The Rodale Institute California Organic Center now has preliminary results from its 2021 CDFA Specialty Crop Grant-supported study exploring cover crops as an alternative to black plastic mulch in organic vegetable systems. 

Looking at organic artichokes and organic strawberries, the Ventura-based California Organic Center compared the performance of these two cash crops in black plastic mulch to their performance in cover crop-residue and living groundcover systems.

Dr. Arianna Bozzolo, Research Director, Rodale Institute California Organic Center

“Specialty crop production in areas like Southern California has often relied on practices such as plastic mulch, intensive tillage, or herbicide use to manage weeds and ensure successful crop yields,” said California Organic Center Research Director Dr. Arianna Bozzolo, explaining the rationale for the study. “These practices have historically offered benefits in terms of weed control and crop yield, but they also come with environmental and sustainability challenges.”

For this study, the California Organic Center grew organic artichokes in five different production systems and then compared them:

• Black PE plastic mulch
• Living groundcover/intercrop of crimson clover 
• Living groundcover/intercrop of perennial white clover 
• Living groundcover/intercrop of perennial Kurapia
• Cover crop residue of buckwheat and peas (terminated by roller crimper)

The Center also grew and compared organic strawberries in five different production systems:

• Black PE plastic mulch 
• Living groundcover/intercrop of white clover 
• Cover crop residue of buckwheat and peas (terminated by roller crimper)
• Cover crop residue of sorghum sudangrass, 50 lbs/acre (terminated by roller crimper)
• Cover crop residue of sorghum sudangrass, 75 lbs/acre (terminated by roller crimper)

Chief among the Center’s findings were that both the white and crimson clover living groundcover improved soil health. However, Bozzolo noted that the clovers had a negative effect on artichoke and strawberry yields compared to when those cash crops were grown in plastic mulch. Of the two clovers, she found that the crimson variety showed the most potential as a living intercropped mulch since it established quickly and didn’t impact yield as much as the white clover.

“I had higher expectations regarding the white clover,” Bozzolo said. “Although chosen for its low growing characteristics, it grew more than a foot, shading the cash crop and it strongly competed with it. … To mitigate the potential negative effects of white clover competition, farmers can adopt management strategies such as adjusting planting density. In future projects we will need to investigate the balance between the benefits of white clover (nitrogen fixation, weed suppression) and its potential competition with the main crop to optimize overall yield in intercropping systems.”

“[Plastic mulch, intensive tillage, and herbicide use] have historically offered benefits in terms of weed control and crop yield, but they also come with environmental and sustainability challenges.” - Dr. Arianna Bozzolo

In terms of the cover crop-residue production systems, Bozzolo said results were favorable, particularly for organic strawberries grown in the sorghum residue, which showed yields comparable to organic strawberries grown in plastic mulch.

“I was positively impressed by the results in the strawberry experiment regarding sorghum weed-suppressive potential and yield,” Bozzolo said. However, she noted that terminating the sorghum cover crop—which was done with the Rodale-invented roller crimper—was difficult as some new growth sorghum shoots emerged post termination and had to be removed by hand weeding.

“The cover crop-roller crimper system has very high potential (the idea of growing your own mulch that can improve soil health, retain soil moisture, and control weeds is a brilliant concept), but a successful cover crop termination system needs to be developed,” Bozzolo said. “The roller crimper needs to be included in a system that can involve other practices such as occultation (silage tarp), undercutting blade, or other practices that support the termination of the plants.” 

Compared to the sorghum, Bozzolo said the buckwheat/pea cover crop was easy to terminate using just the roller crimper. However, she noted that the buckwheat/pea residue decreased significantly over a couple months, leaving space for weeds to grow. The sorghum residue, on the other hand, did not lose any biomass over the same time frame and so was much more successful at suppressing weeds. 

In addition to its work with organic artichokes and strawberries, the Rodale California Organic Center has two other ongoing studies exploring cover crops in organic vegetable systems. One is a 2022 CDFA Specialty Crop Grant exploring the efficacy of no-till, cover crop-residue production systems for short-, medium-, and long-season crops (zucchini, peppers, and eggplants, respectively). 

“I was positively impressed by the results in the strawberry experiment regarding sorghum weed-suppressive potential and yield." - Dr. Arianna Bozzolo

In another project, supported by a CDFA Healthy Soils grant, the California Organic Center is looking at oat/peas and oat/vetch cover crops in organic pumpkin and cabbage production in both till and no-till systems. 

“The primary focus of all our research is soil health,” said Bozzolo of the California Organic Center’s research program. “Soil health remains our top priority, always. Once soil health is enhanced, we are well aware that it leads to a cascade of positive consequences. In all of our experiments, we test the cover crop/roller crimper/reduce tillage system with different cover crops and different cash crops.”