The vineyard and harvest impacts -- positive and negative -- of our unusual September rain

It's definitely been a month with interesting weather. Just three weeks ago I was writing a blog Preparing for a potentially historic California harvest heat wave. And that heat wave was intense, with ten days in a row over 100°F, though we feel like we escaped the worst impacts that were felt in some of the regions to our north. Now, a week after that heat wave broke, we got our first real rain of the 2022-23 winter, and not from some tropical monsoonal moisture that just wandered a little far north, as we sometimes see in late summer here. No, we got a real winter storm plunging out of the Gulf of Alaska, bringing significant rain to the northern two-thirds of California:

More spectacular satellite imagery of unusually early season low pressure system spinning just off CA coast & spiraling bands of showers, with isolated embedded thunderstorms, across NorCal. This low will move little through Tuesday, with showers/iso. t-storms continuing. #CAwx https://t.co/1JUIeQZYOH pic.twitter.com/FX2pWe3acf

— Daniel Swain (@Weather_West) September 18, 2022

While harvest rain is rare here in Paso Robles, it's not unheard of. In the last two decades, the only other year we received September rain was in 2010, but we got rain before the end of October in 2004, 2006, 2021, and most notably 2009, when we received nearly ten inches on October 13th. And it's more common in other wine-growing regions around the world; Chateauneuf-du-Pape, for example, has harvest rain most years, as September and October are the two rainiest months of the year. Walking around the vineyard this morning shows a very non-Californian scene, with mist in the air and droplets of water on the leaves and clusters of the vines that still have fruit. Check out Mourvedre (left) and Counoise (right):

It's not like we got an ocean of rain; we got about a half inch overnight, with potentially another quarter-inch coming today. But the whole experience is rare enough that we've been getting tons of questions about what we expect, and we assembled this morning to discuss what's likely to happen. So I thought it might be helpful if I broke down our risks.

• Dilution. When grapevines get rain late in the growing season, some of that water is pumped into the berries. That reduces both the percentage of sugar and the percentage of acid in the grapes, at least temporarily. As long as you get dry, warm weather after, that dilution is usually short-lived. And given how dry the summer has been, it's not always a bad thing. If you were following us during the heat wave, you know that we pre-emptively gave some water to our most heat-sensitive blocks, figuring that the minor amount of dilution, if it happened, was better than the alternative of having the vines shut down or even pull water out of the berries, creating hard, unripe raisins. Risk of this being an issue: low.
• Rot. Fungus thrives in moisture. Luckily, most of the time during the growing season here, moisture is in short supply. But when it rains it offers any available rot spores a chance to grow and spread. Key to whether or not this happens is what comes after the rain. If it turns dry and you get a breeze, the moisture on the clusters is gone before any rot has a chance to take hold. If not, and particularly if it gets warm and stays humid, things can go south in a hurry. Looking at the forecast suggests that while today will remain cool, overcast, and showery, tomorrow and especially Wednesday look clear, dry, and breezy. That's good. On the flip side, the clusters and berries have already taken some damage from the heat, and are starting to soften, offering more opportunities for rot spores to get inside the grapes than you'd normally see this time of year. Risk of this being an issue: moderate.
• Lack of access. Finally, an issue with wet weather is that it makes it difficult and messy to get tractors into the vineyard. Most times of year that's just an inconvenience; if you can't get into the vineyard to prune, or spread compost, or weed, it's usually not a big deal to wait a few days or even a few weeks. But during harvest, access is more important as grape chemistry can change on a daily basis, and if you can't get into the vineyard to pick you run the risk of missing your window. Mitigating this likelihood is that the grapes don't ripen particularly fast in this weather, and the roughly half-inch of rain isn't enough to saturate the deeper layers of the soil. A few days of dry, breezy weather and this won't be a problem. Risk of this being an issue: low.

 The impacts of the rain aren't all negative. In fact, there are some real positives. They include:

• Helping the harvested vineyard blocks store energy for dormancy. While we do have some worries about the fruit still hanging, the impact of this rain on the two-thirds of the vineyard that has already been picked is nothing but positive. Those vines have expended a lot of resources getting fruit ripe over the past five months. We still have a couple of months of photosynthesis to go before they go dormant with the first hard freeze. It's normal for us to go through after we've finished picking and give the blocks that we can irrigate a bit of water anyway, to help them continue to photosynthesize and store up resources for next year. This rain does that for us, and more broadly than we ever could. 
• Giving the grapevines with fruit still hanging the energy to make a final push. We've often seen when we've gotten early rain that after a few days where numbers (sugar and acid concentrations) retreat due to dilution, the vines then appear to get something of a second wind and make more progress toward ripeness than they had been doing before the rain. It's probably intuitive as to why. These vines have been pushing hard, with their most limited resource being water. Giving them the water that they need helps them photosynthesize more productively, and that photosynthesis translates into ripening.  
• Cleaning the dust off the vines, clusters, and roads. It's been a long, hot, dusty summer. We've been running water trucks from our wetland area over the vineyard roads, trying to keep the dust down so that it doesn't settle on the vines (reducing photosynthesis in the same way in would on a home solar panel) and clusters (where it ultimately ends up settling to the bottom of tanks and barrels as a part of the lees). But while it's impractical to spray off the whole vineyard, that's what this rainstorm effectively did. Hey, we'll take it.

So while the rain isn't an uncomplicated boon, neither is it an unmitigated disaster. What happens over the next few days will determine whether we dodge the risk of rot, or whether we need to redouble our efforts and get the remaining grapes off the vines as fast as we can. If we did have to pick fast, at least everything is pretty much ready. The biggest issue would be space in the cellar, and we're trying to make good use of this break to press off tanks that have reached sufficient extraction to make room for what's coming. But if we get good weather later this week, this could actually end up being a good thing. Please keep your fingers crossed for dry and breezy starting tomorrow.

Meanwhile, we'll enjoy the unexpected misty respite in what's usually a hot, dusty season. I'll leave you with two last photos so you can enjoy it, too.