Preparing for a potentially historic California harvest heat wave
August 31, 2022
So far this harvest season, the weather has been just about perfect. It's been warm (average high temp 93.5°F) but not too warm (highest high just 102.8°F, and just 48 hours over 95°F). Nights have been chilly (average low temp 54.7°F) but not too chilly (lowest low temp 47.7°F). Each warm stretch has been followed by a cool-off. You can get a sense of this by looking at the daily temperature ranges since August 1st:
All that moderation is exactly what you want for your vines as they make that final push toward harvest. Unfortunately, it looks like we're getting something rather different starting tomorrow. A heat wave is coming, and it's a big one. The New York Times is calling it "brutal", the Washington Post is calling it "record-threatening", and the San Francisco Chronicle is calling it "dangerously hot". Here in Paso Robles, our forecast is for a string of days that might touch 110°F:
Many parts of California are looking at temperatures that will exceed seasonal norms by 12-14°C (22-25°F) in what is already a warm time of year. The European Centre for Medium-Range Weather Forecast (ECMWF) gives a great graphical depiction just how widespread the temperature anomaly will be across the western United States. A shout out to Daniel Swain and his blog Weather West, where I found this image, and which I consider required reading for anyone interested in California weather or climate:
A couple of mitigating factors give me some hope that we might escape the heat wave's worst effects. First, it's not quite as hot out here as in the town of Paso Robles. So if the forecast for town is 110°F, it's more likely to be 106°F or 107°F here. Those few degrees help. Second, the vineyard looks really healthy this year. That vigor is going to be put to the test in coming days, but at least we're starting from a good position. Third, it looks like the nights are supposed to cool off. Even at the height of the high pressure ridge, it looks like the town of Paso Robles is going to drop into the 60s. Out here it's usually 3-5°F cooler at night than in town. So the vines will still have a chance to refresh themselves a bit at night. And finally, it's late enough in the year that we have somewhat shorter days and longer nights than we would if it were mid-summer. The twelve hours and 57 minutes between sunrise and sunset is an hour and 38 minutes less heating time than we would have had on our longest day of the year.
On the other side of the ledger, we're in our third year of drought here in California, it's late enough in the growing season that even in a normal year we'd expect the top several feet of soil to be dry and the vines to be under high stress, and we had another heat event in July that already resulted in some losses.
However it shakes out, it's going to be a challenge, and our tools to deal with the heat are limited. The best option is to bring anything that's ready or nearly so into the cellar and get them into tanks or barrels where the outside temperature doesn't matter. Over the last two days, we've brought in nearly 47 tons of grapes, or roughly 10% of what we expect to do this entire harvest season, including these Grenache Blanc bins on our crushpad today:
We'll be continuing to bring in grapes through the wave, starting early in the pre-dawn hours with light towers and headlamps when it's still cool and continuing until probably only around 10am, when temperatures get high enough that we worry about the effects of oxidation on the newly harvested clusters. A big piece of what's coming in next will be Syrah, which is ready to go and looking amazing:
But there are grapes that are still a long way from being ready. Heck, there are grapes like Counoise that haven't even finished veraison. Harvesting those isn't a viable option, and it's important to remember that even with this week's push we're still only about a third of the way through harvest. So we've been doing something you rarely see after veraison and turning on our irrigation lines in our most heat-sensitive varieties, trying to give them the reserves they need to withstand the heat. I posted about this on Twitter today:
Not something you see every day… irrigation on with red grapes on the vines. But as we approach what looks like a potentially historic heat wave, it’s one of the only tools we have at @TablasCreek to help the vines weather the stress. 🤞 that it works. pic.twitter.com/RAiWcM9y10— Jason Haas (@jasonchaas) August 31, 2022
We're trying to avoid raisining, where the vines activate a self-defense mechanism by pulling the liquid out of their berries and using it to replace what they're losing to evaporation and photosynthesis. The resulting hard, sour raisins won't reinflate, costing us production. This Mourvedre cluster shows both that there are grapes still finishing veraison and a few of these premature raisins, the result of an earlier heat wave in mid-July that caused modest losses in Mourvedre:
Those two options are pretty much our entire toolkit, at least in the short term. And, of course, we don't even have one of those options on the 40% of our vineyard without irrigation infrastructure. In the longer term, the farming choices we make can help build the vines' resilience to heat and drought. Focusing on dry-farming builds deeper root systems, which have more reliable access to water and are less impacted by what's going on at the surface. Regenerative farming helps build the organic content of our soils, which then hold more moisture. And Biodynamics (along with the regenerative practices) produces more robust vines that have greater reserves to draw from.
Fingers crossed, please, that it's enough.