When the mercury spikes for multiple days in a row, all living things can be stressed, including winegrape vines. Luckily many varieties available to grape growers are capable of dealing with this heat, but that doesn’t mean there aren’t steps you can take to give your vines a leg up. We got advice from two professional winemakers, from two different climates, about dealing with the heat.
Director of Winemaking: Sergio Cuadra, Fall Creek Vineyards, Tow, Texas
By growing wine grapes in the Texas Hill Country, where Tempranillo or Chardonnay are harvested by the end of July or beginning of August, we should be experts in the impact of high temperatures on our vineyards. Paradoxically, despite scorching temperatures during late spring and summer time we don’t have to do anything in particular to prepare or protect plants against the heat. In fact, by the time of harvest, which is pretty much during August, plants show nice varietal green colors, turgid clusters and good-looking berries, and no heat-related symptoms whatsoever. This phenomenon got my attention almost six vintages ago when I came from my home country, Chile.
To keep it brief, let me say that vines (and a vast number of other plant species) have certain genes called heat shock genes (HSG) that stay unexpressed when temperatures are below a certain threshold which has been set at about 100 °F (38 °C). The Texas Hill Country starts having these high temps during the spring, so these HSG do get activated or expressed into heat shock proteins (HSP) that are put in place to protect other vital molecules that would get damage otherwise. Now, when the HSP are not present, vines are vulnerable to the next heat wave, especially if it’s sudden, as is the case in cooler areas.
Having said that, there are a few considerations to keep in your toolbox when confronting potential temperature spikes in cool, or cooler climates.
Those vines behind in their growth from the rest are usually going to suffer the most from excess heat. So try to bring them up to schedule assuring them good water supply and fertilization as early as possible. The first heat wave of the season is often more damaging than any subsequent ones and this is due to what I explained earlier. With enough stimulus, the HSG kick in and plants get sort of “vaccinated” against future temperature rises.
Now, it’s not so much the heat itself as the vapor pressure deficit (VPD) that skyrockets during a heat wave which causes the damage. In other words, the atmospheric water demand (this is how quickly things get air dried, a combination of temperature, radiation, relative moisture, and wind) may get so high the plant is physically incapable of supply and shuts its stomata as a result. Hence, the tissue ceases to cool itself down, increasing the risk of getting overheated. If this lasts for too long we get heat damage, like burned leaves or scalded berries. Severe heat can be mitigated by preparing the plants and the microenvironment (every bit of relative moisture helps) with a good supply of water. If plants are going to close stomata let that not be due to lack of water but due to a high and unmanageable VPD.
Finally, I’ve noticed clusters that by circumstances are exposed to direct sunlight the entire season are less likely to see heat-related damage than those in the canopy, which reinforces the importance of an early canopy intervention, if needed, rather than late.
Consulting Winemaker: Wes Hagen, J. Wilkes Wines, Santa Maria, California
I start to get concerned days before the heat hits. I like to apply a very deep, long irrigation before a heat spike a few days before the heat with clay soil. If I were working with sandier soil, I’d begin the irrigation the day or night before and irrigate in the nights between hot days. Maximum photosynthetic capacity is 87 °F (31 °C), while vines shut down at 102 °F (39 °C) or so. Vines actually self-regulate quite well in heat, but having water at root level is important to them.
Younger vines are more susceptible to heat stress than mature vines. Keeping on top of irrigation for young vines is more important than mature vines during these weather events. As for yearly growth cycles, I don’t worry about heat too much before veraison, but at the end of the ripening, it can really impact a vintage by sending sugars soaring without the matching physiological ripeness. This can provide a raisin sweetness to the finished wine that often tastes one-dimensional.
I find that proper leaf management can be key when dealing with heat problems and grape clusters. Make sure the clusters are shaded by leaves in the middle of the day’s heat (don’t pull too many leaves); apply water in advance of a heat spike and each night. If you find it is harvest time and heat persists, I would suggest picking at night with headlamps. This is going to keep from bringing the fruit in hot, skins split, and fermentation starting.
When it comes to actual physical labor in the vineyard: Cover up — don’t farm in shorts and a tank top. Take a page from California vineyard workers and cover up, sweat a little — you’ll be cooler and won’t get sunburned. Also find a source of dry ice for the times you have to bring in warm/hot grapes. Drink lots of water or sports drinks before, during, and after vineyard work in the heat, and recognize that dizziness, etc. means it’s time to find a cool room and a cool drink of water.