WCN walnut 3

(originally published December 2017)

There was a time when people thought the world was flat. There was also a time when people thought walnuts could only be grown in class one soil. Both “facts” have been proven wrong.

According to Bill Krueger, emeritus University of California Cooperative Extension (UCCE) Glenn County farm advisor, “research and grower experience has shown with the right preparation and planting system, walnuts can be successfully grown on less-than-ideal soils.”

He went on to say, in the process of planning and planting a walnut orchard, soil evaluation is the place to start.

Katherine Pope, UCCE area orchard systems advisor for Sacramento, Solano and Yolo counties, said, whether planting a new orchard or replanting, getting things off to a good start is essential when considering the investment cost required to develop a successful orchard.

Soil Evaluation/Preparation

Walnuts are deep rooted trees that were traditionally grown in river bottom soils, such as the dark, loamy soils of Vina and Los Molinos which lie along the Sacramento River.

However, research has shown walnuts can be grown on Class III soils when the right soil evaluation and preparation has been made. That is why walnut orchards are now “cropping” up in regions not adjacent to the Sacramento and other rivers, in soils once considered uninhabitable for walnuts.

Krueger said when growers are looking to plant a walnut orchard, soil survey maps are well worth the time to peruse.

Soil surveys are available at local Natural Resources Conservation Service (NRCS) offices, cooperative extension offices, and online at websoilsurvey.nrcs.usda.gov/app.

Soil surveys provide important information, such as soil types, distribution and acreage. “It describes each soil type and provides information about drainage, flooding, exchangeable sodium content, and other details important to successful orchard establishment,” Krueger wrote in his studies. “The soil survey cannot provide every detail that may be necessary.”

He advised using a backhoe to explore the soil, digging pits five to six feet deep in strategic locations to allow first hand examination of the soil. The soil map can take the guesswork out of where to dig and on average about six pits should be dug in a 40-acre field. If the map shows uniform soil, only one pit may need to be dug.

Krueger said to look for stratified soil, compacted zones, hard pans, and clay pans.

“If soil modification is necessary, it will be much easier to accomplish before planting,” he added.

“Modification should be done in the late summer or fall when the soil is dry to ensure the most disruption possible while allowing the winter rains to settle the soil before planting. Touch up leveling or smoothing can be done in the spring before planting.”

However, Krueger explained, leveling is only necessary when flood irrigating, and not necessary when drip or micro-sprinkler irrigation will be used. Shallow ripping will be needed for deep uniform soils. Sometimes, but not always, deep ripping or slip plowing may be necessary for stratified, hardpan or claypan. When dealing with claypan, he reminds growers of special requirements to avoid reseal of the soil.

At times a shallow, loam topsoil is underlain by the heavy claypan which restricts drainage of water down the root profile. Subsoil tillage in some cases allows excess water to drain away from the root zone and safeguards against waterlogging. If roots don't have proper drainage, a whole host of diseases can occur.

One of the ways to deal with less than prime soil, according to Krueger, is to plant trees on berms, especially on heavier soils.

“Ridge berms in the fall for after soil preparation to allow for settling over the winter,” Krueger added.

Berms have the added bonus of concentrating the best quality soil and at the same time deepening the topsoil along the tree line.

Researchers also advise growers to have soil samples tested to determine the chemical properties in the area of consideration for planting. Once tested at an agricultural laboratory, have the results interpreted by a UCCE farm advisor to know what modifications need to be made.


According to research conducted by Bruce Lampinen, UCCE integrated orchard management and walnut/almond specialist, and Janine Hasey, UCCE Yuba/Sutter counties farm advisor, one of the most important aspects of establishing a walnut orchard is its design, and following that, the difficult decision of determining tree and row spacing.

In their article, Proper Walnut Spacing for Light Exposure, the duo of specialists say planting design should provide the tree canopy with maximum exposure to sunlight and allow ease of equipment operation.

Walnut orchards are typically arranged in one of two planting systems that have different methods of management—the standard spaced, or the hedgerow configuration.

Traditionally, walnuts have been planted in widely spaced orchards that allow the tree canopy to expand and fill its allotted space. With the hedgerow design, the trees are planted closer together down the tree row.

In a multi-year study, Lampinen and Hasey utilized an especially equipped ATV Mule to measure and monitor canopy light interception in an effort to learn how orchard design and canopy light relates to yield, and how tree spacing influences canopy development and yield potential.

In sharing what they learned from the study, and how that information can be used, Lampinen and

Hasey said results indicated it takes three to four years to redevelop the complexity of branching that existed before hedging.

Therefore, they wrote, production will be lost for the first few years after hedging Chandler and other varieties.

“We recommend Chandler in a standard spaced orchard for long-term maximum yields (e.g. a minimum spacing on poorer soils would be 25 feet by 25 feet with wider spacing on better soils),” Lampinen and Hasey said.

However, they advised, if a hedgerow design is still desired, then consider Howard variety, which is a less vigorous, smaller tree than Chandler.



When removing an aged orchard for replant, Pope recommends a number of steps especially when preparing to replant that orchard into walnuts again.

If removing an orchard for replant, she said it is a good idea to plan for an 18-24 month transition between orchard removal and planting new trees.

“Trying to rush the operation creates several opportunities for a less satisfactory outcome,” Pope said. “When replanting, first figure out what carry-over problems you'll need to deal with from the last orchard.”

This information can be used in making decisions such as whether or not to fumigate and with what product, what to plant during the fallow period and what rootstock to select for the future orchard.

“There are a few problems that may carry over from an old walnut orchard to a new one planted on the same ground if preventive or corrective steps are not taken,” Pope said, “They are the walnut replant problem, nematodes and crown gall.”

The walnut replant problem, sometimes called the rejection component, is not linked to one pathogen, she explains.

Carryover from a variety of biota from the roots of mature walnut trees to new trees can occur.

To avoid this carryover, Pope recommends killing the roots of the old orchard and rotating in a non-walnut crop for a year.

The root-lesion nematode of concern in California is Pratylenchus vulnus, Pope said.

“Any previous tree crop planting is likely to have hosted P. vulnus,” she added.

Pope recommends sampling for nemotodes. Growers can see the UC IPM Guidelines for sampling details at http://ipm.ucanr.edu/PMG/r881200111.html, and other nematode management information, or speak with a laboratory which assesses samples.

Crown gall, caused by the bacterium Agrobacterium tumefaciens, once introduced into an orchard or field site, has the ability to survive for at least two years in the orchard soil and at least one and a half years in non-irrigated fallow soil, Pope said.

She recommends the use of chloropicrin and 1.3-dichloropropene together in Telone© C-35 to reduce the bacterium populations in the soil.

“In sites with a history of high crown gall incidence, fumigation with Telone C-35 followed by chloropicrin combined with extensive gall removal from the soil should be considered,” Pope added.

To kill the roots, nematodes, and other pests and diseases in trees ready for removal and the soil around them, she suggests the use of both a fumigant and herbicide.

“During the month of October, cut trees a few feet above the ground and within five minutes paint the stump with straight undiluted Garlona3A or a 1:3 mixture of Garlona3A and MorAct or equivalent surfactant,” Pope said. “Leave painted stumps in place for at least 60 days. This time is necessary to allow herbicide to fully circulate and kill as many roots as possible.”

Additional information on this approach can be found at http://ucanr.edu/datastoreFiles/391-53.pdf.

When replanting a walnut orchard, Pope advises replanting on an appropriate rootstock.

“Clonal Paradox rootstocks have different strengths and weaknesses,” she said.

Because some nematodes may survive the soil preparation and sanitation process, VX211, which has shown “some tolerance” to nematodes, it makes sense as the rootstock of choice for the subsequent orchard, Pope says.

“If there is high crown gall pressure, RX1, which showed ‘moderate resistance’ to crown gall, would be a prudent choice,” she added.

The following table provided by Pope, based on data from ongoing University of California (UC) and United States Department of Agriculture (USDA)/Agricultural Research Service (ARS) trials, for preferred rootstocks for problem situations in replanting walnut orchards:

Clonal Paradox Rootstock

Rootstock Vigor*

Site Problems

Crown Gall


Phytophthora/Wet conditions


Highly vigorous

Low resistance

Some tolerance

Low resistance


Moderate vigor

Moderate resistance


Moderate to high resistance**



Low resistance


Low resistance

By: Julie R. Johnson