For Almond Trees, Roots Matter as Water Grows Scarce

We love to eat almonds, but we know a lot less about the roots that sustain the trees producing them – more than 130 million of them growing in California alone. Now, scientists at UC Davis are using a range of technologies – from 3D imaging and genetics to plain ol’ shovels – to figure out how the way we propagate and manage almond trees affect the ability of their roots to explore the soil.

“We aim to give farmers a better idea of how production and management practices impact the way the tree arranges its roots in the soil and, as a result of that, how the roots acquire water and nutrients from the soil,” said Astrid Volder, a professor in the UC Davis Department of Plant Sciences.

Volder’s team is studying how almond root systems develop over time and how researchers can apply what they learn to below-ground root management. The aim is to develop trees that stay productive and healthy in a range of orchard conditions, even amid severe drought.

The Volder team is using 3D scanning to measure how the way different rootstock varieties are propagated and managed immediately after planting relates to the root architecture of young trees. They’re also studying how much of this architecture is preserved in mature trees. In addition, they’re measuring how different root architectures affect how the tree takes up water and its productivity when irrigation is reduced.

Click here to see a 3D video of a young almond rooststock with the finer roots removed. This is a 2-year-old Krymsk 86 variety.

Understanding all this better will help growers manage productive orchards as temperatures rise and water grows more scarce and more saline.

Volder is in her 12th year of a series of projects, funded by the Almond Board of California. Collaborators on the current project include Ph.D. student Rachel Noone; master’s students Fabian Gonzalez and Mat Talton; Chuck Fleck from Sierra Gold Nurseries; and Tom Gradziel, also a professor in the department. Brian Bailey, an associate professor in the department, is assisting with data collection, 3D imaging and developing models for analysis.

Early projects focused on the production of the finest roots – the ones that are most active in water and nutrient uptake. More recent work has focused on the larger roots, which live longer and determine the spatial placement of the fine roots.

“Fine roots are like leaves,” Volder explained. “They live only a short time but take up the most water and nutrients. And like leaves, where the fine roots are located in space is determined by the larger branches.”

Root architecture of international interest

California is the world’s No. 1 producer of the delicious, nutritious nuts. Most of those trees were grown, not from seed, but from tree tissue that is cultured in a laboratory and then coaxed to develop roots and sprout a seedling. It’s called clonal propagation. The advantage of this method is that it assures desired traits – such as vigor, uniform quality and resistance to soil pests – are the same from tree to tree and orchard to orchard.

But there’s a potential downside: Those clonally generated trees originate from tissue taken from a tree stem, and their first roots tend to spread more horizontally before branching into smaller roots. In contrast, the first roots of trees grown from seed tend to grow downward initially; then, branch roots form, leading to an overall deeper root system – potentially giving those trees better access to water during the dry season.

All of this starts when the trees are very young. “It is unclear how long these architectural differences persist as trees mature,” Volder said. “The idea is to see if we can train young trees to grow their roots deeper so that, during dry periods, they can get to water that’s stored deeper down.”

Click here to see a 3D video of a young almond rootstock with all but the main roots trimmed away. This is a 2-year-old Krymsk 86 variety that was propagated clonally.

At a time when water is becoming less plentiful and drought more frequent, that difference matters to the state’s almond growers, who grow 80% of the world’s crop. It’s valued at more than $5.6 billion in 2024, according to the California Department of Food and Agriculture.

This year, Volder discussed her findings as a keynote speaker at the annual conference of the International Fruit Tree Association, held in February in Fresno, Calif. There, she gave the 2026 Wallace Heuser Presidential Lecture on the topic of root growth in orchards and the impact of irrigation, rainfall and drought.

Among other topics, participants wanted to learn more about how root systems develop differently in arid and rain-fed lands, how the timing of root growth is affected by fertilization and vice versa, and how they can mitigate the challenges of climate change, Volder reported.

Volder also presented her work at the IXth International Symposium on Almonds and Pistachio, held in Lleida, Spain, in May. In October, she will present her research at the Almond Board of Australia meeting in Adelaide as an invited speaker.

3D imaging reveals links among root structure, growth, vigor, water scarcity

Most commercial almond trees consist of a fruiting part, called the scion, that is grafted onto a rootstock. Rootstocks are selected to help trees cope with specific environmental challenges such as soil type, wind, disease pressure and soil pathogens. All these factors help maintain high production levels for the scion.

To get there, Volder’s team is studying root system architecture of several varieties of rootstocks from two mature commercial orchards in California and a mature experimental orchard at UC Davis. The trees range from 6 to 20 years old, and some trees have rootstocks that were grown from seed, while others were clonally propagated.

Noone is using 3D imaging to capture detailed root architecture on both young and mature trees.

Click here to watch a video that shows a mature root system in 3D, created by Rachel Noone from hundreds of still images. This is an 11-year-old Krymsk 86 rootstock.

For mature root systems, the team starts imaging the full root system and then trims progressively more roots to delve deeper inside, getting to the oldest roots. Their aim is both to understand the roots’ traits broadly and get to the root-stem connection, where roots can crack and allow pathogens to attack the tree. 

The series of 3D models is then combined into a single 3D model that helps the team better understand the root traits.

Young trees, irrigation and soil microbes

To study young trees, the researchers planted six commonly used rootstock varieties (with no scions grafted on) in a UC Davis test orchard in 2024. Some of the varieties have specimens that were propagated both clonally and from seed.

To measure what happens when the trees get less irrigation, a control group of trees received a normal amount of water, while test trees received 30% less. The scientists measure how much stress the trees feel with reduced irrigation and their stem diameter growth

The team dug up some of the young trees in fall 2024 and more in fall 2025, with another round set for fall 2026. In each round, they also collect soil and root samples to characterize the microbes associated with them.

“We want to see if there’s more disease pressure on clonal rootstocks, because their roots tend to grow much closer together and can cause cracks in the root bark at the point where they attach to each other,” Volder explained. “Both of these situations could give pathogens a chance to infect the tree.”

Then, in addition to scanning and trimming the young roots for their 3D model, Volder and Noone gather the finer root branches for detailed architectural analysis. 

Goal: Manage growth below-ground

The long-term goal of all this work is to understand how management and production practices affect development of the root system as a whole, Volder explained.

“Just like we train the tree canopy to maximize production and lifespan, we can train the root system to optimize the tree’s ability to pull up water and nutrients,” Volder said. “We can even do this for the specific conditions in an individual orchard.

“For example, if you have plenty of water and nutrients, then a root system that is shallow, heavily branched, with high turnover of fine root would be ideal,” Volder added. 

“But such a tree would not do well in drought conditions,” she continued. “When you expect that the orchard will have periods of drought or little irrigation water available, then training young trees to develop several deep roots so they can extract water from deeper sources is the way to go. 

“By collecting lots of data about the responsiveness of young trees to management practices, and measuring how well these traits persist in mature trees, we can develop a model for below-ground management that can advise us on rootstock selection and root training for specific orchard situations,” Volder said.