Sunn hemp and its allelopathic compounds for vegetable production in Hawaii and beyond

2011 Annual Report for SW08-037

Project Type: Research and Education
Funds awarded in 2008: $156,105.00
Projected End Date: 12/31/2012
Region: Western
State: Hawaii
Principal Investigator:
Dr. Inga Zasada
USDA-ARS Horticultural Crops Research Lab
Dr. Koon-Hui Wang
University of Hawaii
Dr. Cerruti R. R. Hooks
University of Maryland
Dr. Ming Li Wang
Jari Sugano
University of Hawaii, TPSS
Dr. Mark Wright
University of Hawaii

Sunn hemp and its allelopathic compounds for vegetable production in Hawaii and beyond


In 2011, we focused on achieving Objectives 3 and 4 where we analyzed nematode toxicity of Crotalaria tissues in vitro using nematode bioassay, as well as detecting monocrotaline in these tissues using HPLC. We had organized an on-farm field day and a student field day to demonstrate the benefit of sunn hemp intercropping with zucchini for soil health as well as insect pest management. A postcard to promote the use of sunn hemp as a cover crop or green manure was printed and distributed. A peer review journal article is published regarding benefits of sunn hemp strip-till cover cropping.

Objectives/Performance Targets

1. Evaluate the impact of using Sunn Hemp (SH) as an organic mulch and green manure, solarization, and SH + solarization on nematode, insect and weed pests during two cropping cycles;

2. Examine how SH and solarization impact soil health, pest and beneficial organisms;

3. Identify compounds in sunn hemp that are toxic to nematodes; and

4. Determine the lethal dosage of SH residue required to suppress nematodes and whether solar heat can enhance its effectiveness.


Bioassay for Allelopathic Effect

Based on 10 field trials conducted in this project for the last three years, biomass of sunn hemp varied from 0.73 to 6.67 Mg/ha of dry sunn hemp residues over approximately two months of growing period (Fig. 1). This variation could be seasonal or due to soil pH. A series of studies were conducted to understand if allelopathic effect of sunn hemp against root-knot nematodes, Meloidogyne incognita, varied by stage of growth, sunn hemp tissues, concentration of sunn hemp and whether the sunn hemp residues were solarized or not. In addition, allelopathic effect of sunn hemp (Crotalaria juncea) were compared to C. spectabilis (Cs), and C. retusa (Cr). Tissues of Cj were collected at one, two, three and four months after planting and partition into stem, leaves, flowers and roots or remained as whole plant tissues. Each testing units were prepared into leachates of 0.1, 0.5, 1.0 and 2.5% concentration. Juveniles (J2) of M. incognita were imbibed into designated leachate for 48 hours and transferred to water for another 24 hours to test for number of nematode revived in water (if the nematode revived, then the suppressive effect is only nematostatic and not nematicidal). Eggs were imbibed in designated leachates for one week and counted for % hatched. Based on factorial analysis of variance, allelopathic effect of Cj was partially nematostatic, i.e. certain percentage revived after transferring to water. In general, Cj suppressed J2 more efficiently than egg hatching (Fig. 2A). One- and three-month old tissues were more suppressive to J2 and egg hatching than two- and four-month old. Leaf tissue was most suppressive to J2, and the result resembled those in the whole plant tissues (Fig. 2B). Less J2 revived in flower extract than those in leaf extract. However, leaf tissue was least suppressive to egg hatching. Significant interaction among Cj age, tissue and leachate concentration occurred (P < 0.01). One – three month old Cj whole plant tissue provided 100% kill of J2 at 1% concentration (equivalent to five tons/acre), whereas two – three month old Cj tissue kill 75% of J2 at only 0.5% (~ 2.5 tons/acre) (Fig. 2C, D). Solarization might have increased the allelopathic effect of SH, but the effect was not remarkable. Among the three species examined, Cj was most suppressive to J2 and egg hatch, followed by Cs and Cr (P < 0.05). However, significant interaction among species, tissues and concentrations was observed (P < 0.01).


Same set of tissue samples for allelopathic bioassay (different ages, plant parts, species and solarization treatment) were subjected for HPLC analysis to quantify monocrotaline. However, most results indicated that concentration of monocrotaline in Crotalaria juncea is low or undetectable (Fig. 3). New literature indicated that another alkaloid might be involved in nematostatic effect of sunn hemp. Thus, we are continue to test sunn hemp tissue for another alkaloid and will include this result in our final report.

Outreach Activities

1. Wang, K.-H. Emerging strategies for controlling plant-parasitic nematodes organically. HOFA Annual Conference. 21 October, 2011. Attendance: 75.

2. Wang, K.-H., L. Kaufman, T. Radovich, and J. Sugano. Strip-till cover cropping and vermicompost tea workshop. Twin Bridges Farm, Waialua, HI. 19 May, 2011. Attendance:30

3. Wang, K.-H. Field trip to Twin Bridges Farm: Strip-till cover cropping and vermicompost tea treatment. Guess Lecture for Organic Food Production. Department of Tropical Plant and Soil Sciences, University of Hawaii, HI. October, 2011. Attendance: 20 students.

We organized an on-farm field day to demonstrate the benefit of sunn hemp intercropping with zucchini for soil health as well as insect pest management. Planting of sunn hemp as a living mulch acts as a trap crop for silverleaf whitefly and significantly reduced the damage of zucchini from this whitefly. Thirty participants in this workshop were able to observe the crop yield improvement of growing sunn hemp as living mulch on zucchini and the additional benefits of sunn hemp, including weed suppression from sunn hemp mulch, enhancement of pollinators in sunn hemp-zucchini intercropping niche and increased abundance of soil nutrient recyclers, the free-living nematodes. Detail output of this demonstration trial is summarized at Our participants include College of Tropical Agriculture and Human Resources (CTAHR) administrators, NRCS agents and farmers, as well as extension agents.

During this period, we printed a postcard (Fig. 4) to promote the use of sunn hemp as a cover crop or green manure and distributed to all county extension agents in our college, our master gardeners, a new sunn hemp seed producer in Molokai, HI, and etc.

A news article entitled “Improving the status of sunn hemp as a cover crop for soil health and pests management” was published in CTAHR Sustainable and Organic Agriculture Program’s newsletter “H?nai‘Ai Newsletter” ( to update the public about our findings from this project as well as several previous sunn hemp projects funded by Western SAREand NRCS. We also included the research outcomes from this project at the presentation made to Hawaiian Organic Farmers Association’s (HOFA) annual meeting entitled “Emerging strategies for controlling plant-parasitic nematodes organically” which reached out to 75 organic farmers in the state.

Peer Review Publication

1. Wang, K.-H., C.R.R. Hooks, and S.P. Marahatta. 2011. Can using a strip-tilled living mulch system enhance organisms higher up in the soil food web hierarchy? Applied Soil Ecology 49: 107-117(doi:10.1016/j.apsoil.2011.06.008 ).

Conference Presentation

1. Wang, K.-H. and I. Zasada. Allelopathic effects of crotalaria spp. against M. incognita as affected by crop age, biomass, heat and species. 2011 Society of Nematologists Conference, Corvallis, OR. July 2011.

2. Marahatta, S.P., Wang, K.-H., and Sipes, B.S. 2011. Effects of Crotalaria juncea on the anhydrobiotic stage of Rotylenchulus reniformis. Oral presentation on fiftieth annual meeting of the Society of Nematologists, Corvallis, Oregon.

3. Marahatta, S.P., Wang, K.-H., and Sipes, B.S. 2011. Integration of sunn hemp cover cropping and soil solarization for reniform nematode, Rotylenchulus reniformis, management. Phytopathology 101:S113 (Abstr).

4. Marahatta, S.P., Wang, K.-H., and Sipes, B.S. 2011. Improvement in sunn hemp cover cropping for reniform nematode, Rotylenchulus reniformis, management. Oral presentation at 23nd Annual CTAHR Student Research Symposium, Honolulu, Hawaii.

Extension Article

1. Wang, K.-H., B.S. Sipes, C.R.R. Hooks, and J. Leary. Improving the status of sunn hemp as a cover crop for soil health and pests management. H?nai‘Ai Newsletter Summer 2011.

Impacts and Contributions/Outcomes

The outcome of this project demonstrated that planting sunn hemp cover crop for one – three months and a generated biomass of five tons/acre could suppress root-knot nematodes efficiently. However, sunn hemp does not always produce this amount of biomass within this growing period. Soil pH could be one reason of low sunn hemp biomass production. Current bioassay result showed that we could still achieve 75% of root-knot nematode suppression if the sunn hemp is at least grown for two – three months. However, four-month old sunn hemp materials do not suppress root-knot nematodes efficiently. Our demonstration trial also reached out to agriculture practitioners, as well as NRCS agents about the multiple benefits of sunn hemp including nematode suppression, soil health management, serving as trap crops for whiteflies and weed suppression. This field trial also serves as a good demonstration trial for CTAHR students to learn about the benefits of cover cropping. Presenting this research results at the HOFA meeting also reached out to organic growers on techniques to improve the benefits of growing sunn hemp cover crop. A new sunn hemp seed producer in Molokai had contacted us at several occasions to obtain information on sunn hemp. Although we cannot estimate how much % of increase in sunn hemp cover cropping as a outcome of this project, we anticipated more growers to use sunn hemp as cover crop either at pre-plant or as barrier crop/companion plant.


Naresh Chand Pradhan
University of Hawaii
21-544 Dilli Bazar
Kathmandu, Nepal, -
Office Phone: 97714436455
Alton Arakaki
Assistant Extension Agent
University of Hawaii
Maui County. Molokai Extension Office
P.O. Box 394
Hoolehua, HI 96729
Office Phone: 8085676929
Steve Fukuda
Couty Extension Agent
University of Hawaii
Wahiawa Extension Office
910 California Avenue
Wahiawa, HI 96786
Office Phone: 8086224185