The simple fact that a meeting was held recently in Tulare County, California’s top table olive producer, on how to thin olives is being seen as an indicator that the fortunes of producers of that crop are poised to see a turn for the better.

Such a meeting would have been unlikely in at least three of the past four years when production plunged to all-time lows.

This year, growers like Jim Wells, a newbie to the thinning notion, are ready to take the plunge. The Tulare County grower believes he could improve his bottom line by chemical thinning as a way to increase the size of olives on his trees for the first time in his 24 years of growing table olives.

“The difference between small and medium or large olives is $600 a ton,” he said, adding that a presentation at the meeting showed that harvest costs alone for an untreated crop “ate up the entire value of the crop.”

Unsettled weather is complicating the matter. On the day the meeting was held, a rain storm swept through the central San Joaquin Valley, wind was to follow and temperatures dropped as much as 25 degrees after a string of 80-plus days. Growers were watching to see if the rains washed pollen bloom off trees.

After all, you don’t need to thin if Mother Nature does it for you, something that has happened on some of those grim years. Whatever the extent of harm from that day’s rain, it will remain a last minute decision the grower must make after taking into account criteria that include temperatures likely to be felt just days after applying the chemical.

“Right now, the crop looks heavy,” said Adin Hester, president, Olive Growers Council of California, a Visalia-based organization of family farmers producing olives for the black ripe canning industry. While he’s bullish on the crop, he’s not sure about the cooler temperatures that are periodically cropping up in the Valley. But he adds that the cooler temperatures could make it an optimum time to apply the thinning materials.

If temperatures are excessively warm in the days after spraying, excessive removal of fruit can result, said Bill Krueger, the University of California Cooperative Extension farm advisor from Glenn County who spoke at the meeting. If temperatures are unusually cool, chemical thinning may fail.

Tulare County grower Dan Dreyer said he has had some success in using sprays to thin his crops “and some times when there was no effect at all.”

Dreyer said there could be a crop this year statewide of as much as 100,000 tons. Hester said three of four recent crop years were failures, including an all-time low in 2006 of 14,000 tons, “a year when Mother Nature did the thinning and some growers didn’t bother putting a ladder into trees.”

Last year, he said, the crop was the second worst on record at 22,000 tons. Hester said the 2007 crop was “normal and there was a half a crop in 2009.”

“Our growers are on their knees now,” Hester said.

Management of fruit size can be achieved by pruning or chemical thinning, and there were proponents of each approach in the audience for the meeting that was held at the Lindcove Research and Extension Center in Exeter.

Dennis Burreson, director of field operations with Musco Family Olive Co. in Tracy, said the practice is more commonly used in northern California because “the grower there can have the courage of their convictions because the fruit tends to be a size to half a size smaller [in the south Valley]; the south has the advantage of better size.”

Krueger added that there is also the potential for higher temperatures in the south Valley.

Gene Welch, central and southern California field manger for Bell Carter Olive in Corning, said he favors pruning over chemical treatment, recalling a time in the mid-1980s when some growers had painful experiences with chemicals that cut the crop unexpectedly short.

Welch said it’s unclear how big this year’s crop will be because of cutbacks growers made in pruning, in irrigation of their orchards and in use of nutrients. There’s also concern that growers are not being as aggressive as they had been in spraying for the olive fruit fly.

Last year’s yields, by all accounts, were dismal. Wells got three quarters of a ton per acre on land where he has gotten as much as 8 tons. Dreyer got a third of a ton.

Krueger said there has been some freeze damage in northern California where temperatures hit as low as 15 degrees. He acknowledged “the fruit is born on last year’s shoots,” and he believes pruning – which takes fruit and leaves – is not as effective as chemical thinning.

Krueger says thinning results in larger, more profitable fruit. He said it also means more consistent yearly crops and earlier maturity, meaning there’s less competition for harvest labor and the crop is less likely to fall victim to cold weather in the fall. He also says it keeps harvest costs down.

Chemical thinning is achieved with the use of the plant growth regulator naphthaleneacetic acid (NAA), which is absorbed into the leaves and fruit. An abscission layer forms during the two weeks after application, causing some fruit to drop.

Pruning plus chemical thinning is recommended for crop control in the Manzanillo variety, but chemical thinning is not recommended in Sevillano. NAA is marketed as Liqui-Stik Concentrate by Platte Chemical Co. The chemical can be applied as a diluted spray 12 to 18 days after full bloom.

A fruit size method can also determine when it’s best to spray. This can be determined by folding a standard 2-by-3.5 inch business card in half across the narrow dimension.

“When 11 to 16 fruit can be placed side-by-side across the card, it is time to thin,” Krueger said.

Too early an application will overthin. Too late an application will yield unsatisfactory results. An application during bloom will destroy the crop. NAA should not be used on water stressed trees, Krueger said.