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ST. HELENA — With harvest bringing the grapegrowing season to an end in Napa Valley, the crux of the winemaking process has shifted now to the winery, as grapes come in daily and are crushed in the first steps toward the finished wines of 2018.

But even as activity slows in the vineyards once the year’s fruit has been plucked, growers are already laying the groundwork for the 2019 vintage of Napa wines.

“It’s amazing how much we’re already thinking about 2019 before we’ve even got the crop in,” grapegrower Caleb Mosley said. “But you have to. If you don’t, you’re already behind schedule.”

That preparation today includes tasks like post-harvest irrigation to soothe stressed out vines, preparing new vines to grow in the next season and tackling any growth issues ahead of the next year.

Growers’ decisions made today will affect how a year’s worth of grapes and wines will turn out several vintages down the road. With years’ worth of consequences at stake, growers’ choices can affect long-term differences between grapes even on the same sites, growing in vineyard blocks just steps away from each other.

On a recent afternoon, a group of workers were toiling in the white soil of Howell Mountain at a site perched above the valley, carefully grafting buds to rootstock in a newly replanted vineyard. The site is tended by Michael Wolf Vineyard Services, where Mosley is senior viticulturist.

Most grapevines are essentially two-part plants: a rootstock to start and a varietal like Cabernet Sauvignon, which is grafted onto the rootstock as a bud and then grows into a vine. Rootstocks come in a wide variety of types and can greatly affect how vines will grow and how their fruit eventually turns out.

Moving methodically down a row of thin plants, the workers cut into each rootstock, peeling back the outer layer to place a bud into each cut. The buds are held to the rootstock with rubber bands and the base of each plant is covered with soil. In the months to come, the bud will fuse to the rootstock and begin to grow into a new vine.

By making their grafts now instead of next spring, the team is setting up the vines to begin growing with the new season, instead of having them take a late start in May or June of next year, Mosley said.

“This will put it on the right cycle of growth from the very beginning.”

For large-scale vineyards, grafting this way is impractical as it all must be done by hand. Generally in larger vineyards, growers will use pre-grafted vines.

But for his company, Mosley said, “We just feel like it’s the best way to establish a vineyard.”

In a few years’ time, those vines will be producing Howell Mountain Cabernet Sauvignon growing from the rootstock, which was specifically chosen for its resilience to drought and its ability to thrive in the dry mountain soils.

Rootstock choices like this are made in part to promote healthy vine growth, though across the valley at a hillside site above St. Helena, two side-by-side vineyard blocks grow today out of a decision made more to coax out different tastes from the same site.

Earlier in the day, Mosley walked through the two blocks, which had yet to be harvested. Separated by only a few dozen steps, one block is older, with vines that have been producing for years; the other was replanted only a few years ago.

In replanting the younger block, the company chose to orient its rows to make better use of how sunlight falls on the site, while a winemaker-influenced decision led to the use of a different rootstock for the younger block.

The rootstock chosen for the younger vineyard block tends to make for tighter and heavier clusters of grapes than those in the older block. Packed together, the younger vines’ clusters host berries only about 75 percent the size of the fruit in the older block.

Owing in part to the rootstock choice, at this point in the growing season, the younger block’s Cabernet grapes are further behind than those in the older block just steps away, and were notably less sweet that day, with a taste loaded on acid and gripping tannins.

Why the contrast on the palate? In part, the rootstock for the younger vineyard has trouble mining potassium from the soil, Mosley said.

One of the main sugar-carrying molecules in the plant, potassium helps sugar move from the vine’s leaves and into the fruit, leading to sweeter flavors. Less potassium means slower-moving sugars and later ripening. Add to that, the rootstock generally has a later growth cycle than its neighbor vines.

This comes as a plus for winemakers though, as grapes like these can lend themselves to fresher, more acidic wines. For growers, it means more maintenance and inputs like extra potassium throughout the growing season to get the results they want from the grapes.

But while growers can deliberately shoot for stark differences among the fruit from a single site, in others they are also currently at work sorting out irrigation issues and tricky soil profiles just to get a uniform quality in a vineyard’s grapes next year.

On the same afternoon, Mosley walked through a vineyard tucked into the eastern hills near Calistoga, where work is being done to tackle a water problem ahead of next season.

A small piece of toe slope drifts down from the adjacent mountain and into the far side of the vineyard, depositing tight, sticky clay underneath a patch of vines. With its small soil particles, clay is good at holding water. Too good, in fact, as the vines growing in it struggle to wrest the needed amount of water from the soil as the growing season goes on.

In turn, the vines in this patch of the vineyard remain parched year after year and ultimately yield markedly less fruit than the rest of the vines just feet away.

Because the majority of the vines sit on more productive soils and are able to take in the amount of water they need, the drier vines in the clay portion pose an irrigation conundrum.

Normally, every vine is fed by one drip line that, when turned on, irrigates every vine on the line at the same time. But turning on the drip line repeatedly just for the vines that need more irrigation would be a waste of the water falling on already healthy vines. Extra water could also push healthy vines to grow more than needed, potentially taking a toll on the overall quality of the fruit.

“You wouldn’t want to irrigate just to appease that small amount of vines there, blowing the quality that you have locked in here,” Mosley said.

But stressed vines can’t be ignored. The lack of water has the grapes hanging on the dehydrated vines at half the size today of those in the rest of the vineyard. In turn, the affected fruit is far more tannic and acidic. For winemakers, picking all of the fruit at once from a site like this, with portions showing dramatically different grape health, would result in disjointed flavors throughout the eventual wine.

The fix: a second drip line, running alongside the first, with small emitters that will let water out only in the areas of vines that need the extra water.

Double drip line systems like these are a newer concept, Mosley said. “But it’s catching on in Napa, especially in the higher end vineyards to make sure that every vine is reaching its full potential.”

Though the problem has existed for years, solutions in any vineyard naturally come down to the economics involved, Mosley said.

“Everything that you want to do costs money. You’ve got to make sure that you have a well-defined plan, you know what you’re going to do, and we’re going to put it into the budget next year. We’ll know the exact number of vines we’re going to put on the system, we’ll know the exact amount of drip tube; make sure there’s no surprises.”

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Wine Reporter / Copy Editor

Henry Lutz covers the local wine industry. He has been a reporter and copy editor for the Register since 2016.