Midway through a long-term field trial aimed at making the most productive use of space and sunlight in an almond orchard, it’s clear that tree vigor impacts yield in high density orchards.
But, the study is scheduled to run another 10 to 12 years, until the trees reach the end of their productive life, and by then the findings may change, cautions Roger Duncan, University of California Cooperative Extension Farm Advisor for Stanislaus County. He’s directing the study, in a 37-acre commercial orchard in the eastern part of the county.
The trial began in 1999 when the grower planted Nonpareil and two pollinators, Carmel and Sonora, grafted onto Nemaguard, a moderately-vigorous rootstock used in California for many decades, or Hansen, a relatively new peach/almond hybrid rootstock developed by the UC. Hansen supports much faster tree growth.
The trees are growing in rows 22 feet apart. Within each row, the distance between trees varies between 10, 14, 18 or 22 feet. Tree densities range from 90 trees per acre for the 22x22-foot spacing to 198 trees for the 10x22 spacing.
When Duncan began the study, 18x22 feet (110 trees per acre) was a common almond spacing in the northern San Joaquin Valley. Since then, many growers have moved to tighter patterns, planting trees 14 or 16 feet apart within rows and spacing rows 21 or 22 feet apart.
Duncan has been tracking the Nonpareil and Carmel yields on the two different rootstocks planted at spacings in his study.
By the fifth leaf, Nonpareil yields on the more vigorous Hansen rootstock totaled about 2,500 pounds per acre, regardless of in-row tree spacing. However, tree spacing did make a difference in yield of the Nonpareil on the less vigorous Nemaguard rootstock.
The Nemaguard trees planted the farthest apart (22x22 feet) produced 2,000 pounds of nuts per acre — 500 pounds less than trees planted closer. Yields with intermediate tree spacings were in the middle.
These results show a linear relationship between yield potential and canopy development, Duncan says, and since then, these trends in production have held up.
By the end of the 2011 harvest, Nonpareil on the Hansen rootstocks in the 10x22-foot spacing (the highest tree density) had produced a total of 27,117 pounds of nuts through the 12th growing season. That compares to the cumulative yield on the tightest spacing, 10x22 feet, of 26,695 pounds — just 422 pounds less. Trees spaced 14 or 18 feet apart had similar cumulative yields.
With Nonpareil on the more vigorous rootstock, there was essentially no difference in yield between any of the four tree spacings, Duncan says. That’s because the large Nonpareil trees on the vigorous Hansen rootstock filled their space in the orchard early in the life of the orchard, minimizing the yield effect of closely planted trees.
In contrast, tree spacing has had a significant effect on yield of the smaller Carmel variety on the less vigorous Nemaguard rootstock. In this case, cumulative yield with the tightest tree spacing (10x22 feet) totaled 26,827 pounds. Meanwhile, the comparable yield of Camel on Nemaguard rootstock with the widest tree spacing (22x22 feet) was 23,023.
At an average market price of $1.50 per pound during that period, the 3,804-pound difference in production represents a gross income difference of just over $5,700 per acre for the 12-year period. Yields of Carmel on Nemaguard with intermediate tree spacings were intermediate.
“The bottom line, so far, is that if you have a very strong, very fast-growing almond tree, tighter tree spacings won’t affect cumulative yield much, “Duncan says. “But, with a less-vigorous rootstock and variety, planting density can have a significant economic impact.
“This trial is currently in its 13th growing season, so we’re
only about halfway through the lifespan of the orchard. We don’t have all the answers to our questions, and as the trees get older, yield dynamics might change. Some people are predicting that yields of tightly-planted trees might decline faster than widely-planted trees. But, my guess is that yields may decline more on widely-spaced trees.
“Low yields in older orchards are generally due to missing trees, not shading of lower wood. If I had to make a guess, I would predict there will be bigger holes in the areas with the most widely-planted trees and less open space as tree spacing gets closer.
“Also we may find that, even if there isn’t much difference in yields between the various tree spacings over the long term, the trees planted closer together may out-yield the others in the early part of an orchard’s life. That would help a grower pay back any loans more quickly.”
Up to this point, Duncan has seen some definite advantages to the denser stands other than yield. For example, wide spacing encourages trees to grow larger. And those trees are more susceptible to structural failure, such as scaffolds splitting or trees blowing over in strong winds.
He hasn’t noticed any difference in susceptibility to diseases and insects as spacing varies. However, because closer spacing limits tree size, spray coverage may be better for more effective insect pest and disease control.
The smaller trees on the tighter spacings are easier to shake. The canopy of a tree on a 22x22-foot spacing, for example, is almost twice as large as that of a tree planted in a 10x22-foot pattern, he notes.
“It’s much easier for the energy of the shaker to disperse all the way through the trees,” Duncan says. “As a result, harvesting leaves fewer mummies to harbor navel orangeworm over the winter.”
So, based on what’s he’s been seeing, what spacing would Duncan choose?
First, he would evaluate what kind of vigor he would expect, given the varieties and rootstocks that would be used and the strength of the soil. It’s all about capturing as much sunlight as you can, as early and for as many years as you can, without creating worker and food safety issues or logistical problems in the orchard,” he explains.
“I’d go somewhere in the middle,” Duncan says. “The 10-foot spacing within a row is overkill unless you’re on a very low vigor site, using something like plum rootstock. In this field trial, trees planted 14 feet apart down the row have yields and other benefits similar to the tightest spacings, without the planting costs of extreme high density. And, with trees planted 22 feet apart in the row, you will almost certainly have reduced early yields and probably reduced cumulative yields, too, unless the trees are extremely vigorous.
“Plus, with big trees, you have more structural problems and more potential for navel orangeworm infestation. We’ll have to wait and see on all this — ask me again in another 13 years.”