Cover Cropping in Almonds Exploring Benefits and Tradeoffs

Depending on a grower's goals, the soil mix might be right choice for a cover crop. Photo courtesy: Cynthia Crézé

Almonds are a major part of California’s landscape, and consumers and policymakers are paying attention to how they’re grown. Many orchards are having issues related to soil degradation, such as lack of irrigation water infiltration and ineffective gypsum applications, and producers are increasingly out of solutions.

Cover cropping, while not widely implemented in the state of California, has garnered the attention of an extensive team of expert researchers from a handful of universities in the University of California system, along with the University of California Cooperative Extension, and the Almond Board of California. Together, they tested a soil mix and a pollinator mix across various soils and climates in the Central Valley with the goal of improving the overall efficiency of orchard resources. While the interest is increasing amongst growers, it has yet to really spark a demand.

“Cover cropping adds complexity to the operation,” says Amélie Gaudin, assistant professor of agroecology at University of California (UC), Davis, about why growers may be slow to accept this method into their practices. “It’s not trivial for growers and should be tailored to what their goals and main production constraints are.

Those goals can take a number of forms, but there are a few specific ones that have been subjects of the cover cropping studies, with some having undergone more research and others still waiting for results. Cover cropping can affect numerous processes in the orchards, from pollination and pest management, to chemical and water usage, to soil health, weed control, and yield.

The (Potential) Benefits

Boosting Soil Health

Soil health—along with the physical, chemical, and biological benefits various cover crops might have on soils—has been the primary focus of these studies. With compacted and degraded soils in California, almond orchards need viable solutions to improve soil conditions. “Growers who want to build soil health usually use compost and gypsum, and should consider winter cover crops,” says Gaudin.

Improved soil health has a ripple effect for other aspects of the orchard. Major nutrients and chemical properties, such as pH and cation exchange capacity (CEC), are positively affected, as is the increase in microbial activity and nutrient cycling. The stability of soil aggregates increases, making soils more resistant to water or wind erosion, and increases water infiltration.

As soil health improves, so does the actual cover crop itself, since it, too, feeds from the same soil that it is in place to protect. This has affectionately been termed a “virtuous cycle,” where such a system can actually provide long-term support for the soil.

Boosting Bees, Suppressing Pests

Every orchard has organisms that support it, like bees, and those that don’t, like parasitic nematodes or navel orangeworm. Choosing the best cover crop for an orchard comes down to knowing the soil and the details of the trees.

“It really depends on your pest complex and your crop conditions,” says Houston Wilson, Ph.D., assistant cooperative extension specialist with the Kearney Agricultural Research and Extension Center. “The goal is to reduce pesticide use. However that’s done, we’re trying to find non-chemical alternatives, and we want to go with what seems the most promising given the economics right now.”

There are a lot of ways to look at how pests could be affected by a cover crop, with most of them still being researched. As for pollinators, a carefully planned flowering cover crop can provide a haven for bees by providing food and shelter both during and after almond bloom season and help ensure effective pollination. Though, unpredictable weather changes, sanitation, and mowing make planning a little more challenging, so does the possibility of peak cover crop bloom happening post almond bloom.

Brassica species appeared to be a favorite for bees over other species of cover crop, and those same plants can simultaneously inhibit parasitic nematodes in soils, as brassica and mustard have shown to be effective biofumigants. Certain species of cover crops are more effective than others, and what might be effective against root lesion nematodes might not be effective in suppressing ring nematodes. Different species of clover covers have different effects—rose clover effectively suppressed both the root lesion nematode and the ring nematode, for instance.

Yield & Margin

At roughly $30-40 per acre for cover crop mix, the seed may come in at a reasonable price tag, but it’s ultimately up to the grower as to how much further it goes from there in terms of cost. Do you have a drill? Will you contract out the mowing and maintenance? Will it really be worth it financially if you can save on some resources?

In studies conducted on three orchards in California’s Tehama, Merced, and Kern counties, yield increases ranged from between 94-225 pounds per acre after just one year of cover cropping. The attributions to this result are several, with each being a benefit to the orchard in and of itself.

Low Maintenance Weed Management

Cover crop and weeds compete for space and resources on the orchard floor. The upside is that cover crop usually wins out if it is well established. The downside is that low rainfall and limited light due to mature, full canopies can lead to poor cover crop establishment, which can limit the effectiveness of weed suppression. In studies conducted with a soil mix and a pollinator mix, both were effective in reducing the pressure and diversity of winter weeds.


Minimizing Tradeoffs

Cover crops in almond orchards is not standard practice, even though it is in other regions and could become the standard in California. There are a number of factors that are still being considered and researched in terms of whether or not it’s really worth it for growers to add this to their standard tool kit of orchard maintenance.

“It’s important to be honest about potential tradeoffs,” says Gaudin. “It’s might not always be a win-win if management is not optimized.”

There are two major factors weighing in on this: water and cover crop maintenance.

Researchers are currently playing numbers close to the vest when it comes to water usage and storage in soils, as experiments are still underway. While they continue to investigate the question of whether or not a cover crop will increase or decrease water usage, and by how much, there have been some promising results, especially in terms of infiltration rates.

In theory, with improved soil conditions also come improved water infiltration. Taking it a step further is the question of whether or not improved infiltration can help save on irrigation water in the spring, and enhance tree water status prior to harvest.

A series of growers decided to give cover cropping a try. Their orchards were mature—16 years old—and they were using micro irrigation and experiencing pooling at the surface. “From a grower’s survey, we learned that [cover cropping] wasn’t just about soil health, but about water conductivity,” says Cynthia Creze, a Ph.D. student at UC Davis. How an orchard is using its water is an equally important factor in the grand water discussion. “A lot of growers use gypsum just because it’s common practice. But it isn’t working for them, and gypsum is expensive.”

Then there is the matter of biomass: how to control it, what to do with it, mowing frequency, and what the best methods are for winter sanitation. More research is needed to develop best management practices that work for both growers and the environment.

Each orchard is different, and planning a cover crop can be beneficial for growers, but they must know their goals going in. Whether it’s to improve soil health and water conductivity, or increase pollinator populations, research continues in order to determine what the best steps are.

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