Designing a Regenerative Systems Approach for Sustainable Turmeric Production

Final report for GW22-242

Project Type: Graduate Student
Funds awarded in 2022: $29,521.00
Projected End Date: 07/31/2023
Host Institution Award ID: G135-23-W9212
Grant Recipient: University of Hawaii Manoa
Region: Western
State: Hawaii
Graduate Student:
Principal Investigator:
Dr. Theodore Radovich
University of Hawaii, Manoa
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Project Information


In 2018, turmeric was reported to have the second highest farm value in published crops ($1.2 million) in Hawaii (USDA NASS, 2020). Despite its value as a crop, organic turmeric producers in Hawaii are limited by agricultural land availability, as well as fertilizer and pesticide options. As global agricultural production demands continue to climb and the need for regenerative, limited-input agricultural systems becomes more urgent, sustainable intensification via a thorough understanding of ecological processes merits critical examination.

Cover crops implemented in an intercropping system are a valuable tool for increasing productivity and profitability through decreased reliance on external inputs, increased yields, and enhanced ecosystem function. We propose to approach the issue of sustainable intensification with cover cropping through a collaborative partnership with a local turmeric cooperative. The intention of this project is to improve production ease and economic prospects for turmeric producers, in addition to conserving and enhancing natural resources, through the pursuit of novel cover crop research.

This collaborative project co-developed with a turmeric grower and processor will test 5 different cover crop species mixes as intercrops within a turmeric production field. Data on biomass (quantity and C:N ratio), ecological function (weed suppression, free-living nematode populations, and CO2 respiration), and turmeric yield as influenced by the different mixes will be collected over a total of 2 turmeric seasons. Project results will be communicated to growers, agricultural professionals and the public through multiple in-person and online communication styles and materials (see education plan for details).

Project Objectives:

Research objective:

  1. To identify optimal species composition for intercropping cover crops in turmeric (Curcuma longa cv. Roma) production systems in Hawaii by assessing (1) biomass production, (2) ecological function, and (3) turmeric yield by 5 May, 2023.

Educational objective:

  1. To disseminate project results to 200 local producers, as well as agricultural professionals, researchers, students, and interested community members through a series of workshops, online videos, and newsletter extension bulletins.


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  • Kevin Flanagan - Producer


Materials and methods:
Graduate Student Alina Wood stands in a field of turmeric and cover crops.
Graduate Student Alina Wood stands in a field of turmeric and cover crops.

This project is being conducted at the Waimanalo Research Station on Oahu, HI, and was initiated in June 2021. The proposed design is a randomized complete block design, with 5 treatments, 1 control, and 3 replications.


The objectives of this project are: (1) to identify cover crop species mixes that are beneficial for intercropping and crop rotation within organic turmeric (‘Roma’) production systems in Hawaii, and (2) to effectively communicate data and results to local farmers, agricultural professionals, educators, and students in order to support expansion of organic turmeric production.


In order to determine optimal cover crop species mixes, 5 treatments (each consisting of a distinct combination of 3 different cover crop species) are being tested as intercrops. These will essentially serve to prepare next season’s turmeric beds once cover crops and turmeric crop are rotated. Treatments are as follows: (1) smart radish, sunn hemp, piper sudangrass; (2) early flowering chia, red ripper cowpea, white wonder foxtail millet; (3) early flowering chia, sunn hemp, piper sudangrass; (4) black oil sunflower, red ripper cowpea, piper sudangrass; (5) smart radish, sunn hemp, white wonder foxtail millet. The control treatment consists of plastic weed cover.


During the first turmeric growing season, preliminary data are being collected from only the cover crop alleys throughout the growing season as well as at time of turmeric harvest. The first growing season began in June 2021 with the first turmeric crop planting. Two cycles of cover crop mixes have been planted and termination of the second cover crop planting is expected in March 2022 at time of turmeric harvest.


Identical data will continue to be collected during the second turmeric season. Turmeric crop and cover crop mixes will be rotated, with 2 turmeric rows planted in each (former) cover crop alley. Three cycles of cover crop mix plantings are expected within the second turmeric season.


Prior to the first cover crop planting, soil samples were taken and analyzed for pH and mineral content by the Agricultural Diagnostic Service Center (ADSC) at the University of Hawaii at Manoa in order to assess soil status.


Metrics for analysis include biomass (quality and quantity), ecological function, and turmeric yield. Biomass wet weight, dry weight, and C:N ratio data will be taken at each cover crop termination (approximately every 3 months). In order to achieve this, 3 quadrants (1 ft2) will be placed in each cover crop alley prior to mowing of alleys. Biomass within each quadrant will be cut at soil level (i.e. excluding root biomass) and then separated and weighed by species (weed biomass will be kept and analyzed as a single group). Composites will be made for each cover crop species (as well as for weed biomass) per alley, and these will be oven dried to obtain dry weights.


Ecological optimization will be assessed by observing weed suppression, free-living nematode populations, and CO2 respiration. Weed suppression data will be collected weekly by estimating percent canopy cover of cover crops using a 1 ft2 quadrant. In addition, light penetration will be recorded using a 3-sensor light meter placed above and within the quadrant at soil level. Soil samples for free-living nematode analysis will be collected 2 weeks after each cover crop termination (approximately every 3 months). Nematodes will be extracted via Baermann extraction method. CO2 respiration data will be collected at each cover crop termination (approximately every 3 months) using the Solvita test kit (field test or lab test yet to be determined). Test comparison of the field kit versus the lab CO2 burst kit will be conducted prior to first sample analysis (expected at time of first turmeric harvest) in order to determine which test is most suitable for this project.


Last, turmeric yield data will be collected at time of turmeric harvest. Methods of turmeric harvest will be determined and facilitated by the producer, using an innovative custom harvesting machinery.

Research results and discussion:

Detailed results including tables, graphs and figures are presented in the attached bulletin. Results from this study underscore one of the major advantages of employing mixed species cover crops in agroecosystems. As expected, cover crop species that are already known to be well-adapted to Hawaiʻi, specifically sudangrass, sunn hemp, cowpea and radish, outperformed species not widely utilized in Hawaiʻi, but recommended by mainland suppliers, such as millet, chia and sunflower. Planting a mixture of sudangrass, sunn hemp, and radish stood out as a strong candidate for the most successful cover crop mix in terms of generating cover crop biomass with C:N ratios < 20:1, which can support net nitrogen mineralization. However, treatment 1 did not outperform treatments 3, 4, and 5, each of which contained only 2 dominant cover crop species, yet exhibited numerically higher total percent coverage. Notably, treatment 3, consisting of sudangrass, sunn hemp, and chia (a weak broadleaf), exhibited numerically highest % cover crop and lowest % weed among treatments 1-5. Contrary to our initial hypothesis based on the ‘Three Sisters’ concept, this outcome suggests that a mixture’s overall success does not necessarily rely on the presence of a greater number of robust species, and two species may be sufficient. However, our findings do not suggest that a lesser number of species in a mixture guarantees a superior outcome, as exemplified by the suboptimal performance observed in treatment 2, which only included 1 well-adapted species (cowpea). 

Intraspecific and interspecific competition may have influenced the outcomes observed in this study. Treatments with only 2 strong cover crop species were observed to have initial germination and establishment, which might have eased competitive pressures due to more available space. To mitigate competition and encourage maximum provision of ecosystem services, seeding rates can be adjusted based on the success of each species (Bybee-Finley et al., 2022). As seeding rates can be a barrier for producers when it comes to cover crop implementation, determining optimal seeding rate becomes crucial for ensuring ecosystem functionality while minimizing production expenses (Bybee-Finley et al., 2022). Prior research indicates that decreasing seeding rates for highly competitive species, while keeping rates for less competitive species near monoculture rates, could mitigate competition in cover crop mixtures (White et al., 2017). 

Nematodes are recognized as valuable bioindicators of soil health (Ferris et al., 2001). Results presented here indicated better soil food web structure with higher SI, EI, and CI in the bare ground control compared to all the cover crop mix treatments.  

Our results may have been shaped by two key factors. First, cover crop biomass was not left to decompose in cover crop alleys where soil samples were taken. Instead, the cover crop biomass was blown into the turmeric ridges. It is unclear if the cover crop biomass blown into the turmeric rows benefits turmeric growth, since it was not part of the measurements taken in this study. This would have lessened the benefits of cover cropping, let alone cover cropping mixtures, on soil health. Second, the bare ground control alleys, which were covered by black plastic weed mat, were not subjected to tillage between trials, unlike the cover crop treatment alleys where cover crop residues were disk plowed and seeds were drilled into the soil. In addition, the use of wood chips in the field prior to covering the bare ground control alleys with weed mat resulted in high carbon inputs with slow decomposition rate, thus potentially allowing for a more structured soil food web due to a sustained nutrient availability from the wood chips. Consequently, the control treatment may have had more carbon inputs and less disturbance, leading to a better soil ecosystem. 

Bacterivores and fungivores are essential biological elements in the soil nutrient cycle, facilitating the release of soil nutrients (such as nitrogen) from bacteria and fungi, and promoting mineralization (Ingham et al. 1985). Conversely, predators and omnivores indirectly affect nutrient cycling via predation of bacterivores and fungivores, thus enhancing overall soil health and productivity (Yeates & Wardle, 1996). Interestingly, our results indicate that predator abundance was positively associated with % legume cover. This is consistent with prior research conducted by Wang et al. (2011), which documented that predatory nematode abundance was augmented by sunn hemp when grown in greenhouse trials. Fungivore abundance, conversely, was positively correlated to % broadleaf cover in this study. 

Relating nematode indices against cover crop cover percentages suggested that in our tested agroecosystem, the cover crop alleys did not optimize nutrient enrichment (EI) or soil food web structure (SI). This outcome may be a consequence of tilling and the removal of cover crop residues. This result emphasizes the importance of incorporating cover crop biomass directly into the soil prior to subsequent cash crop planting in order to benefit from cover cropping. While our cover crop alleys did allow the root systems of the cover crop alleys to sustain beneficial microorganisms within the root rhizosphere, the incorporation of green manure, or keeping the cover crop biomass in place, is an indispensable aspect of cover cropping.

Participation Summary
1 Producers participating in research

Research Outcomes

Recommendations for sustainable agricultural production and future research:

Our study indicates that the inclusion of just 2 species of cover crop in a mixture might be sufficient for soil health improvement. However, to capitalize on multifunctionality and an array of agroecosystem benefits, utilizing 3 species offers additional insurance. Our results suggested that local producers in Hawaiʻi might benefit from using cover crop species that can thrive in local conditions. In addition to the well-adapted species utilized in this study (sudangrass, sunn hemp, cowpea and radish), there are numerous species reported to be well-suited to Hawaiʻi. Species performance can vary substantially due to climatic conditions, therefore elevation is a significant determinant of cover crop productivity. Sudangrass, sunn hemp, and cowpea are considered low elevation species, while radish can thrive in both low and high elevations. Further investigating how these regionally appropriate species can be integrated into the ʻThree Sisters’ model at both low and high elevations can provide a more detailed understanding of the effectiveness of intercropping mixed species cover crops in Hawaiʻi. 

5 New working collaborations

Education and Outreach

10 Consultations
1 Curricula, factsheets or educational tools
1 Online trainings
1 Published press articles, newsletters
2 Tours
5 Webinars / talks / presentations
2 Workshop field days

Participation Summary:

50 Farmers participated
50 Ag professionals participated
Education and outreach methods and analyses:

We disseminated project results to  local producers, agricultural professionals, researchers, students, and interested community members through a series of workshops, online videos, newsletters, and a research and extension bulletins and refereed journal articles. The SOAP team including the Hawaii State WSARE PDP coordinators served as the projects Educational advisory group ( Specific project activities and outputs conducted to achieve the educational objectives include:

  • 2  workshops (10/7/22 & 11/10/22) for current and aspiring growers to feature advances in regenerative turmeric production, mechanical harvest and processing. 
  • Online video posted to the Sustainable and Organic Agriculture Program (SOAP) YouTube channel (original posting 7/12/23, reposted 10/29/23). 
  • A podcast on Cover crop use and regenerative agriculture practices was aired 2/8/23 on ( Graduate student Alina (Iliadis) Wood interviewed participating grower Kevin Flanagan. 
  • A Research and Extension Bulletin describing concurrent cover crop cultivation to promote ecosystem function and crop quality was developed (7/12/23).
Education and outreach results:

Our outreach objective was to disseminate project results to 200 local producers, agricultural professionals, researchers, students, and interested community members through a series of workshops, online videos, newsletter and extension bulletins. We included a radio interview/podcast as part of our outreach efforts (past estimates put listeners of the show between 2,000-6,000 each week). While we may have fallen short of that goal in direct contact of individuals during the project period, indirect contacts are estimated to exceed 1,000. Additionally, the products generated from this project will continue to be utilized in programming beyond the project period. 

15 Farmers intend/plan to change their practice(s)

Information Products

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.