Stress kills. Right? Well, not if you’re talking almond trees and it’s the right kind of stress.
David Doll, also known as The Almond Doctor, spoke to growers about water stress in Fresno in late June, sharing tips on how to manage tree stress and irrigation for almonds from now to hull split.
First he covered traditional concerns about too much water stress.
“What we try to do with good irrigation is reduce the amount of stress on these trees,” he said, explaining that that maximizes the carbohydrates the plant can produce. “We're trying to let the plant do what it can do at the best of its ability.”
That’s difficult in mid-summer, he said.
“We have several challenges we face--one is we have really high water usage in this period, two inches a week. This is a lot of water these trees are using,” Doll told the growers. “We also struggle with lack of irrigation system performance, poor distribution uniformity and poor design. For irrigation systems, the best day of performance is the first day.”
“You're shooting yourself in the foot by not investing in your irrigation system,” Doll said.
Some irrigation system problems are self-inflicted, he said, when growers try to save money with poorly designed and underdesigned systems, such as using narrow pipes.
Why is proper irrigation critical in the summer?
“This is not just due to temperature we're experiencing, but also the crop stage,” Doll said. “The trees are growing and need a lot of water and the heat is asking for a lot of water as well.”
Doll called out the importance of irrigation for almonds soon after harvest:
“Once you get into post harvest, irrigate as soon as you can,” he said. “Eight inches of water from mid-August to the end of September...is the strongest influence on these trees (going into the next season).”
Planned deficit irrigation through the season
Doll said that planned deficit irrigation can reduce hull rot problems.
“If you're not battling hull rot, you don't need to apply this much stress in June and July,” he said. “But I don't know that many people who aren't battling hull rot.”
Another benefit is deficit irrigation can ease the harvest process by making trees across the orchard more uniform.
“Generally this practice also makes harvest a little easier,” Doll said. “It helps dry out the areas where you have more water applied by the irrigation system. You're bringing those wet areas into more uniformity with the dry areas.”
Doll said that an applied irrigation deficit around hull split helps. Trees draw down some soil moisture, growers then detect tree stress, and then it’s appropriate to go back to foliar irrigation.
He said executing this strategy will differ depending on the location and the field. Past overwatering can mean deficit irrigation takes a longer time to draw down soil moisture.
“We tend to put a lot of water on...it can take anywhere from two to four weeks to dry out that soil to get a stress level we want,” he said. “In June, to get the stress we need, we can give a mild deficit.”
How much of a deficit? Doll said 15% is appropriate.
“Deficit irrigation may not lead to stress right away,” Doll said. “We want to maintain deficit irrigation through this part of june til we get to that target stress level.”
The target summer stress level is negative 15 bars midday Stem Water Potential at 1% hull split. Too much stress can impact the weight of the kernels, and cut yields at harvest. Doll said one grower who ran a 20 bar deficit in June and July saw a 25% reduction in crop at harvest.
“After two weeks of moderate stress, you should go back to full irrigation,” Doll told growers. “I've done enough pressure bombing to know, it's really hard to keep these trees between 15 and 18. Don't worry if you've made a mistake here and there, it's not the end of the world.”