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Growing Stress-Free Tree Nuts

By Tamara Scully




If a grower is looking to produce nuts, preventing nut tree stress goes a long way toward preventing tree nut grower stress.
Photo by Dr. Cloud/shutterstock.com.

Those growing tree nuts for commercial production may have to dig a bit deeper to find resources geared specifically to their needs, rather than to the growers of peaches, pears, cherries, apples and other tree fruit. Nut production, while common in some parts of the country, often takes a backseat to other tree crops. Many times, nut trees are treated as home gardening crops, not well-suited for commercial production. Most of the country's commercial nut production is centered in California.

According to U.S. Department of Agriculture (USDA) statistics, there are 1,247,000 acres of tree nut production, which includes pecans, walnuts, hazelnuts, pistachios, macadamias and almonds. The only states with enough significant commercial production to report are: California, for all but macadamia nuts; Oregon, but only for hazelnuts; and Hawaii, exclusively for macadamia nuts. Georgia, Texas and New Mexico produce a small percentage of pecans. Of course, there are many other types of tree nuts not grown commercially in the U.S., such as shagbark or shellbark hickories, chestnuts and butternuts. There are also small commercial nut growers throughout the nation.

If a grower is looking to produce nuts, preventing nut tree stress goes a long way toward preventing tree nut grower stress. A few orchard experts weighed in on the best methods to get nut trees, and growers, off to a good start.

Establishing tree nut orchards

"Serious orchardists always use grafted cultivars to optimize their orchard production and management," said Michael Gold, associate director, Center for Agroforestry, University of Missouri. "Seedling trees are only for the hobbyist." This is due to excessive variation in every trait, including yield, nut size, maturity date and disease resistance. As with fruit trees, grafted nut tree cultivars will bear fruit in two to three years. A nongrafted seedling nut tree will take up to 12 years to produce, and its yield and crop will not match the production from a cultivar.

Preventing stress begins with adequate site selection and preparation. Well-drained soil, preferably loamy with a pH of 6 to 7, is the ideal soil for almost all tree nuts. Deep, fertile soils are optimal for tree nuts, and establishing orchards on appropriate sites is the key to success. Planting in soils that can't support the deep root system of nut trees, or in soil that is prone to drought or does not drain well, can cause tree stress.

"Here in Missouri, with summer drought a common reality, we recommend the use of trickle irrigation to keep the trees adequately watered during times of stress," Gold said. "Site selection is perhaps the most important criteria to consider for orchard establishment. The trees will be here for many years, and especially as they mature - and you want them to yield maximum levels of nuts - you need a good quality site."

Different nut trees do have varied tolerances for water and soil characteristics. Pecans can tolerate seasonal flooding, while chestnuts will not tolerate flooding at all, Gold noted. Fertile, well-drained, deep soils are the absolute best for nut production, just the same as for annual crops, he emphasized. Orchards, however, can be located on hilly terrain without erosion problems.



According to USDA statistics, there are 1,247,000 acres of tree nut production, which includes pecans, walnuts, hazelnuts, pistachios, macadamias and almonds. Shown here are black walnuts.
Photo by Tamara Scully.

The irrigation system will impact site selection. David Doll, nut crop pomology farm advisor, University of California Cooperative Extension, explained, "If flood irrigation is used, soils need to be deep, without any layering that would reduce rates of infiltration. In soils with micro irrigation systems, soils can be limited, but limitations should not be within 3 feet of the root zone. Basically, in these soils, you are farming the soils above the limiting layer. This is critical for water and nutrient management."

Water management

"Trees are stressed primarily by one thing: lack of water," Doll said. "Proper management of irrigation water is critical to reduce tree stress and maximize tree growth and yield."

Nut trees have deep taproots, with fibrous roots maturing slowly. Too much, too little or erratic availability of water will stress the trees. Upon planting, the trees should be given water unless the soil is saturated. Watering at planting will help settle the soil, fill in small soil air pockets, and allow the development of a healthy root system. It is also important not to crowd the roots, and to allow them to grow outward from the tree to prevent root girdling and reduced tree vigor. Tree grafts must be maintained above the soil line.

"Watering too little will lead to reduced nut size, increase the percentage of shriveled nuts, reduce vegetative growth, reduce fruit bud development and retention for next year's crop, and increase the presence of mites and other insects that are attracted to stressed trees," Doll said.



Young chestnuts at Stouffer Farm near Napton, Mo. This is an alley cropping system, with winter wheat as the annual crop.
Photo courtesy of Bill and Sue Ellen Stouffer.

Too much water will cause the same issues.

"Excessive water creates anaerobic conditions within the soil. Roots respire and need oxygen, so without oxygen, roots die, starting with the fine feeder roots," Doll explained. "These fine feeder roots are responsible for 90 percent of the water and nutrient uptake, so killing them also reduces tree vigor."

At different times during the trees' growth cycles, the need for water changes. During kernel development, a lack of water can cause reduced crop weight and shriveled kernels, and will sometimes cause the tree to abort nuts. Not allowing soils to dry adequately before rewatering can also cause damage.

Fertility

Water management is also nutrient management. Nutrients move into the tree via water uptake. Appropriate fertility management goes hand in hand with water management. Once the trees begin to bear, nutrients are removed through the crop, and this must also be taken into consideration.

For first-year trees, fertilizer application should not occur until the tree leafs out. Fertilizers should never be put directly into the planting hole. Care should be taken so fertilizer is not applied to the trunk to prevent burning. Minimal amounts of nitrogen are needed in small applications during the first year of establishment. Excessive fertilization of young trees is a common problem, often resulting in tree loss. Fertilizing too close to the root zone will also cause tree loss. In addition to nitrogen, zinc is an important nut tree nutrient, and soils with a pH above 7 can contribute to deficiencies.

"From the second year onward, providing the trees with fertilizer will greatly aid their growth and development," Gold stated. As the orchard matures, "maintaining fertility is essential. Soil testing and leaf analysis will provide information as to macro and micronutrient levels."

The timing of nutrient application is crucial. Applying nitrogen after nut kernel formation will direct it to the hull, and excess nitrogen in the hull contributes to hull rot. After harvest, the tree's need for nitrogen to develop bud growth requires appropriate fertilizer applications. Fertilizer applications are calculated based on the crop's yield, the nitrogen needed to maintain tree growth and any limiting soil characteristics. Applications are timed to coincide with the needs of the tree, depending on seasonal growth stage and tree maturity, as well as the type of nut tree.



Stressing trees in the late summer and fall can lead to reduced blooms the following year.
Photo courtesy of David Doll, University of California.

Orchard maintenance

Nut trees take time to mature, and some trees are very large. The mature size of the trees needs to be taken into consideration when establishing the orchard, although the actual plant spacing may be much more crowded initially. Selectively harvesting trees as they mature (and thereby offering a wood crop) allows adequate spacing and resource availability for the mature trees, while providing for a larger crop during the first years due to a higher-density planting. Crowded trees will drastically decrease nut production, and trees shaded by their neighbors will need time to recover from this stress. Thinning the orchard as soon as lower limbs touch is recommended.

"Trees must never be permitted to compete with each other for light," Gold stressed. "The branches from one tree must not grow up against their neighbors. Without full access to sunlight for every single orchard tree, yields will decline. Thus, orchard spacing is a consideration throughout the life of the planting."

Orchard spacing will vary if an alley cropping system is used. In this type of agroforestry, a tree crop is planted in wide rows, with an annual crop planted in between for harvest. In this manner, the long-term tree crop - in this case nuts - is supplemented with an annual crop, providing income immediately from the newly established orchard. While alley cropping will produce less of a nut crop per acre than traditional orchards, the benefits of maintaining more diversity - wildlife, disease and pest control - may be realized.

No matter the planting system, weeds or competing crops must not be allowed to drain nutrients and water away from the trees. Alfalfa and fescue, Gold said, will stunt the growth of nut trees. The typical orchard system consists of grass alleys between rows of trees. Mowing the grass and maintaining weed-free rows is standard practice.

"You must have good weed control to get the trees off to a good start. Minimizing competition from grasses on the orchard floor by keeping a weed-free zone [circle or strip] around the base of all the trees also helps to conserve soil moisture," Gold explained. Weeds can also offer habitat to rabbits or other pests, which can cause damage to young trees.



Different nut species require different pruning practices. Hazelnuts, for example, grow more like shrubs, in contrast to trees like the black walnut shown here.
Photo by Tamara Scully.

"Deer fencing or caging individual trees is also essential to keep deer from browsing and 'buck rubbing' the orchard trees," Gold said. "Cages keep the bunnies away as well, but if there are no competing weeds around the trees, the bunny problem should not be a major concern."



Proper planting techniques should be used to reduce tree damage. Improper planting can lead to kinked and girdled roots, reducing plant growth and vigor and creating variability.
Photo courtesy of David Doll, University of California.

"It is important to remove, if possible, all nuts that drop from the trees onto the orchard floor to keep pest numbers way down," Gold said. Maintaining orchard sanitation is the best disease prevention.

Proper pruning is essential to developing a productive tree and maximizing the crop. As with fruit trees, nut tree pruning is done on dormant trees in late winter to remove broken branches and create the proper form for maximum crop yield. Different nut species require different pruning practices. Hazelnuts, for example, grow more like shrubs, in contrast to black walnut trees. Tip-pruning young trees can help create a denser canopy and more leaf area.

Keeping the crop load balanced, without overcropping the trees, is also important to prevent tree stress. Overcropping can cause reduced kernel size, as well as limb breakage or dieback and even complete tree death in overly stressed systems. Nut trees tend to be alternate bearing, which can be exacerbated by a heavy crop load. A tree's cold tolerance can also be impacted by overcropping. Thinning nut trees can be accomplished via tree shaking at the proper time of kernel development.

Preventing nut tree stress is the first line of defense in preventing grower stress. Viable commercial nut orchards need good management practices to increase the chances of successful nut production. Many of the factors that go into establishing a healthy, stress-free orchard can be optimized with proper site selection and planning well before the first tree is planted. Well-drained soil, appropriate tree spacing, irrigation planning, proper pruning, weed control, adequate fertilization and routine orchard maintenance go a long way toward preventing tree stress. Avoiding common problems is the best way to reduce nut tree and grower stress while maximizing profit and longevity.

The author is a freelance contributor based in New Jersey. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.