The Pacific Northwest has been under an unusual heat spell over the past 14 days. Portland is seeing peak temperatures between 90 and 100F and eastern Washington is passing 105F every day. For late June/early July those are unheard-of temperatures.
Along with the high peak temperatures come warmer nights than usual (above 70F in eastern Washington) and no rainfall. In fact, The Oregonian reported that June in Portland was the warmest on record, with the warmest overnight low ever recorded and currently 27 consecutive days without rain and likely more to come. The chart below shows cumulative Growing Degree Days in eastern Washington (June 28th end) where every daily average degree above 50F are accumulated. It usually gives a good picture of how much heat vines are getting. Heat is important because it speeds up vine growth and ripening, it increases evapotranspiration of plants (thus requiring more water), and combined with high sunlight intensity, it can burn canopies and grape clusters. One can clearly see on the chart below that we are getting plenty of heat; 2014 (the hottest season on record) ended at 3,082 GDD and we are currently running 221 degrees year to date above 2014. So is that good or bad? I would say overall it could be bad but we won’t know for sure for a few weeks – why?
My first concern with excessive heat is to have a large disconnect between technical ripeness (sugar and acidity) and flavor ripeness. Heat speeds up sugar accumulation in berries (from faster photosynthesis) and reduces the amount of acidity – so one might expect this vintage to have high sugar and low acid sooner in the growing season. On the other hand, most flavor development is time-dependent with long hang time favoring riper and more intense flavors. For those reasons we could expect that grapes might be technically ripe but with little flavor, forcing a compromise between sugar, acid, and flavors. But we won’t know that for some time because this heat is pretty early in the game. Also the heat is so extreme that the vine actually shuts down above 95F making any heat above 95F unimportant for GDD comparison (I would probably take it out of the calculation if I would have my say). So perhaps ripeness will be disconnected – perhaps not.
My second concern is the hydric stress the heat is putting on the vines. The plant’s evapotranspiration (ETP) is very high at 95F and even if stomatas are closing above 95F there is still much ETP going on. The closing of the stomata trigger the vines to shut down as discussed above. The demand for water can be so great that some vineyards might not be able to provide the water on time, resulting in weak vines. Weak vines can have small berries, small canopies, and will be more susceptible to disease later in the growing season. While this might be good for reds, it is not conducive to quality white wines. I am also worried about the Yakima Valley running out of water toward the end of summer, which would impact the amount of water available for the winter and the 2016 growing season.
My third concern is sunburn. Too much sun on berries can burn the skin and the skin is where most flavors are located. Additionally, too much sun exposure can result in thicker skins for whites, which always means higher tannin content in white juices. There is also ample evidence that while a little sun improves terpene (think lilac) content in Riesling, too much leads to precursors of TDN (which smells like petroleum). So generally, a warm year means high levels of tannins, lower flavors, and potential TDN – hard conditions for drier styles of Rieslings. What we don’t know is if the early heat will trigger all of this or not. Again, we need to wait and see.
The positive side of this heat business is the lack of disease in vineyards.
We all hope for a bit of a reprieve in the heat here and we have to start thinking about the potential problems associated with this crazy weather — be they ripeness issues, long term impact of hydric stress or extreme sunlight, and heat impact of berries.