As season goes by and the first berries enter veraison, 2015 promises to be a very hot and dry vintage. Despite a late bud break early April, the 2015 season is now turning into a very early vintage, catching up with 2011. This catch up is explained by temperatures in May, June, and July, which are above the seasonal normal and in fact the warmest ever recorded by weather stations. Indeed, looking at the latest vintages, the average temperature in Bordeaux since May is 19.3°C in 2015, versus 17.9°C in 2014, 17°C in 2013, 18.6°C in 2011 and 18°C in 2010.
As of July 21, 2015, we recorded 7 days in advance in thermal time (degree day accumulation in base 10, calculated since April 1) in the Médoc, 10 days in Léognan and 12 days in the Libourne compared to last year.
After a relatively rainy winter -except in the Libourne where rainfall was lower (300mm)- , we are currently experiencing a second period of drought. The first period took place between May 15 and June 10, stopped by the stormy episodes of June 10, 11 and 12. The second period started right after these storms. It is remarkable to note that it rained less than 5mm since July 1st while heat waves crossed the Gironde.
From our experience and the literature, we call “heat wave” a climatic event which generates a water Vapor Pressure Deficit (VPD) greater than 3,5kPa. To recap, the VPD is the pressure that must be exerted on a volume of air to extract the first drop of water. The hotter and drier the air, the higher the VPD. When the VPD exceeds 3,5kPa, it causes water loss in leaves and fruit to the atmosphere, even under conditions of stomatal closure. When the soil is still rich in water, this kind of event is not worrisome for the plant. When soil moisture conditions are limiting (ex: June 26, 2011, when VPD reached 5.2 kPa in Libourne), there may be a sudden onset of leaf burn or later in the season we may observe some maturity blockages.
To date, the consequences of these successive episodes (8 days of heat waves in Bordeaux) are to be monitored carefully, especially with vines that have a limited root system or limited water holding capacity. Indeed, intense heat waves combined with drying soils may cause significant vine water deficits. The Merlot planted on filtering soils (gravels, sands) suffer more from the lack of water and show predawn leaf water potentials below -5 bars on some areas. The Merlot on clay or Cabernet-Sauvignon grapes (an isohydric varietal that is more resilient to drought) tolerate this period much better . In all cases, a rain in the near future would be beneficial to promote ripening in good conditions and a good start of maturation.