The adequate to high numbers of chilling hours recorded across California during the current winter season are overall good news to nut and tree fruit growers, according to University of California farm advisors.
As of Jan. 31, an average of 764 chilling hours were recorded in Kern County during the '06-'07 winter, according to Kern County Farm Advisor Mario Viveros. The '05-'06 winter numbers were lower at 522 hours. Both levels paled to the 930 hours recorded in '00-'01. While higher chilling hours are important to almonds, levels as low as 300 hours are still beneficial.
“A higher number of chilling hours can be good or bad for almonds. Higher chilling numbers can result in an extremely heavy, higher density bloom for about 2-3 weeks compared to the average bloom period of a month,” Viveros said.
However, the shorter bloom may not provide sufficient time for bees to cross-pollinate blooms, depending on the weather. “Bees respond to temperature. Their cue is 55 degrees.”
He predicted a late bloom this spring because of the lack of warm weather after the dormancy period, and a good bloom overlap between the main variety and the pollinizer.
“I suspect that once we have a mean temperature (heating units) of 50 degrees, the buds will be ready to go,” Viveros said.
He expected a good bud break resulting in good bloom overlap between the main variety Nonpareil and the pollinizer. Varieties that require more chilling hours would bloom with varieties that require less.
“The Carmel variety, one of the main pollinizers for Nonpareil, has a higher chilling requirement than Nonpareil,” Viveros said. “But since we have not accumulated enough heat units before bloom, the Nonpareil is just waiting for the units to appear while the Carmel accumulates more chilling hours to break dormancy.”
UC farm advisor Mark Freeman in Fresno County said three study models are often used to record chilling hours: one measuring hours below 45 degrees (including hours below 32); another between 32-45 degrees, and a model recording figures from 32-60 degrees.
On Jan. 30, one of those showed 900 chilling hours in the county. Last year's chill total also averaged 900 hours.
Freeman predicted this season's high chill numbers would spark an early February bloom in almonds, and a mid-March bloom in walnuts and pistachios.
Most people would agree that optimal chilling hours are 32-45 degrees, Freeman said.
“This range happens a lot in the Central Valley when fog helps keep temperatures in the 32-45 range. When high rainfall generates fog and dew, the fog halts the sun's radiant energy from hitting the buds that indirectly provides more chilling. You don't have the sunlight hitting the buds,” he said. “This absorbs a lot of heat and makes buds even warmer.”
According to Freeman, some researchers conclude that hours under 32 degrees have no benefit. Chilling between 45-60 is not as beneficial as between 32-45 degrees.
Almond grower Don McKinney of Madera, Calif., believes chilling hours under 32 degrees are the most valuable to trees.
“When we have chilling hours under 32 degrees for a good length of time, the result is a more uniform bloom in the spring,” McKinney said. “The tree will not be productive if they don't take a couple of months off.”
He called his comments unscientific but based on 36 years of almond growing experience.
“I am really skeptical whether chilling hours below 40-45 degrees truly place trees in deep dormancy and rest compared to good hard freezing weather. I'll take 100 hours under 32 degrees any day over 400 hours under 45,” McKinney said.
UC farm advisor Harry Andris of Fresno County said 900 chilling hours are excellent for tree fruit. End of season chill hours there could reach 1,100-1,200. Typically chill hours are measured from Nov. 1 thru Feb. 28 in California.
Andris said high chilling hours usually result in compact fruit blooms, baring any rain or frost problems.
“Good (low) chilling hours and a large number of them produce fairly uniform buds that develop and bloom at the same time. This results in a shorter time frame for open flowers and reduced chances for disease. More uniform fruit is created.”
Uniform size saves growers time and money by reducing needed thinning. Thinning is the second most expensive cost of growing fresh shipping tree fruit, he said.
The UC provides daily chilling hour information by California county annually from Nov. 1 - Feb. 28 on its Web site, located at http://fruitsandnuts.usdavis.edu. Click on weather services.