Interaction of paint, winter chill, sunlight and temperatures and their cumulative effect on growth of young walnut trees is not fully understood, but UC researchers report progress in evaluation of management choices that affect tree architecture.
Dani Lightle, UCCE orchard systems advisor in Glenn County, Calif., said recent issues with paint and uneven shoot break on walnut trees in the Central Valley raised questions about how paint affects carbohydrate reserves and bud break. Painting young trees and choices in heading were included in a research trial that was established in 2015 at the California State University Chico University Farm with first leaf Chandler and Solano on Vlach clonal rootstock.
Lightle said uneven leafing in the spring of 2014 was observed in young walnut orchards, most notably in the northern growing areas. It was not uncommon, she said, to see half the tree- the northeast half- with 1 to 2 inches of new growth while buds were just beginning to swell on the southwest half. Other growers had trees that were painted, breaking evenly around the tree up until where the paint stopped, but uneven above that point.
She said that after recommending growers apply paint to young trees, weather conditions during the 2014-15 winter and spring led to trees that leafed out where there was no paint, but the buds didn’t break or were delayed in painted zones.
Warm winter temperatures and a lack of foggy, cool days were cited as the most likely reason for unevenness of bud break and leafing. This spring, Lightle said, leaf out has been strung out longer than normal but most of the significant delays were in waterlogged sites, which is expected. There were no unusual issues observed with unevenness in leafing out in the northern Sacramento Valley.
The research trial did not point to any consistent conclusions about how paint affects bud break, Lightle said. Temperature fluctuations during both springs appeared to affect bud break. There were no long- term effects from the unevenness, she said. Research continues on how carbohydrates move in the tree prior to bud break.
An evaluation of painting and heading practices commonly done in young walnut orchards found there were no obvious visual effects of paint on the length of shoots on the northern versus southern exposure of trees. Headed trees with no paint had longer shoots than unheaded trees with 50 percent paint, though these differences disappeared later in the season.
The recommendation is to lightly paint or whitewash the southwest side of the trunk as high as possible to minimize unevenness. The paint will also protect from freeze or winter sunburn.
The research also found that headed and unheaded trees did not have a different trunk caliper diameter, but trees that were re-trained with a side shoot as the central leader were significantly smaller at the end of 2016. Headed trees with no paint had longer shoots than trees with side shoot re-trained or unheaded trees with 50 percent paint, showing that the headed trees leafed out earlier.
Lightle said the goal with new trees is to have the tree as tall as the stake by the end of the growing season. Some growers are not heading trees, she said, and trials are now looking at long-term effects. The research found that headed and unheaded trees did not have a different trunk caliper diameter in 2016. Yield data will be collected at harvest this year.
Heading young walnut trees is the more traditional style of training. It establishes a branching structure and keeps the tree growing to fill space. No heading and minimal scaffold selection is a newer method of training that researchers believe leads to earlier fruit production.