Promoting Adaptive Management With 'Tropic Sun' sunn hemp (Crotolaria juncea) in Hawaii for Ecological Strategies in Weed Control, Nematode Suppression and Nutrient Management

Final Report for EW08-013

Project Type: Professional Development Program
Funds awarded in 2008: $53,768.00
Projected End Date: 12/31/2011
Region: Western
State: Hawaii
Principal Investigator:
Dr. james leary
University of Hawaii at Manoa
Dr. Brent Sipes
University of Hawaii
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Project Information


Sunn hemp can be used as a green manure cover crop to control weeds, nematodes, and other pests, add nutrients to the soil, and prevent soil erosion. These benefits from sunn hemp act to increase soil health in agricultural fields. Overall, fields cover cropped with sunn hemp exhibit a more robust and complex soil nematode community—a sign of a healthy soil ecosystem. Innovative growers have recognized the benefits of sunn hemp and are seeking seeds and desire collaboration with researchers as these producers integrate sunn hemp into their individual cropping systems.

Project Objectives:
  • To establish field day demonstrations for maximizing sunn hemp establishment and biomass productivity.

    To demonstrate the qualities of sunn hemp as a surface residue.


Sunn hemp (Crotalaria juncea) is a leguminous cover crop adapted to tropical and subtropical areas. Sunn hemp was originally recommended by the Soil Conservation Service for erosion control but its benefits extend even further (Joy and Peterson, 2005). Sunn hemp generates copious amounts of biomass (7 t/ha of air-dried organic matter at 2 months of growth), and produces 150 to 165 kg/ha of N under favorable conditions (Rotar and Joy, 1983). The plant has a well-developed root system, with a strong taproot. Sunn hemp produces allelopathic compounds that affect soil borne organisms and weeds (Wang et al., 2001; Jasy and Koshy, 1994; Fassuliotis and Skucas, 1969; Jourand, 2004). Soil incorporation of sunn hemp results in higher soil nutrient content, greater organic matter, and an increase in cation exchange capacity (Marshall, 2002).

Sunn hemp originates in the Indian subcontinent and has been grown for centuries as a green manure, livestock feed, non-wood fiber crop, and for soil improvement (Annon., 1999). Under favorable environmental conditions, sunn hemp could be an adequate fertilizer to meet most of the N, P, K nutrient requirements for many vegetable crops. A sunn hemp green manure has the potential to reduce fertilizer and pesticide needs for growers. While Indian farmers have embraced and benefitted from the wonders of a sunn hemp cover crop, US grower adoption of sunn hemp has been slow. Consequently, the objectives and goals for this project were to facilitate grower awareness and incorporation of sunn hemp into their cropping systems where appropriate. Our specific objectives/performance targets are listed below.


Click linked name(s) to expand/collapse or show everyone's info
  • Dr. Theodore Radovich
  • Jari Sugano
  • Dr. Koon-Hui Wang

Education & Outreach Initiatives



To better leverage funding and accomplish the goals of the project, activities were combined with several other projects (Table 1).

Field plots were established on cooperator farms (Khamphoute, Keleikoa, and Kamiya) and on university research stations. Land was prepared, sunn hemp planted, and established. In all test, pre- and post-sunn hemp soil samples were taken and assayed for plant-parasitic and free-living nematodes (Marahatta et al., 2011). Soil health was calculated based upon the free-living nematode numbers and types (Marahatta et al., 2011). In those trials monitoring soil health, weed pressure was also observed and recorded. After sunn hemp was grown for the time desired in each test, the cover crop was mowed and incorporated into the soil. The cash crop was then planted.

Field days were held at the Keleikoa farm, the Poamoho research station and the Waimanalo research station. The field days were held in conjunction with other projects focused on sustainable agricultural production to boost attendance. At the four field days (two held at Waimanalo), one researcher briefly presented the objective of the trial and any current results from assays. The researcher was then available to answer questions.

To maintain sunn hemp on growers and agricultural professionals radar, a postcard was developed. Development of the postcard arose from informal evaluations conducted during the the field days, interviews with cooperators, and discussion among agricultural professionals. Agricultural professionals pointed out that information on sunn hemp is available in a range of media - online, video, and print. A postcard highlighting sunn hemp’s benefits was developed to meet this need for reminding people. The postcard was developed, printed and distributed. County agents were sent a supply of the sunn hemp postcards to offer to clientele. Sustainable agricultural professional in the state were provided postcards as well.

Literature Cited

Anonymous. 1999. Sunn Hemp: A cover crop for southern and tropical farming systems. USDA NRCS Soil Quality Institute. Technical Note No. 10.

Fassuliotis, G. & Skucas, G. P. (1969). The effect of pyrrolizidine alkaloid ester and plants containing pyrrolizidine on Meliodogyne incognita acrita. Journal of Nematology 1, 287-288.

Jourand, P., Rapior, S., Fargette, M. & Materille, T. (2004). Nematostatic effects of a leaf extract from Crotalaria virgulata subsp. grantiana on Meloidogyne incognita and its use to protect tomato roots. Nematology 6, 79-84.

Joy, R. &. S. Peterson. 2005. Sunn hemp. USDA NRCS Soil Quality Institute. Plant Guide.

S.P. Marahatta, K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2010. Strip-till cover cropping for managing nematodes, soil microarthropods and weeds in a bitter melon agroecosystem. Journal of Nematology 42:111-119.

Marshall, A. J. (2002). Sunn hemp (Crotalaria juncea) as an organic amendment in crop production. M.S. Thesis. University of Florida. Gainesville, FL.

Reeves, D. W., Mansoer, Z. & Wood, C. W. (1996). Suitability of sunn hemp as an alternative legume cover crop. Proceedings of the New Technology and Conservation Tillage 96, 125-130. University of Tennessee Agricultural Experiment Station, Jackson, TN, U.S.A.

Rotar, P.P., Joy, R.J., 1983. 'Tropic Sun' sunn hemp, Crotalaria juncea L. Research Extension Series 036 University of Hawaii, Honolulu.

Wang, K.-H., R. McSorley, R. N.Gallaher, N. Kokalis-Burelle. 2008. Cover crops and organic mulches for nematode, weed, and plant health management. Nematology 10: 231-242.

Outreach and Publications

S.P. Marahatta, K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2011. Integration of sunn hemp cover cropping and soil solarization for plant-parasitic and free-living nematode management. Nematology (accepted September 2011).

S.P. Marahatta, K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2011. Effects of Crotalaria juncea on anhydrobiotic state of Rotylenchulus reniformis. Nematropica (submitted August 2011).

S.P. Marahatta, K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2010. Strip-till cover cropping for managing nematodes, soil microarthropods and weeds in a bitter melon agroecosystem. Journal of Nematology 42:111-119.

K.-H. Wang, B.S. Sipes, and C.R.R. Hooks. 2011. Sunn hemp cover cropping and solarization as alternatives to soil fumigants for pineapple production. Acta Horticulturae 902:221232.

S. Marahatta, K.-H. Wang, and B. Sipes. 2011. Effects of Cortalaria juncea on the anhydrobiotic stage of Rotylenchulus reniformis. Journal of Nematology 41: in press.

S. Marahatta, K.-H. Wang, and B. Sipes. 2011. Effects of Cortalaria juncea on the anhydrobiotic stage of Rotylenchulus reniformis. Journal of Nematology 41: in press.

S. Marahatta, K.-H. Wang, and B. Sipes. 2011. Integration of sunn hemp cover cropping and soil solarization for reniform nematode, Rotylenchulus reniformis, management. Phytopathology 101:in press.

I.-C. Wang, K.-H. Wang, and B. Sipes. 2011. Nematode community analysis for soil ecosystem health prediction. Phytopathology 101: in press.

S.P. Marahatta, K.-H. Wang, and B. S. Sipes. 2010. An improvement in marigold cover cropping by targeting vulnerable stages of root-knot nematodes. Journal of Nematology 41: in press.

Outcomes and impacts:

The field days were attended by approximately 100 people. Producer/growers represented 20-25% of the attendees. Agricultural professions composed of county agents, NRCS personnel, state government and private consultants constituted 75-80% of the attendees at the four field days. The field days were very successful in reaching agricultural professions with a penetration of 50% of the appropriate individuals in the state attending.

Agricultural professionals responded positively to the field demonstrations. Selected field day participants were debriefed after the field days in loosely structured interviews. Evaluations and assessments found that most of the innovative and likely early adopters of sunn hemp were doing so. We found it relatively easy to secure cooperators for field sites/demonstrations. Clientele was contacting us for sources of sunn hemp seed. Agricultural professionals indicated their knowledge of and access to a fairly wide body of sunn hemp cultivation and benefits information. We concluded that it would be appropriate to move to other means of conveying information on sunn hemp rather than demonstrations. We needed to reach a broader audience of clientele than those that would come to field days.

A wide range of information is available to grower clientele on sunn hemp. Media is available in print, video (, and on-line formats ( Growers have information available on the cultivation of sunn hemp and on the benefits of sunn hemp. Growers may be at the stage of needing reminders and encouragement to incorporate sunn hemp into their cropping systems.

Project Outcomes

Project outcomes:

The current project has had several accomplishments. In the short term, field demonstrations provided growers a chance to have first-had experience with sunn hemp growth and utility. In the long term, the availability of a postcard with information on sunn hemp should aid more growers in adopting sunn hemp into their operations. In addition to grower increases in grower awareness and adoption, research has continued to demonstrate the benefits of sunn hemp.

This project contributed additional scientific information on sunn hemp benefits in controlling plant-parasitic nematodes and enhancing the soil food web. While sunn hemp cannot eliminate plant-parasitic nematodes, weeds, or all insects, the cover crop does reduce the pest pressure in the subsequent cash crop. The Enrichment indices (EI) associated with sunn hemp indicate an abundance of bacteria-feeding nematodes which signifies bacterial decomposition in a soil will more likely rich in nutrients. The value of EI tends to follow the trend of sunn hemp biomass. We have continued to demonstrate that sunn hemp must be treated as a crop to be grown successfully.


Potential Contributions

Through the postcard and presentations at scientific meetings, many agricultural professionals have gained additional understanding and familiarity with sunn hemp. Agricultural growers in the state have also gained appreciation and understanding of the benefits of sunn hemp from field days. Growers will also continue to be informed of benefits from sunn hemp use as a cover crop or green manure over several years after the termination of the project from the postcards that will serve as reminders and refreshers.

Future Recommendations

More widespread adoption of sunn hemp as a cover crop or green manure will depend upon availability of seed and cost analysis. Sunn hemp seed is sometimes difficult to secure in sufficient quantities. This could be a substantial barrier to widespread adoption by growers if they are unable to secure planting material. Future projects might need to work with entrepreneurs who are willing to grow commercial seed. Producers needs better economic analysis to understand how sunn hemp can improve their profitability. Economic analysis and clearer cost/benefits for growers might encourage adoption at a faster rate.

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.