are your vines are in the wrong place. To paraphrase Smart: you think you’re in the wine production  business when you are actually in lumber.     

No comments:

Post a Comment