By SEAN ELLIS
EMMETT, Idaho -- Two plates of green, seedless Emerald grapes sat on a table side by side during an organic grape field day Sept. 27 at Rocky Fence Vineyard.
The grapes on the right dwarfed the ones on the left.
The main difference in the sizes, RFV owner Mike Medes explained, was that the larger ones were sprayed regularly with gibberellic acid, a natural growth regulator that uses a naturally occurring acid in grape seeds to stimulate growth.
A few dozen people gathered at Medes' vineyard to see the results of a two-year study he conducted on how to grow organic table grapes bigger in Idaho. The results were right there in front of everyone and the conclusion was simple.
"Those look nice," Medes said about the smaller grapes. "But if you want to make them larger, gibberellic acid."
Spray timing and mixture rates for the gibberellic acid were important, he said, but too much spraying prevented full coloring and slowed ripening.
A detailed breakdown of the techniques and recipes Medes used to grow the grapes larger can be viewed at his website at www.rockyfence.com .
Medes said he tried several techniques but found gibberellic acid was by far the most beneficial.
Keeping canopies open was important to allow the acid to make contact with the grapes, he said, and removing little blooms was important to make room for bigger berries. Cutting off a portion of clusters post-bloom provided more energy to the remaining berries.
"I know I will be changing a lot of my ways next year because of (Mike's work)," said Joe Morton, who also grows organic table grapes in Emmett.
Medes also shared the results of a separate study on how to prevent or reduce powdery mildew in organic table grape vineyards. Canopy management was really important to allow good penetration for spraying, he said.
"Keep the canes from overlapping because overlapping is where powdery mildew really likes to take off," he said.
Adhering to a good spraying schedule was also important, he added. Powdery mildew, a fungal disease that can significantly reduce yields, over-winters in trunks but it is not cost-effective to try to kill it while it's hiding there because it lives off live tissue, he said.
He advised people to only spray living tissue and not the ground and trunks.
"Wait until you see some green and spray until you see a little dripping," he said, adding that he sprayed at 10-day intervals after that, less after rain. "Adhere to that practice, it worked for me."
Good horticultural practices were also important, he said. "Unless you do good canopy management, you're wasting time and money."
Both project were financed by specialty crop grants provided by the Idaho State Department of Agriculture and University of Idaho officials verified the results.
"This project looks like a great success," said ISDA Director Celia Gould.