Predicting the onset of budbreak, bloom, and other phenological stages, is a very active topic of research. Recently, new results regarding the effect of winter temperature at hastening the onset of budbreak have been reported.
Parker et al (2013) have suggested that the effect of heat accumulation on vine phenology could be more accurately described by “initializing” the temperature clock on March 1st (in the northern hemisphere) and also by lowering the minimal temperature threshold from 10 degrees C to 0 degrees C. According to this calendar, a “late” varietal like Cabernet sauvignon will reach flowering around 1300-1350 degrees days; whereas an “early” varietal like Chardonnay will reach the same stage around 1220-1250 degrees days.
“We need to look at extreme temperatures to understand the precocity of the season”
To understand the precocity of the season, we actually need to look at extreme temperatures (minimum and maximum) during wintertime. 2015 thermal time profile shows not only more heat accumulated, but also warmer nights and days. This in turn affects vine phenology.
Zapata et al (2015) have shown that base temperature (ie. the minimal temperature above which vine “wakes up” and starts a new cycle) is varietal specific. It appears that a Chardonnay “wakes up” when temperature is at or above 8ºC while a Cabernet Sauvignon will start to “wake up” for base temperature at or above 13ºC. These findings gives a better insight to explain why we observe up to one month of difference in budbreak date according to varietal.
So what’s next?
After a very cool month of May, thermal time is now closer to the 2014 heat accumulation pattern, even lagging behind 2014 in some California sites. That is certainly giving us a little break and it stretches the length of the period for bloom and set. Across Napa, we see a lot of sites reaching their peak leaf area. It is now time to watch carefully your plant transpiration and the gradual apparition of water deficit…
NB: for those who want to dig further on Spring weather, you can read this interesting article from Greg Jones in Wine Business Monthly