The recent heat wave experienced after the rain last week (June 24th -25th) illustrated very well a classic misconception about vine water use. For years, people have considered that “drippy” leaves or “folded” leaves were a reliable sign of soil water deficit. However, after the last rainfall in Napa, while leaves were showing signs of water deficit, very few vineyard measurements pointed towards a lack of moisture in the root system. If you are still in doubt, just check the look of your shoot tips: if they are growing this should dissipate any skepticism… Shoot tips do not survive under soil water deficit! In conclusion, if your vine was still growing after the rain, it did not need any irrigation, even during a heat wave such as the one we just experienced (June 28th- July 1st).
Over the last few days in California, all of us have noticed the cupping of the leaves, turning away from the sun during the warmest hours of the heat wave. It would be wrong to attribute those symptoms to soil moisture deficit… The shoot tips would have been long dead and dry if soil moisture supply had been limiting for those “stressed” looking vines. I am not saying “vines were not experiencing any water stress” during the heat. I am just saying that the visual symptoms of water stress were caused by the climatic conditions and not by a lack of soil moisture. Now, we should always remember: the folding of the leaves is not only a plant response to drying soil!
For sure irrigating under a heat wave makes us feel good. Because we love our vines, we treat them like humans… and all humans like to get a drink when it is warm outside. But, even if you only have an empirical approach, everyone will also agree that a little bit of water deficit early during the season will improve wine quality, particularly reds. This is true whether you are in Bordeaux, in Argentina or in California. This is not even a secret in the wine world. Therefore the current period is a dilemma for vineyard management. We need to fight our natural tendency and have clearly in mind the benefits of limiting vine water supply now to improve wine composition later.
Due to the dry winter, we already see that 2013 is bringing huge disparities between vineyards in terms of leaf area development. It means that the impact of practices will be extremely strong on fruit quality at harvest. Winemakers and vineyard managers who have dared to trust vine measurements over a natural tendency to “give a shot of water” during the heat wave should be rewarded once the wine is made. Today we are at a stage where veraison is just around the corner. I actually saw the first pinkish berries on July 1st. Very soon, there will be less obvious benefits attached to vine water deficit and its positive effects on wine quality. The “good” wine is being made now in the vineyard!
In conclusion: Trend of napa valley vintage so far…
This past week was clearly decisive in shaping the vintage. In fact, there should be two kinds of vineyards in 2013. In one hand, the most audacious vintners will have successfully imposed water deficit before veraison with its expected rewards on wine quality. Most likely those vineyards went “dry” through the heat spell. In the other hand, more conservative vintners will have – intentionally or not – delayed the onset of water deficit if unneeded irrigation was applied. Already visually, we can tell that pre-veraison water deficit has reduced berry size. It is too early to speak about flavor intensity. However, once the wine is made, we should also expect more intense and more complex flavors from vineyards where water deficit was imposed early during the season.
In fact, there is a direct relationship between flavor synthesis and water deficit before veraison. It can even be explained at the gene level. This was recently very well demonstrated for Cabernet Sauvignon… but this will be for another discussion.
Please visit Fruition sciences website for more information.