Sunburn and silver linings: Taking stock after 2024's first heat wave.

The cool start to the growing season feels like eons ago. For the last three weeks, since June 21st, we've been in a classic Paso Robles summer heat wave. It didn't start out all that dramatic; between June 21st and June 30th the average high temperatures were about 93°F. That was a lot warmer than the first two-thirds of the month (average high 86.5°F) but only about three degrees above average for the season. But since the calendar flipped to July, it's been scorching. The average high in July so far has been 104.4°F, with nine of the eleven days topping the century mark and a maximum high of 111.9°F on July 6th. That average is nearly 13 degrees warmer than normal in what is already one of our hottest seasons. The full picture to date:

Grapevines evolved in a hot, dry climate, and do pretty well up to about 100°F. But above that there start to be consequences. The biggest of these is sunburn. If it's really hot for an extended stretch, exposed grape clusters can suffer direct damage as cell membranes break down, compromising a berry's skin and allowing the liquid inside to evaporate away. The result is hard, brown, sour raisins, as in this Marsanne cluster:

The temperatures required to cause this sort of cellular damage in grapes is typically around 125°F. Even in a Paso Robles heat wave we don't ever see ambient temperatures this hot. But fruit that's exposed to the sun can see temperatures 20°F-25°F higher than the ambient air. So, when the temperatures top 100°F, we start to be at risk.

It might seem straightforward to avoid sunburn by leaving more canopy arching over the grape clusters. And we try to do that. But this desire is at odds with what we need to do to keep mildew pressures at bay. In June, we're typically working to remove any lateral shoots and tuck the canes in our trellised blocks up into the wires, all to open up the fruit zone to the free flow of light and air and the lower mildew pressures that come with good air circulation. So it's always a balance between addressing these two risks. For the same reason, we typically orient our vineyard rows in a north-south axis, perpendicular to the prevailing west-east winds. That also allows all the clusters to get some sun exposure, which is good for ripening. In a heat wave, though, this means that the clusters on the west side of a row, which are exposed to sun in the afternoon when it's hotter, are at more risk of sunburn than those on the east side of the rows. These next two photos are taken in the same Grenache block but on opposite sides of a row. First, the east side, which looks great:

Unfortunately, the west side of the row doesn't look as happy, with sunburned berries on the outside:

The impact of the sunburn is uneven across different varieties. Grapes with more vigor, longer canes, and more sprawling shape (think Vermentino, or Bourboulenc, or Syrah) are mostly fine. I'd estimate that we're seeing less than 5% sunburn damage in those varieties of grape. Grapes with less canopy, shorter canes, and less vigor (think Counoise or Mourvedre) show more damage, particularly in younger blocks with less well-developed cordons. In some younger blocks I saw as much as 50% loss from sunburn, though overall across those grape varieties I'd estimate it's more like 25%.  And then there are the grapes like Marsanne, Grenache, Roussanne, and others who sit somewhere in between in growth pattern and damage. Overall, across the vineyard I'd estimate something like a 15% loss.

Unfortunately, the tools that we have to deal with sunburn are limited. There are some vineyards that are experimenting with widespread misting to bring temperatures down in peak heat times, but the infrastructure is expensive and it uses a lot of water. Others put shadecloth out to protect the fruit zones, but that's cumbersome, needs to be replaced every couple of years, and creates a ton of plastic waste that goes straight into the landfill. If it were later in the year, we could turn on our irrigation, and we've done that in some of our most stress-prone blocks, but that's more to stave off any later loss of vigor than it is a way to address the temperature of grape clusters exposed to the sun.

In terms of future options, we've talked about redeveloping vineyards along an east-west axis instead of north-south, so that the fruit zone is always shaded by the canopy. We've also talked about moving our rows farther apart from the 8 feet they are currently, so we can leave the canopies to sprawl more without worrying that they'll start to interconnect and make it difficult to get in and see what's going on. But those changes will have to wait for new plantings. Meanwhile we'll have to make do with half-measures like perhaps only tucking the canes on the east side of a row and letting those on the west side sprawl and provide more shade.

There are three pieces of good news. The first is that we're starting from a position of health. Thanks to two consecutive years of good rain, the vineyard looks healthy, and the canopies are still green and lush. I drove around almost the entire vineyard today and the vines don't look ragged or otherwise stressed, and everything is still a vibrant green:

The second piece of good news is that sunburn at this time of year doesn't typically have a negative impact on fruit quality and can even be an asset. Quantity will be negatively affected, sure. But in terms of quality a bit of sunburn can act like a fruit-thinning pass, increasing the intensity of what remains. And finally, the third piece of good news is that we had some quantity to spare. I spoke to Viticulturist Jordan Lonborg yesterday and he was confident that even with the losses we were going to see a healthy crop above last year's levels. He was even thinking that this might save a fruit-thinning pass and the labor costs that would entail.

I'll leave you with one photo, so as not to have your lasting impression be that this was a disaster. Here's a Grenache block, photographed from below. This is the west side of the vine row, and you can see the clusters ripening, green and plump, while the intense green of the canopy duels with an even bluer sky:

This heat wave is forecast to break tomorrow, and next week to have temperatures average or below. We'll all be grateful. Next stop: veraison.