Cover cropping has been growing in popularity as conventional growers of field and row crops have learned of its benefits, and the practice has become widely accepted outside of the organic and biodynamic realms. Benefits include enhanced soil organic matter and carbon sequestration and increased water-holding capacity, along with prevention of erosion and runoff and reduction of pathogenic microbes in the soil.
But how do you utilize cover crops in a tree fruit orchard?
Tianna DuPont, Washington State University tree fruit Extension specialist, has been conducting field research examining the use of cover crops in orchards. She’s formerly of Penn State Extension, where she worked with cover cropping in both vegetables and small grains. DuPont has taken her knowledge in both areas and combined it to assist orchard growers. She was recently interviewed on Perennia Food and Agriculture Inc.’s “Orchard Outlook” podcast.
When DuPont asked tree fruit growers about their primary concerns, how to successfully replant, why replant issues occur and the long-term effects of replant disease were common inquiries.
Replant disease is related to soil health, and cover cropping is one proven way to combat soil health issues.
Dr. Mark Mazzola, USDA, along with other researchers, has been studying the role of soil health in apple replant disease, and has concluded that a “preponderance of evidence attributes apple replant disease to plant-induced changes in the soil microbiome including the proliferation of soilborne plant pathogens.”
The challenge is to utilize cover cropping in a long-lived, perennial tree fruit orchard, and to enhance the soil health and decrease replant disease concerns without negative impacts on tree productivity, DuPont said. Tree fruit growers obviously aren’t able to re-establish their crops each season, and the manner in which cover crops can be utilized to benefit soil health, without impacting yield, is an emerging science.
“What is soil health, and how does it relate to my bottom line – yield, pack outs and profitability?” are the grower’s primary questions, ones which Dupont’s field research aimed to answer. “Which if any soil health indicators were related to yield and pack out” was the aim of the trial, she said.
Dupont enrolled just over 100 orchards and had growers identify their best and their most challenging side-by-side orchard blocks. They matched these blocks based on scions, rootstocks and training systems, comparing statistically similar blocks, and measured 21 indicators of soil health. Of the 110 orchards in the study, three-quarters of the blocks identified by the growers were similar enough statistically to compare what issues might be derived from soil health rather than other factors such as irrigation or crop load.
The data indicated that soil health differences were impacting growers’ best and worst orchard blocks, with water-related factors and root health factors being seen as primary concerns. Percent sand and available water capacity, as well as root health ratings and the presence of Pratylenchus penetrans nematodes, were all related to yields.
“That’s where we had significant effects on yield overall,” Dupont said.
Dupont also reviewed pertinent research on cover crop in tree fruit orchards, finding eight papers on the topic. In six of the eight papers, cover cropping was detrimental – at least in the short term – to tree fruit productivity. Dupont hypothesized that the negative results were probably due to competition in the root zone, as the studies were examining cover crops planted within the tree row.
“We want to be careful about how we use cover crops in an orchard system, so that we are getting the benefits of the organic matter and the living roots, but without that competition,” which can result in short-term yield loss, she said.
Using Cover Crops
To capture the benefits of cover crops in orchards with less risk requires properly selecting the cover crop and proper timing of the crop. If seed goes in too late and the cover crop doesn’t mature to provide enough biomass, any benefit is reduced.
Pre-planting the ground to cover crops prior to planting or replanting the orchard is ideal, Dupont said. But for growers in dry areas where water is a limiting factor, this can be a challenge.
There are several nematodes that effect apple trees, and cover crop selections to biologically remediate the soil to eliminate or decrease nematode loads have been shown to be effective. Research trials have shown that rapeseed and other Brassica seedmeals can inhibit pathogenic nematodes.
DuPont encouraged growers to consider a year-long, pre-plant cover crop approach where sudangrass is summer planted and followed by rapeseed in autumn and spring. The crops are chopped and incorporated, releasing the volatile compounds which will reduce nematode loads and enhance soil health.
“That orchard is going to be in the ground for a long time,” and taking a year to build soil health and decrease the chances of replant disease, and reduce nematode populations, can provide long-term benefits, DuPont said.
Planting a cover crop when there is not enough water available, either via rain or irrigation, is not going to result in enough biomass to justify the time, labor and expense, whether the cover crop is meant to reduce nematode load or to enhance organic matter or both.
Cover crop fertility also has to be considered. Mustard family cover crops which provide bioremediation require a lot of nitrogen to properly grow. However, the fertility provided to the cover crop is going to be recycled back to the orchard, so growers need to consider how that added nitrogen will impact the tree fruit crop.
Sulfur is important as it activates the enzymes responsible for the nematode suppression capabilities of the Brassica cover crops. If soil sulfur concentrations are too low, the desired result won’t be achieved.
Another key factor for bioremediation is the immediate incorporation of the chopped cover crop. The plants’ cells need to be macerated to activate the compounds for bioremediation to occur. After chopping – not just mowing – the cover crop, incorporation into the soil within a few minutes is needed to capture these active components. Dupont recommended two tractors be used: the first to cut and flail, the second following behind to disc or rototill the material into the soil. Finally, using irrigation or a cultipacker to seal the soil is ideal.
“Mow and blow” is another strategy that can be used to incorporate some benefits of cover crops into the tree fruit orchard. In this technique, the cover crop is grown between orchard rows and mowed down and blown onto the trees. This provides organic matter to the in-row soil without having the cover crop compete with the trees. The mown and blown cover crop residue also acts as a mulch, retaining soil moisture. This technique has been shown to increase soil matter in the tree row by 20% and increase the growth of young trees by 10%, DuPont said.
With a little planning, cover crops can be used in tree fruit orchards to provide protection from replant disease, enhancing tree performance and yield.