Light Is Essential for Good Almond Yields

Findings from a  six-year study conducted by the University of California (UC), Davis researchers reveal that light interception is the deciding factor in how many almonds a given tree will produce.

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“The complete study (conducted between 1995 and 1998, and again between 2001 and 2006) documents the life cycle of spurs in almond trees,” Theodore DeJong, one of the research team members, says. “Individual spurs rarely produce fruit in two subsequent years, and their productivity decides the actual crop. We found that spurs have to be in the light to bear fruit. Heavy shading in a canopy can lead to a greater proportion of the spurs dying.”

DeJong is a Professor Emeritus and Pomologist with the UC Davis Department of Plant Sciences and a specialist with UC Cooperative Extension. He says the conclusions regarding almonds may also be true for other spur-bearing crops, such as apples and pears. But unlike those crops, although canopy management is important, almond growers do not need to prune.

“Basically, you’re managing a spur population. Many almond pruning studies have been done in the last 50 years. Few have ever documented increased yield because of pruning,” he says. “In fact, you get a slight or significant decrease in yield when you prune.”

DeJong says this is because sometimes growers prune in sections of the canopy where light was adequate. This means that they cut off replacement spurs that would have borne fruit after mature spurs stopped doing so.

“When you prune replacement spurs, you won’t see a substitute for them grow again for another year or two,” he says.

DeJong conducted the study, called “Spur Dynamics and Almond Productivity,” with Bruce Lampinen, a specialist with UC Cooperative Extension, Sergio Tombesi, a professor of sustainable crop production at the Catholic University of the Sacred Heart and a former graduate student in plant sciences at UC Davis at the time of the study, and Samuel Metcalf, a research staff member in plant sciences at UC Davis. The study was conducted at Wonderful Almonds in Shafter, CA. The almond varieties in the study included ‘Nonpareil,’ ‘Monterey,’ and ‘Price,’ which were planted in alternating rows.

New Growth Essential for Spurs
The research team focused on the interaction of spurs in mature orchards.

“Spurs that do not bear fruit reserve their energy to produce fruit in the following year. When a spur does bear fruit, the leaf area on that spur tends to be lower the following year,” DeJong says. “It doesn’t bear again because it doesn’t have as much energy the next year.”

Spur mortality is inevitable, and the remedy isn’t as easy as you think.

“Some of the spur population dies every year. That’s why you can’t just increase the yield by increasing the amount of water you give the tree,” he says. “You have to have some new growth to provide locations for replacement spurs.”

It’s also important to keep growth in balance.

“You don’t want too much growth, because then less sunlight gets to older spurs. You need to encourage growth in spurs farther down in the canopy,” DeJong says. “If you give the tree a lot of water and nitrogen, you just speed up the process of spur dying and spur replacement.”

Study’s Broader Reach
Light interception studies help almond growers understand what other factors may affect yield.

“Walking the orchard is important. It helps growers know why they may not be achieving their maximum yield. It is a good idea to walk the orchard during the middle of summer. That’s when 85% of light interception occurs. Every inch of light produces up to a 50-pound yield potential,” says Sebastian Saa, Senior Manager of Agricultural Research of the Almond Board of California, which funded DeJong, Lampinen, Tombesi, and Metcalf’s study.

Saa says different variety and rootstock combinations of almonds have different maximum yields. An individual almond tree is a combination of a variety, the canopy, and a rootstock.

“Knowing what we learned from DeJong and Lampinen’s work, we use that information to play with different varieties and see if they can produce more,” says Saa.

Experimentation is limited, however, because 90% of California’s almond crop is produced on fewer than 20 varieties.

DeJong adds that knowing more about light interception’s role does not mean irrigation, water quality, and nitrogen fertilization are not important.

“There’s a whole plethora of different circumstances that can result in almond trees producing fewer almonds. For example, spur mortality rate increases with late season water stress,” he says. “Trees depleted in carbohydrates going into the dormant season are not as strong as they should be going into the spring. It took us three years of research to see that.”

Education is Key
DeJong says educating growers about the study’s findings will help change best practices, but it’s a challenge.

“The percentage of people that actually pays attention to what we do is not always that great, he says. “I gave talks at the Almond Conference. There’s maybe 200 to 300 people in the room. There are thousands of almond growers. Not too many people read the scientific papers that we put out.”

DeJong and the other researchers published their findings in the journals Tree Physiology and Horticultural Science. He hopes spreading information from his findings will reduce growers’ costs.

“Almonds are a very high-value crop. Growers are making good money. They’re susceptible to having many people come down the road telling them, ‘You can grow more almonds if you buy this product I’m selling,’” he says. “The light interception studies show them that spending $100 more an acre will probably make no sense.”

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