Today, we review some of the aspects of the 2017 growing season in Napa. While Bordeaux suffered some very serious frost events earlier this year, followed by unprecedented heat waves (read more here), Northern California has endured a series of heat waves following an extremely wet winter.
Growing degree day accumulation and plant development stages
In spite of the heat wave which struck earlier in June, 2017 looks very much like 2014, 2015 and 2016 vintages in terms of thermal time or in terms of evaporative effects combining light, wind speed, and vapor pressure deficit as measured by ETref.
The speed at which growing degrees days have accumulated in Napa this season isn’t very different from previous years. Since 2014, each vintage reached similar thermal time scores: almost 700 degrees days by the solstice. Consequently, the cold and wet winter hasn’t detoured the path for 2017. Given the similarity of 2017 to the last 3 vintages in terms of temperature, the dates at which bloom, set and later on veraison and sugar loading occur should look very similar to those past vintages.
The climatic demand on grapevines is also similar. The evapotranspiration, or ETref is also similar to past vintages at this time. Like 2016, the start of this growing season had quite of cloud cover, which lowered the evaporative demand and transpiration rate early in the season.
Early heat wave and effect on plant water use
What marks the 2017 growing season apart from recent vintages is the high frequency of heat waves. (We discussed heat wave and its measurement in terms of vapor pressure deficit using data from the Bordeaux region as an example here).
By looking at the severity of vapor pressure deficit, we can characterize the severity of a heat wave. As figure 1 shows, this high frequency of heat waves in 2017 hasn’t occurred in the past.
Because vine water status at this stage of the season is generally high (particularly in deep rooted vineyard site where large amounts of water during the winter provides a lot of water supply), cavitation in response to heat wave is not the main concern, except maybe on vineyard sites with small root reservoir . For more info: we’ve discussed this here.
Irrigation strategy after heat wave
In terms of water supply (particularly in shallow rooted vineyard sites where amount of plant available water is limited regardless), heat wave can have a tricky effect on vine water use after it is over. During a heat wave event, the rate of transpiration increases markedly. The speed at which water molecules are being pulled through and out of the plant accelerates. Thus, soil water reserves that would have lasted the vine a week or more are used in a few days.
- What to do in practical terms? If plant transpiration rates can increase during a heat wave, then you need not to worry. The rise in transpiration reflects vine ability to access more water to meet vine extra water needs during that heat event.
- Keep in mind: the rate at which water is transpired during a heat wave event is much higher. It means that the level of water available in the root reservoir is also depleted at a faster rate. Thus, following the heat wave, it is wise to check your plant-based transpiration ratio to get an early water deficit warning.
- Anticipating a brutal decline in vine water use, by monitoring a fast decline in plant-based transpiration ratio, will help protect your vines from sudden water deficit symptoms. We have classically observed visual symptoms of water deficit after the heatwave, caused by a more brutal restriction of soil water supply.
Because of the fast temperature accumulation, vineyards are likely to experience a similar timeline for plant and fruit development as in 2014, 2015, and 2016 if weather indexes keep pointing to the same trajectory. However, the high frequency of heat waves justifies a particular attention to transpiration ratios, earlier in the season compared to last year. In situations where root reservoir sizes are limited, the water supply may soon be limited and – paradoxically despite the wet winter-, 2017 may display more severe symptoms of water deficit than last year in some situations.
To be continued.
Our 2017 Seminar Series is underway! If you are a current client and would like to attend one of the upcoming seminars, please let us know at [email protected].