By TIM HEARDEN

Capital Press

ARBUCKLE, Calif. -- University of California researchers are continuing to assert that walnut growers may not have to prune their trees as much as many do.

A multi-year test plot at the UC Cooperative Extension's outdoor Nickels Soil Lab here still shows trees that are minimally pruned yield the best tonnage per acre, advisors say.

"I was very skeptical ... when we started this study" five years ago, UC farm advisor Janine Hasey told about 60 growers during a workshop at the lab March 5.

"Obviously, I've been watching this whole plot with a lot of interest," she said. "The data ... makes a compelling case that we need to re-evaluate decades of practice."

The UCCE is conducting an ongoing test of different pruning techniques on hedgerows of Chandlers and other walnut varieties commonly grown in the Sacramento Valley.

The researchers have consistently found that many trees that were trimmed sparingly or not at all have produced a bigger yield of nuts than trees that were cut more aggressively.

In the 2012 harvest, trees that were minimally pruned and lightly irrigated yielded an average of 2.4 tons per acre, while heavily pruned trees produced an average of 2.08 tons per acre.

In fact, trees that were not pruned or headed at all have at least held their own, the researchers say. For one thing, pruning can disrupt a tree's pattern of branch development, said Bruce Lampinen, a UC-Davis pomology specialist.

On unpruned trees, the branches become strong from having the crop on them in season after season, he said. With pruned trees, some weaker branches could break off during heavy crop years or in more severe weather, he said.

Unpruned trees tend to grow in alternate years, and farmers need to be careful during "off" years not to over-water them, Lampinen said.

"We had the most extensive growth last year in the unpruned trees," he said.

The farm experts have cautioned that it's still early in their research and proper pruning levels can depend on many factors, including soil and overall farming practices in an orchard. Hasey also warned growers not to abruptly stop pruning trees that they have previously pruned more aggressively.

Hasey said she and other researchers have encountered some resistance from growers to their minimal-pruning methods, including one in the San Joaquin Valley who said they've "gone over the deep end" after reading about the experiment in an industry journal.

Hasey said it was difficult at first to leave a block of trees unpruned.

"It was very hard for me to just walk by those trees," she said. "I've never done that in my whole career."

But she said she's never recommended heavy pruning, although many growers were doing it.

"You do the math," she said. "You nave no labor and no brush versus on the pruned trees, you have labor and brush."

Online

University of California Cooperative Extension walnut research: http://walnutresearch.ucdavis.edu/