Feeling Hot, Hot, Hot: Sizzling Summer Heat & Its Effects on Wine


Ole. Ole. Ole. Ole.The Merrymen had it right when they said it was feeling hot, hot, hot. The 2021 growing season has been off to a good start. The vines are looking good with a nice fruit set and canopy development, albeit warm. Like June, July started off in the triple digits at our Walla Walla winery and this heatwave is just getting started.

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Few living things enjoy being in the triple digits for an extended period of time, and that includes grapes. Normally at this point in the growing season we want to stress the plants a little bit to reduce berry size while the berries are going through cell division, but during a heatwave like what’s going on right now, we want to be careful about stressing the plant out entirely.

Excessive heat for extended periods can be detrimental to grape development as it can cause heat stress where the plants get dehydrated. And, heat stress can lead to water stress. If a vine has access to water, it will continue to respire which is when the glucose sugar produced during photosynthesis combines with oxygen to produce usable energy to fuel the growth of the vine. If there isn’t enough water, respiration ceases which causes the cessation of photosynthesis and delays fruit development.

When photosynthesis stops, we can see a hindrance of berry growth, as well as delay the accumulation of sugar and development of phenolics, or the flavor and tannins in a grape. Ultimately, it’s delaying the entire growth process of the vine and can lead to a delay in harvest in September.

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So if it’s so hot, what can we do to fight it?

Right now, our Washington viticultural crews are making sure things don’t get too dry at all the vineyard sites we source from to avoid grape dehydration, which could start breaking down the acids in the grapes.

We’re also managing the canopy to avoid things like sunburn on the grapes, which will affect the color and phenolic development of the fruit and can make the grapes more susceptible to disease. We also want to prevent the grapes from shriveling in the sun because at that point raisined grapes are useless to us during harvest.

With proper canopy management, we can use the vines' leaves to act as our umbrellas to produce shade for the grape clusters. The berries of the grapes don’t like to be over 90 degrees, so the canopy can help shade the fruit creating a little microclimate, but the canopy still needs to act as solar panels by letting in some dappled sunlight for photosynthesis. Many of the viticulturalists and vineyard crews across the state are manipulating the canopy of our blocks of grapes to create more shade by lowering the trellising wires.

According to winemaker Chuck Reininger, all vineyard sites we source from for our REININGER, Helix and CPR brands are looking fine right now. They have good canopy development and decent fruit set. Depending on the vineyard site and the varietal, we can see some great things out of hot weather right now, like low yields and a higher concentration of flavors and tannins in the small berries. We just need to carefully monitor water supply these next couple weeks to make sure things still stay on track for another fantastic harvest this fall.

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