In this third installment of our four-part “Optimal Yield Series,” Almond Board of California’s (ABC) Sebastian Saa discusses conditions and practices for achieving a high percentage of fruit set.
Once bloom comes to a close, almond growers will start to see the fruits of their labor — literally: almond trees will enter the period where some flowers set fruit, while others fall to the ground.
“There’s two distinct periods that affect the percentage of flowers that will set fruit: before petal fall and after petal fall,” said Saa, associate director for Agricultural Research at ABC.
Saa wants growers to understand the full range of factors that contribute to converting flowers into harvestable fruit on the tree, both pre- and post-petal fall. Since petal fall has occurred in the vast majority of orchards, growers should focus on what they are experiencing in the orchard now, followed by the pre-petal fall factors leading up to this point.
Petal fall to early-fruit growth factors
The start of petal fall marks the onset of an important competition between different parts of the almond tree, ultimately impacting the percentage of fruit that will set.
“At this point, everyone is growing — the roots, leaves and fruit — and they are each trying to be the winners,” Saa said. “How do we mitigate the competition, which isn’t necessarily bad, in the tree? How do we manage it so the tree isn’t perplexed and starts dropping fruit?”
With almond trees grabbing significant amounts of nutrients and water through their roots at this time of year, supplying them the right quantities is paramount to meeting their biological functions.
Saa points to ABC tools and resources available to growers to help calculate nutrient requirements, such as the Nitrogen Calculator available at the California Almond Sustainability Program (CASP) website, SustainableAlmondGrowing.org. Based on individual grower-specific data, it provides a total nitrogen fertilizer recommendation, along with recommended amounts by crop growth state. Saa also says that early leaf tissue sampling, combined with the results of yield estimates, can help growers effectively plan in-season nitrogen demand. Read more about early leaf tissue sampling here.
In addition to feeding your trees the appropriate nutrients, care and attention to water management at the beginning of the irrigation season is essential. Saa says it can be tricky for growers to calculate tree evapotranspiration (ETc) at this stage of early fruit growth, with cloudy days, changing temperatures and a tree canopy still growing.
“You don’t want to apply too much water and starve the roots of oxygen. Too little or too much water can be detrimental,” Saa said.
He recommends tools, such as soil moisture monitors and a pressure chamber, to help growers calculate water demand. ABC’s Almond Irrigation Improvement Continuum provides detailed information for use of these tools to help effectively and efficiently manage irrigation and scheduling practices. ABC’s “The Irrigation Station” YouTube series also features helpful how-to videos related to irrigation management.
Saa emphasizes that growers need to listen to what their trees are telling them, especially during the three phases of fruit drop. The first drop occurs shortly after bloom when defective flowers fall from the tree, with the second drop happening about a month later when pea-sized flowers, mostly unpollinated, fall. The third and final drop — referred to as “June drop” — occurs six to seven weeks after bloom.
“The first and second fruit drops may be a reflection of what happened with weather conditions and flower health,” he said. “The third drop will be the result of competition. Walk your orchard and look at the floor during these drop periods. Doing so will help you identify the limiting factor.”
Saa said this year he envisions a lower first and second fruit drop as weather conditions were optimal throughout bloom and the following month. That said, he thinks the industry could see a higher-than-normal third drop, especially if the initial fruit set is high and then weather conditions turn cloudy, rainy and cold during April.
“Still, if you do see a huge third drop, that demands attention to your management practices — something is out of balance.”
Provided that tree nutrient and water demand was properly managed, growers should see nice new shoots, with good leaf size and complete hull development around the first week of May.
“The hull is like a balloon. You want large balloons on your tree in early May and then the process of filling up those balloons with kernels begins,” Saa said.
In the conclusion of our “Optimal Yield Series,” Saa will focus on the factors that will fill the “hull balloon” to capacity, generating maximum kernel weight.
How did we get here? Bloom to petal fall factors affecting fruit set
In addition to playing an important role now for setting fruit, adequate nutrients prior to or at the beginning of bloom can make a positive difference for healthy flower development, leading to better fruit set.
“Boron is essential for a healthy flower and recent studies show that zinc provides positive benefits during fruit set.”
Another early sign for successful fruit set starts with tree varieties that bloom around the same time in the orchard.
“Synchronization or overlap of your pollinizer varieties with Nonpareil trees is key,” Saa said. “Optimal sync equals a higher probability of fruit set. If your varieties were out of sync, the flowers of Nonpareils weren’t receptive to the pollen of the pollinizer varieties when the bees were flying with it.”
Growers have long known that bee hours during bloom are essential to a successful pollination season, with favorable weather promoting more bee activity. This includes temperatures above 55 degrees and dry, non-windy conditions. This past bloom, most almond orchards throughout the state experienced ideal weather conditions, so bee hours were not (likely) a limiting factor in pollination.
“Having a huge number of healthy flowers is also relevant to achieving a successful fruit set,” Saa said.
While growers can’t control the weather, there are steps to promote healthy bee hives to increase the likelihood of bee hours during inclement weather.
“Having eight or 10 frames instead of six frames per hive can help. Bees will be more likely to leave the hive and harvest pollen — even in less than ideal weather conditions — because they don’t have to stay in the hive to maintain internal temperature,” Saa said.
A cover crop that blooms at the same time as almond trees can also encourage more bee activity, Saa noted. He recommends that growers consult ABC’s recently updated Honey Bee Best Management Practices for the latest information on maintaining a safe, healthy and productive environment for bees.
Following pollination, protecting and promoting flower health are the next steps to increasing the percentage of fruit set in the orchard. While the abundance of rain this winter and early spring is a blessing to the state’s overall water picture, it also brings a higher probability for aggressive fungal attack. Saa says that growers need to take a close look at their IPM approach, especially concerning the specific fungi they may be dealing with and sprays they can apply to fight their effects.
“Following your IPM plan rather than just spraying based on the calendar is critical, especially because of the rain season we’ve had.”
The fourth and final installment of the “Optimal Yield Series” will focus on maximizing kernel weight. Part one of this series focused on the fundamentals of understanding the principle factors that drive yield, followed by a deep dive in part two concerning the influences for healthy flower growth.
Have questions about any of the topics covered in this series? Email Saa at ssaa@almondboard.com.
