Rootstocks are life. Varieties pay the bills.

Of all the decisions tree nut growers make when planting a new orchard, rootstock choice has to be the most important one, said nurserymen who develop and provide the young trees for new orchard plantings. Variety choices are driven with payback in mind and are somewhat easier to make.

“Absolutely, rootstocks are the most important consideration for a new orchard. If you get it wrong, the variety doesn’t matter,” said Agromillora’s Cliff Beumel.

According to UC Davis Fruit and Nut Research and Information, rootstocks are bred to grow in different soil types and conditions, and provide anchorage, vigor and resistance or tolerance to soil borne pests and diseases.  However, no individual rootstock is tolerant of all factors that can impact production. The strengths and weaknesses of each rootstock should be considered in the context of a specific orchard location.

Match Rootstocks to Site

Matching rootstocks to the environmental conditions in an orchard site is not an exact science. The person making the decision has to factor in soil type, disease pressure and soil pests at the specific site along with climatic conditions. For example, trees planted where strong winds are common need better anchorage. The type of orchard management, conventional or organic, and how much time a grower or manager can spend ‘doctoring’ trees can also have an impact on the rootstock decision. There is no ‘perfect’ rootstock that fills every need, all nurserymen interviewed for this article agreed, but with careful consideration, sound choices are often made.

“A lot of growers come to us already knowing what rootstock they want or need, but we play a role in helping them with the decision,” said Reid Robinson of Sierra Gold Nurseries.

Mark Crow of Orestimba Nursery explained that when growers have an interest in new plantings its best to collect as much information before they choose a rootstock. That information includes what has been planted at the site previously, what diseases have been diagnosed there and if there is any micro climate effect at the site. What nearby neighbors are growing may also be a consideration.

“Every piece of ground has some unique characteristics that should be recognized,” Crow said. “We are able to provide our growers with data from our own test blocks of new varieties and rootstocks. As a farming family, we want to see successful orchards.”

Crow is a sixth-generation grower, and hopes to pass on his knowledge to his children and their customers.

“We will always strive to find the best solution for the grower,” he added.

Burchell Nursery’s Ron Boone said growers either are knowledgeable about conditions at the prospective orchard site or they will ask the nursery to assess the site and make recommendations. Burchell will consider the soil texture, drainage and water table among other things to make a recommendation. The salt load in the soil and the water source for irrigation also has to be taken into account in a rootstock choice, Boone said. Since the drought, salt has built upon some ground or it may be marginal to begin with. Infiltration tests and soil mitigation may be necessary before even getting to rootstock selection. He said they also like to see results of soil tests to determine the levels of soil borne pests.

In California, Beumel said, there is a wide range of soils and growing conditions, just as there is a wide range of choices in rootstocks. Some have limitations, but there is a place suited for each one.

Nurseries should provide growers with good, solid scientific information about rootstocks and show real-world examples of their success, he added.

University of California Walnut Improvement Program has information on walnut rootstocks and varieties. Information on almond and pistachio rootstock and variety trials is also available.

Variety Decisions

Scion varieties are chosen for many reasons, including rootstock compatibility, pollination timing, harvest timing, accommodations to harvest equipment availability, weather conditions and finally, advice from the processor. Many of these decisions will also have financial implications.

Like walnut and pistachio rootstocks, the scion varieties chosen with them will have long lives. Almonds have a shorter turnover, but can be affected more by environmental conditions. There are limited choices in pistachio varieties, but the earlier maturing varieties have gained in popularity to space out harvest.

Factors to consider when selecting a walnut variety include climate and insect pest pressure. Walnuts require a period of winter chill to break dormancy and initiate leaf and flower. Pistachio varieties also have a chill requirement.

In addition to considering how a potential scion cultivar would fare in local climate and pest conditions, growers should consider other characteristics of each cultivar including leaf and bloom date, timing of harvest, nut quality and bearing habit.

Boone said that in a growing region with early rains, nuts from a later harvesting variety might end up being laid down on wet ground. That risk can be mitigated with an earlier variety.

The size of the block to be planted can also come into play. In almonds, Boone said they would recommend a grower with a smaller block use a self-fertile variety. It will be easier to get a harvest operator to come once rather than two or three times to get different varieties. If the new block is planned near an existing variety, it is often practical to plant the same variety.

Location, Location, Location

“An astute grower will make this decision based on location and attributes,” Beumel said. While prices and yields are important, they won’t necessarily reach their potential if the growing conditions for a variety are not optimal.

Almond variety trials conducted by UCCE advisors are a source for information on new varieties (for detailed almond trial information, see article in September 2020 West Coast Nut.) These trials can provide information on time of bloom, bloom weather, bud failure, hull split and harvest maturity as well as productivity, yield data and nut quality. Much like rootstocks, UC advisors note there is “no perfect variety.”

First, choose the main variety, then the pollinator, they advise.

Jereme Fromm of Dave Wilson Nursery brings with him experience as a field rep with Blue Diamond. Fromm said he spends time discussing needs and specifics of potential orchard sites with growers, but also encourages them to seek some input from their processor when choosing a variety.

“Ask them what sells; that needs to be part of the conversation,” he said.

Growers should also think about what varieties are already in their portfolio and if it makes sense to diversify or have a more homogenous operation. Management costs and harvest efficiency should be considered, Fromm added.

Harvest timing or spreading out harvest are often cited as drivers of growers’ variety choices. Beumel said more growers are thinking about spacing out their harvests so that not all orchards are ready at the same time and may have to wait for equipment availability. Insect damage or an untimely weather event could affect crop value in those instances.

Robinson said there are a lot of regional components to choosing a variety and it is often a business decision. Growers should do a lot of homework and speak with other growers who already have the variety they are considering in the ground.

“We can provide information and perspective for specific rootstocks and varieties.”