Kerman, Calif. grape grower Steve Kister is among the growing legion of raisin producers changing to dried-on-the-vine (DOV) production to reduce labor costs and increase production.

Ten years ago he began switching from conventional hand-harvesting/field tray drying of raisins to the mechanically-harvested DOV by ripping out blocks of old Thompson seedless vines, fumigating the soil and replanting with young, more productive vines of several different non-Thompson varieties better adapted to DOV.

With the switch, Kister began using cover crops in his 300 acres of DOV raisins, either perennial ryegrass or common vetch. This contributes to better soil fertility and vine nutrition and improves soil structure, tilth and water penetration.

The cover crops have lowered his machinery and fuel costs by reducing the amount of tillage needed to control weeds. He’s now using more cost-effective herbicides.

“My goal is to go virtually non-tillage with an overhead DOV system,” says Kister, a third generation Fresno County grape grower.

DOV raisin production offers several environmental and economic benefits, he notes. “Our carbon footprint is much less with DOV, because it uses only solar power, not paper, to dry the raisins. The flat overhead DOV is a full, overhead canopy and therefore has more shade, which reduces evaporation and makes irrigating more efficient. It also has potential for higher production.”

Earlier this year, Kister pulled out one of his last conventional Thompson vineyards, a 40-acre block. He converted to DOV with the Selma Pete variety grafted onto more costly, but also more vigorous, nematode-resistant rootstock.

Despite a decade of experience with his DOV system, Kister continues to learn. Eight years after planting his first DOV vineyard, he discovered nematodes attacking some Fiesta vines. Fiesta also ripens a little later than his other varieties, which increases powdery mildew pressure.

The later maturity can also interfere with his goal of cutting all canes by Aug. 15. That stretches the drying period to about six weeks before harvesting the raisins. “Fiesta sets a lot of fruit,” Kister says. “But, some years the grapes aren’t quite ripe by mid-August. The longer you wait for them to get ripe, the harder it is to get them dried on the vine by the end of September.

“Our Selma Pete grapes mature early, and their quality has been great every year.”

However, supporting the high production potential of Selma Pete has been a challenge. “A lot of us with flat overhead systems are trying different practices in an effort to maintain high production. I’m still working on my fertilizer program — like many other growers, I’ve found that Selma Pete needs higher levels of zinc than other varieties.”

There are now 18,702 acres (bearing and non-bearing) of DOV raisin production systems in the Central San Joaquin Valley, according to the Sacramento Field Office of the National Agricultural Statistics Service, amounting to 8.9 percent of the total raisin-type grape acreage.

Fiesta and Dovine varieties had the highest percentage of acreage using the overhead trellis, at 52.6 percent and 45 percent, respectively.

Approximately 44 percent of the total raisin-type acreage planted since 2002 was managed with this system. Most California raisins, however, are still produced by field drying.