To prevent a phytotoxicity interaction between sulfur and Stylet oil, he applies a biofungicide, or Kaligreen, to control powdery mildew before transitioning from sulfur to the horticultural oil. Usually one treatment is enough to protect the vines between the last sulfur application and start of the Stylet oil treatment.

The high calcium carbonate levels in his soils imparts certain unique qualities — like minerality — to the wine made from his grapes not found grapes grown on alluvial soils, Glenn notes. But his vines struggle to grow in these high-pH conditions.

“We always battle nutritional issues to produce as much shoot, leaf formation and leave area as possible so that the vines can produce grapes most efficiently,” he says.

The vineyard’s primary red varieties are Mourvèdre, Syrah, Grenache and Counoise, while the major white grapes are Roussanne, Marsanne, Grenache Blanc, Viognier and Picpoul. Tablas Creek Vineyards is one of the few in California to grow Tannat and Vermentino. Last month, Glenn and his crew planted about a half-acre of Picardan vines. That increased total worldwide acreage of this French variety by about 50 percent, he reports. “It's supposed to be a mid-ripening variety, with good acidity, that is well-suited to our hot, dry, low-fertility sites,” Glenn says.

His most widely-grown white grape – Roussanne – provides his biggest fertilizing challenge. Heat stress seems to throw Roussanne vines into a tailspin, and they turn yellow, he reports.

“We’ve called in various agronomists and held seminars trying to figure out what is going on,” he says. “But, we’re still looking for a silver bullet that will solve the problem.”

In the meantime, he’s trying to keep his soils as fertile as possible

“We’re really proactive in the Rousanne blocks,” Glenn says. “After harvest we fertilize with 2 to 5 tons of organic compost per acre. We also apply various organic fertilizers in foliar sprays. We’ll spray foliar fertilizers in conjunction with our fungicide sprays or apply the nutrients through the drip system where possible. We want to keep the vines green and happy so that they will ripen the crop effectively.”

Dealing with uneven ripening is another challenge in his Roussanne blocks. “We may have to hand-harvest a particular block three to five times before we can get them all,” Glenn says. “We tolerate all the frustration and worry this variety causes, because it ends up making such beautiful, age-worthy wines.”

High demand for water from grape growers and cities in Paso Robles has dropped water tables 50 to 100 feet recently, he adds. That has only heightened concerns about wells going dry soon.

“Farmers are planting more vineyards and municipalities want larger water allocations,” Glenn says. “Within the next 10 years water is going to be a real point of contention between them.”

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