California walnuts are on a roll. Like almonds and pistachios, walnuts have been identified as a nutritional, healthy food item.

Things are so good that Mintel, a global market research firm, reported that walnut snack sales in Europe have doubled in the past five years and are up 89 percent over the previous year, according to the California Walnut Marketing Board.

The same company reported last fall that walnuts were the most frequently mentioned nut on restaurant salad menus during the first quarter of 2006. In fact, mentions were up 33 percent from 2005 in the 550 restaurants surveyed, of which 350 were chain restaurants.

Good news indeed for California walnut producers.

But, many are not happy, due to prospects for the small 2007 crop that apparently will be delivered to dehydrators and marketers this fall from the state's 215,000 bearing acres.

The first crop estimate isn't due for two months, but growers are already putting preliminary numbers to the crop set, and the outlook for virtually every variety is down — some precipitously from last season.

No one knows exactly why. Theories abound, and no one is discounting any of them.

Scientists and researchers may be talking about 2007 for a long time to come. Seemingly, there should be a good California walnut crop in the making. But there isn't — or at least when you hear growers like Joe Conant of Wheatland, Calif., go down the list of varieties he farms, delineating how far each is down compared to last season.

“Chandler down 20 percent, and we were down last year; Hartley down 10 percent; Tulare, a crop disaster with only 20 percent of a crop; Vina down 70 percent; and Serr — terrible.”

Dave Kewaya, a major grower in the Orland and Chico areas who also operates a dehydrator, doesn't argue with Conant's observation. He might differ only on how far off this year's crop will be compared to 2006.

They agree that only one variety, Howard, has managed to escape the litany of weather-related maladies.

Conant says his Howards are only down about 10 percent. Keyawa is propping up the limbs of his Howard walnuts to keep them from breaking.

“Howard is the only bright spot this year,” says Keyawa.

“We've been knocking our heads against a wall trying to figure out what happened to cause such a short crop.”

It's definitely not insects, all agree. Walnut Blight has not been bad this season, either.

It's all about weather.

The frustration comes partly because most tree and vine growers in the state say last winter was one of the best “chilling hours” years in several seasons.

But, shoot growth was erratic this year, a telltale sign of low chilling hours. According to the University of California, walnuts need 500 to 700 chilling hours. Keyawa believes it is closer to 1,000 to set a solid crop.

By comparison, almonds require only 400 to 500, pistachios 800, and French plums 800.

Conant and Keyawa believe growers didn't recognize that it may not be just chilling hours, but how you calculate them.

Conant knows the weather away from the thermometer can make a difference.

“Fog makes for more effective chilling hours,” he says. “There is a big difference in chilling hours when you have a low of 30 degrees followed by a daytime high of 70, compared to when the low is 32 and the high tops out at only 48 because of the fog.”

The fog is a result of rain. According to UC Cooperative Extension Farm Advisor Franz Niederholzer, “There was no measurable rain here in January — and when you have no rain, there is no fog.”

There are three different ways of calculating chilling hours. The two simplest and most common are to accumulate the number of hours below 45 degrees or the number of hours between 32 and 45 degrees.

The other is more complicated and more telling on the counting of this crop's chilling hours. It's called the Utah method, and uses the concept of relative chilling effectiveness and negative chilling accumulation.

It adds chilling hours like the other two, but subtracts chill units when the temperature gets above 61 degrees.

“If we use the Utah model, we find that we had as few as 500 chilling hours. We need 1,000 plus,” says Keyawa.

“It got cold enough to kill some of my citrus trees, and all my 10-year-old avocados, but there weren't enough chilling hours for my walnuts,” says Conant.

The excessive heat last summer is another culprit growers and fieldmen identify as a possible culprit.

The lack of nut set is also being attributed to excessive pollen. “My patio was covered with pollen this spring,” says Keyawa.

High pollen load has been identified as one of the reasons for the long-standing problem of pistillate flower abscission (PFA), which causes poor nut set, primarily in the Serr variety. Some growers believe PFA may have caused problems in many of the other wind-pollinated walnut varieties.

Glenn County UCCE Farm Advisor Bill Krueger says too much pollen can create just as many nut set problems as too little.

Conant and Keyawa both say catkins were in abundance this spring, another oddity that may be a reason for the short crop.

“I had probably four times as many catkins than normal,” says Conant.

“They covered the roads around orchards,” says Keyawa.

Catkins are the male flowers, and when they are excessive, it takes energy away from the female buds. Some believe last year's hot summer contributed to heavy numbers of catkins this year.

Niederholzer also points out that the weather during bloom this year was very hot; temperatures were high and moisture was low.

“You have to have moisture to hydrate the flower,” he says. “In prunes, research has shown temperatures above 80 degrees hamper pollen germ. I think this may have hurt walnuts in the Sacramento Valley this spring. We had a heat spike of 83-85 degrees when prunes were blooming in the southern Sacramento Valley, and there is a very short crop.

“On the other hand, prunes in the Chico and Red Bluff areas missed that spike by three or four days, and the crop there is better,” Niederholzer says. “We had weather in the lower Sacramento Valley in March that was more like May.”

Growers also are wondering if detrimental weather during bud differentiation may have had an impact.

“They used to think bud differentiation happened in September,” Conant says. “Now research has shown it's way before that. I wouldn't be surprised if we weren't seeing bud differentiation in May.

“We saw fruit three-eights of an inch in diameter fall off this year, long after PFA should have been a problem. This could be related to the lack of chilling, or intense heat last summer during bud differentiation.”

No one has definitive answers about factors that reduced this crop significantly in many areas, which only makes it more frustrating when considering what could have been.

“Look here,” Conant says, as he checks a Chandler block, pointing to where he would normally see young nuts. “There should be two, maybe three, nuts here. We're seeing a lot of nothings this year, or maybe one nut. There are very few doubles.”

“We used a lot of Surround last year to protect against sunburn, but not this year,” lamented the veteran Sutter County producer.

There is no good time to come up short, but with a crop like walnuts — on a marketing roll due because of health benefits — this year is particularly disheartening.