Why Paso Robles is So Well Suited to Late-Ripening Grapes

This morning, when I got back to the winery after a week on the road, my first order of business was to check in on how harvest was going. I was happy to learn that things picked up a bit last week. After more than two weeks of chilly fall weather, it had warmed back up, with eight days of perfect ripening weather: daytime highs between 83°F and 93°F, and lows between 41°F and 51°F.

And still, when I asked Chelsea how she was feeling, she responded, "this is definitely the first October 1st I can remember where we haven't been stressing about tank space."  Although harvest picked up from the glacial pace it was in mid-September, we are still waiting on most of our Marsanne, Roussanne, Mourvedre and Counoise.  Why? Blame the cold nights. Here's Neil, this morning, next to our first pick of Mourvedre. It was 52°F at 8:30am:

We're used to this here, but most of the Mediterranean world is finishing up harvest about now. Beaucastel's Facebook page (for example) shows that they brought in their last fruit on September 29th:

It's not like this year is an outlier for us, either.  Over the last 15 years, we've averaged a last pick off the estate on October 29th, and our earliest-ever finish was October 7th in 2013.  Six times in those 15 years we were still picking in November. 

To explain why grapes take so long to ripen in Paso Robles, I'll have to detour briefly into some basic plant physiology. Bear with me here, or just skip to the end of the bullet points if you'd like the conclusions without the chemistry. There are a number of different processes which limit a grapevine's ability to photosynthesize at low temperatures. These include:

• The tendency of plants to close their stomata (pores in the leaves) in response to cold, limiting respiration and the uptake of CO2
• Carboxylation (sorry for the long, technical term) is the first stage of photosynthesis, whereby CO2 molecules are turned into an acid known as 3-PGA. Carboxylation efficiency declines as temperature declines
• The electron transport capacity of plants is reduced at low temperatures
• An enzyme known as Rubisco, essential to the first step of carbon fixation in photosynthesis, is inefficient at low temperatures

So, in essence, at cold temperatures, plants take in less CO2 and are less efficient in turning the CO2 that they do take in into the starches that fuel both plant growth and fruit ripening.  Grapevine ripening proceeds most efficiently between 30°C and 35°C (86°F and 95°F).  It drops dramatically below 25°C (77°F), and reaches zero at 10°C (50°F).  A summary graph from a technical paper published in Plant, Cell, and the Environment shows the combined effects pretty clearly:

For context, take a look at the temperature curve for the most recent 24 hour period:

You can see that while it did get warm, topping out around 30°C (86°F) yesterday afternoon, it only lasted until sunset just after 6pm.  By 8pm it was already down to 20°C (68°F). It bottomed out at 6.4°C (43.6°F) at 6am and wouldn't rise back up above 20°C until noon today.  So, over the last 24 hours, our vineyard spent 5 daylight hours over the 25°C temperature at which photosynthesis happens efficiently (2pm-6pm yesterday). Five other daylight hours (9am-1pm today) saw temperatures at levels where some photosynthesis can happen. Two daylight hours (7am-8am today) saw no photosynthesis at all because it was too cold.  And for 12 hours the sun was below the horizon. 

We are far from the only, or even the most extreme, location in Paso Robles.  The temperature grid from the Paso Robles Wine Country Alliance shows other areas that dropped near freezing last night.  Most show diurnal temperature swings of 40°-50°F. 

So, what does all this mean? That once you get into the end of the growing season here in Paso Robles, it's hard for grapevines to do too much photosynthesizing. That's a benefit, because you can get the last little bits of ripening on your late-ripening varieties slowly, so they continue to build complexity without accumulating too much sugar.  In general, the longer your grapes can stay on the vine before they get to the ripeness levels you want, the more complexity your wine has.  That's why a generally accepted bit of wine wisdom says that the best examples of different grape varieties can be found at the northern limit of their ripening range. So, the best Sauvignon Blancs tend to come from the Loire, and not Bordeaux. The best Pinot Noir tends to come from Burgundy, and not the Languedoc.  And the best California Chardonnay tends to come from cool coastal pockets where the fog slips in from the Pacific, not from the Central Valley.

Of course, at some point, you do need to get things ripe.  Grapes that don't make it to good ripeness produce wines that are green and bitter: no one's idea of a pleasurable drink. But here too Paso Robles has an advantage: that we don't tend to get our first serious rain until mid-November.  If we need to wait, we wait.

Hopefully, this particular waiting game is over for a while. But if it's not, I'm still confident we'll be OK. Thanks, Paso Robles.