On a warm day in late June, Bob Schiebelhut walks between the rows of wine grapes at Tolosa Winery’s Edna Valley vineyards just south of San Luis Obispo.
The dry soil crunches beneath Schiebelhut’s shoes as he bends over to pick up a small grape seedling that is waiting to be planted.
Although it’s a little late in the season to be putting new vines in the ground — supply chain issues set him back several weeks — Schiebelhut has high hopes that the seedlings he’s planting now will produce high-quality wine grapes in a few years.
He knows that the varietal, pinot noir, tends to grow well in the valley’s mild climate. And he expects the rootstock, or root system, onto which the pinot vine is grafted will hold up against diseases and pests and navigate the various soils on his property.
Diseases, pests and soil type aren’t the only factors Schiebelhut has in mind. He also selected this particular rootstock for its tolerance to drought conditions.
“We have to make sure our (groundwater) basin can become sustainable,” Schiebelhut said. “Right now we’re in overdraft.
“So if we cut our water usage, which we will be doing, especially since the other guys are replanting their vineyards with the same rootstock — we’re going to make a real impact.”
SLO County wineries see less annual rain for grapes
Schiebelhut’s vineyard sits above the San Luis Obispo Valley groundwater basin, which has seen water levels decline in recent decades due to overpumping.
Like many other winegrowers in San Luis Obispo County, Schiebelhut knows that bringing the basin into balance is necessary for the survival of his crop and business.
It’s also important for his personal drinking source. His house is just down the street from Tolosa’s vineyards.
“The wine grower industry in general ... has been trying to find any and all paths to water conservation,” said Matt Turrentine with Grapevine Capital Partners in San Luis Obispo. He’s a board member of the Shandon-San Juan Water District.
Rain gauges in Paso Robles and at Cal Poly in San Luis Obispo both show that average annual rainfall has steadily trended downward over the past 50 years.
San Luis Obispo used to receive roughly 25 inches of rain annually, according to data from the university. Now it’s lucky to receive 20 inches.
Wine growers in the Paso Robles area once depended on getting a little more than 15 inches of precipitation a year, according to the California Department of Water Resources data. Lately, they’ve had to make do with an average of about 12 inches annually.
That means growers must look for grape rootstocks that are not only resistant to diseases and pests, but also can tolerate decreasing rainfall.
Growers are typically lucky to get 20 to 30 years of good harvests off one wine grape plant, so many have dug up their old vines and replaced them in recent years.
When replanting, growers select certain grape varietals depending on the wine they want to produce — from pinot noir and petit sirah to chardonnay and cabernet sauvignon.
Everything above the ground is considered the scion.
The rootstock — essentially everything below ground — doesn’t grow fruit. Instead, that part of the plant specially bred to withstand certain soil conditions so that the scion can produce good grapes.
Growers graft the scion and rootstock together by taking cuttings of each, making notches so the cuttings can be pushed together then sealing the juncture with tape or wax. The two sections grow together as the rootstock develops roots and feeds nutrients to the canopy and fruit.
One rootstock that’s popular with local growers is 1103 Paulsen, or 1103P, which was reportedly first bred in the late 1890s by a grower in Sicily, Italy.
Though other rootstocks bred around the same era and in the decades following can be more resistant to drought, 1103P reigns supreme in California because of its ability to grow in unfriendly soils and resist many diseases and pests, all while tolerating dry conditions and producing stellar wine grapes.
“(1103P has) always been a popular rootstock, but ... in the last 10 years, it’s become a huge majority of our sales,” said Andrew Jones, vice president of sales for Bakersfield’s Sunridge Nurseries. It’s one of the largest grapevine nurseries in the nation.
Jones noted that 1103P doesn’t do well in all regions.
In wetter regions, the rootstock will create a vine that’s too “vigorous,” meaning it concentrates too much its energy on developing a robust leaf canopy instead of just producing grapes, he said.
In mild areas such as much of San Luis Obispo County, it performs quite well, Jones added.
Many growers are hopeful the 1103P rootstock will save them water because it needs less irrigation to produce a good crop of grapes.
Researchers work to develop better drought-tolerant rootstock
The 1103P rootstock is drought tolerant because of the way it responds to wet and dry conditions differently from other rootstocks said Megan Bartlett, a researcher and viticulture and enology assistant professor at UC Davis.
It’s likely able to protect itself from shriveling up during dry periods and it may grow extra quickly after even the slightest rainfall, according to Bartlett. These traits have become so popular and necessary for winegrowers that Bartlett and others at UC Davis are searching for the exact genes that make 1103P able to grow well in dry conditions.
“We’re still at the stage of, ‘OK, what seems to be associated with drought tolerance or drought sensitivity?’ ” she said, adding that the research is still in the early stages.
Bartlett said the end goal is to find which genes in rootstocks give them drought tolerance. Researchers could then breed rootstocks with the precise genes needed to create a more drought-tolerant plant than 1103P, she said.
Finding genes related to drought tolerance is much more difficult than finding genes related to disease resistance, Bartlett said.
“You expose something to disease and it either survives or it doesn’t,” she explained. “With drought, there’s a lot of different aspects of the structure or the anatomy of the plant that help it survive.”
Jones said he’s hopeful the UC Davis research as well as his nursery’s in-house rootstock breeding research pans out.
“In 15 to 20 years when we start replanting these vineyards that we’ve been planting over the last couple of years, we’re going to need a new rootstock,” he said. “It’s a bit like the vegetable growers with crop rotation. You never plant lettuce after lettuce, or a new disease will find its way in.”
Roughly more than 2,000 acres of vineyards in San Luis Obispo County have been replanted with the 1103P rootstock this year alone, Jones said. That accounts for more than 4% of the total wine grapes planted in the county.
Turrentine said that it’s likely that about 90% of the vineyards in the area have been replanted with 1103P in the past decade.
“It’s certainly not going to solve the groundwater problem,” he said. “But it’s part of a toolkit of responsible actions.”
CORRECTION: This story was updated with the correct spelling of Bob Schiebelhut’s name.
