Grapes and Heat

Published August 9, 2017

We are going though our first major heat wave of the summer, with triple-digit temperatures for 3 or 4 days in a row. I haven’t found very many Oregonians who like heat waves, but the discomfort does give us something to talk about.
I very much appreciate people’s concern about how the grapes are doing in these conditions, so thank you to all who ask. But you can rest easy knowing that grapevines are really tough. They go into a kind of low-activity mode as a response to excessive heat. They don’t grow new tissue, and they don’t work on ripening the crop. I can totally relate.
This little delay will affect the harvest schedule, but we had such ideal weather in July that I’m not worried about it. Ask me in late September though.

One problem caused by these heat waves is sunburn on the clusters. It usually shows up on the west side of the vines where the still-green grape berries are exposed to afternoon sun. Once sun-burnt, the berries shrivel up and turn brown and crispy. Kind of like a cross between Raisins and Grape Nuts cereal. Unfortunately, they stay attached to the cluster so we have to sort them out in the winery. We will know soon how much sunburn happened.
One of the vine management practices commonly used in Oregon is ‘leaf pulling’, whereby leaves are removed from the area where the grape clusters are hanging. Most farmers only remove leaves on the east side of the vine, because the west side needs the leaves for sun burn protection.

Sun burn problems show up in less-vigorous clones. For example, we get virtually no sun burn in the huge jungley Pommard vines, but have to worry about the 777 vines because they have less foliage.

There is one good thing to say about heat waves: they kill Powdery Mildew. Grapes are very susceptible to infection with Powdery Mildew. All grape farmers spray during the growing season to control this fungus, but it is nearly impossible to have complete spray coverage, so we always have a little of it somewhere in the vineyard. When mildew gets into green clusters, they are pretty much ruined. But Powdery Mildew dies above 95 degrees F. It only takes 15 minutes above 95 to die, though. So the perfect heat wave would be 16 minutes long.

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