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

2009 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


We completed 6 vegetable field trials in Hawaii to demonstrate the use of sunn hemp (SH) in conventional cover cropping system as well as when integrated with soil solarization (Sol) to improve nematodes and weeds management. We demonstrated that sunn hemp in a strip-till living mulch system suppressed initial population densities of plant-parasitic nematodes and weed population densities, while enhancing beneficial free-living nematodes and soil microarthropods involved in soil nutrient cycling. This cover cropping system was able to extend the nutrient enrichment effect of sunn hemp cover cropping towards the end of a vegetable cropping cycle, a phenomenon that had not been shown in a conventional sunn hemp cover cropping system. Solarization resulted in certain level of soil disturbance to the soil communities (base on nematode community analysis). However, disturbance from Sol did not last and dissipated towards the end of a cash crop cycle. This disturbance was further reduced when solarization was integrated with sunn hemp (SH+Sol). Higher heat were generated in SH+Sol as compared to Sol alone. Sol significantly suppressed weeds up to 3 months after crop planting. We hypothesized that SH+Sol would suppress plant-parasitic nematodes more efficiently than SH alone. However, we observed a trend that Sol reduced the nematode suppressive activities of SH. Further experiments are underway to understand SH allelopathic effects and its interaction of soil solarization. Outreach activities include a field day; a video about the use of sunn hemp to improve soil health was uploaded to YouTube; two online newsletter articles on the University of Hawaii Sustainable and Organic Program were published; research concepts and outcomes were published as downloadable online articles in the PI’s webpage, and a peer review journal article were published. A bonus of this project is that we also convinced some fruit crop farmers (papaya and pineapple) to try out sunn hemp cover cropping in Hawaii.

Objectives/Performance Targets

Our approach is to use sunn hemp (SH) as an interplanted cover crop and green manure, solarization, and combine SH with solarization within the same field. Specific objectives of the proposed research are to:

1. Evaluate the impact of using 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.



Six field trials were conducted; five of them were at commercial farms whereas one of them was at the UH Poamoho experiment station. Trial I was conducted from October 2008 to May 2009 at Benny’s Farm in Lanai to demonstrate the use of sunn hemp cover crop and its integration with solarization for nematodes, weeds and soil health management for tomato production. Trial II was conducted at Khamphout Farm in Kunia, Oahu for two cropping cycles. In the first cycle, sunn hemp was planted as cover crop and was compared to another nematode suppressive cover crop, marigold and bareground preplant treatments for management of nematodes and soil health on bitter melon. In the second cycle, we split the main plot (cover crop treatments) into soil solarization vs not sub-plots followed by eggplant planting. This cover crop and solarization was initiated in June 2009 and is ongoing. In addition to these two trials that were set up according to description in the proposal, new farmers requested to participate in our sunn hemp project for nematode management. Most of these farmers are only interested in using sunn hemp as cover crop and are currently reluctant to perform soil solarization that will delay their planting period for another 6 weeks. Thus, three more sunn hemp field trials were conducted without the integration of solarization. Trial III was conducted at Osutji’s Farm from April to July to demonstrate the use of sunn hemp to manage reniform nematodes on beet in Hawaii Kai, Oahu. Due to complication of the farmer planting different varieties of beet and resulted in unexpected outcomes, a greenhouse pot experiment was follow up using soil from Osutji Farm. Trial IV was conducted at Aloun Farm’s pineapple field in Kunia, Oahu to demonstrate use of sunn hemp cover crop as green manure and for nematode management during the summer months from May to July 2009. Trial V was initiated at Kaleikoa Farm, Kaulapuu, Molokai in September to demonstrate the use of sunn hemp as living mulch in papaya planting patch to suppress weeds, preserve soil moisture and enhance beneficial nematodes. Trial VI was recently initiated in November 2009 at University of Hawaii Poamoho Experiment Station where sunn hemp was planted to provide sunn hemp tissues that will be used for the assay of allelopathic compounds.

All demonstration trials showed that sunn hemp planted in spring, summer or fall generated significant amount of organic biomass (>9 tons/acre of wet biomass) over 2 or 3 months of growing period.

Trial I: Solarization in the fall in Lanai did not generate sufficient heat to suppress weeds and root-knot nematodes due to heavy rain fall in that season. However, planting of SH (alone or in combination with Sol) (FIG. 1) improved soil health conditions as indicated by enrichment index, structure index and channel index of nematode community analysis. SH also significantly increased numbers of soil micro-arthropods such as predatory and oribatid mites, and collembolan involved in soil nutrient cycling. At the end of the tomato crop, SH alone or SH+Sol increased tomato plant growth (measured by stem diameter), and SH resulted in significantly higher percentage of healthier plants at the end of the experiment (Fig. 2). Soil nutrient analysis indicated that planting of SH (regardless of integration with Sol) consistently increased N, P and Mg content in the soil prior to tomato planting. Longer period of sunn hemp planting in the SH treatment (5 months) as compared to that in the SH+Sol (<3 months) contributed higher C content in the soil prior to crop planting. We expected significant root-knot nematode suppression would have been observed if soil data were collected few weeks after sunn hemp incorporation. This is because sunn hemp suppresses root-knot nematodes by releasing monocrotaline when soil incorporated. Root-knot nematode pressure in this field was very high, reaching >1400 root-knot nematodes/250cm3 soil at 4 months after tomato planting. More plants in control plots were showing severe wilting with root-galling. Healthier tomato plants in SH or SH+Sol plots were an indication of earlier root-knot nematode suppression and enhancement of beneficial soil microorganisms in the soil that resulted in more nutrients available for the tomato plants to uptake.

Trial II: Sunn hemp cover crops were strip-tilled in Cycle I followed by intercropping of bitter melon with SH living mulch. This SH cover cropping system significantly suppressed root-knot nematodes prior to bitter melon planting and resulted in healthier plant growth of bitter melon based on measurement of stem diameter. Root-gall index of bitter melon was also lower in SH plots at termination of bitter melon crop. Severe damage from melon flies interfered with our yield data collection. Sunn hemp as living mulch consistently enhance beneficial nematodes (bacterivorous and fungivorous nematodes) after strip-tilling of sunn hemp, and continued to maintain higher enrichment index (an indication of soil nutrient enrichment) towards the end of bitter melon crop. After 2 months of fallow period, each treatment plots were split into two and were either solarized or not. Eggplant was planted in cycle II. Numbers of root-knot nematodes were very low at initial stage of eggplant planting. Opposite trends of SH treatment effect were observed between Sol and non-Sol plots (i.e. SH reduced root-knot nematodes in non-Sol treatment but increased the numbers of root-knot nematodes in the Sol plots). This is opposite from out hypothesis that SH+Sol will suppress root-knot nematode more efficient than SH alone. However the effect was not significant. It is possible that SH+Sol does not have biofumigation effect. We will further examine SH+Sol effect with freshly incorporated sunn hemp residues. Although Sol reduced bacterial and fungal feeding nematodes initially, this impact did not last until 3 months after eggplant planting. Despite the minimum effect of SH+Sol on nematodes, SH+Sol significantly increased soil temperature (maximum temperature = 74?C) as compared to Sol alone (maximum temperature = 62?C) and resulted in significant reduction of weed population densities up to 3 months after eggplants were planted (Fig. 3).

Trial III, IV and V are demonstration trial without extensive data collection. Similar trends to Trial I and II were observed by participating farmers. We are conducting Trial VI to perform studies related to objective 3 and 4.

Impacts and Contributions/Outcomes

A workshop was held on 29 September 2009 at Waimanalo Experiment Station in collaboration with two University of Hawaii CTAHR extension specialists, Drs. Ted Radovich and James Leary. The workshop is co-organized by extension agents, Jari Sugano and Steve Fukuda and Associated Specialist that work with Hawaii underserved farmers, Dr. Sabina Swift. A total of 15 agricultural personnel attended the workshop. This participant encompassed farmers, other CTAHR extension agents, master gardeners, Syngenta representatives, and CTAHR graduate students and faculties (besides the organizers). A comprehensive handout was prepared for this workshop. It is now posted on CTAHR Sustainable & Organic Program website ( The new concept of sunn hemp cover cropping integrating with solarization is also introduced online at ( Our sunn hemp videos ( and CTAHR extension articles (SCM-21, PD-28) generated last year were distributed during the workshop. Additional sunn hemp extension articles were distributed to Lanai home gardeners class thought regularly by CTAHR extension agent, Alton Arakaki, at Benny’s Farm. We hold a display booth entitled “The Wonderful World of Nematodes” at the Leeward Community College Annual Science Fair where we talked about damage of plant-parasitic nematodes and benefits of beneficial nematodes. Extension articles on sunn hemp were again distributed mainly to home gardeners.

Two conference presentations were made by the PI and a graduate student. The graduate student received a second place award for his poster presentation at College of Tropical Agricultural Science (CTAHR) student symposium. A peer reviewed article and a newsletter article ( related to this project were published. A video “Sunn hemp for soil health management” was posted on YouTube (

Publication: McSorley, R., K.-H. Wang, J.J. Frederick. 2008. Integrated effects of solarization, sunn hemp cover crop, and amendment on nematodes, weeds, and pepper yields. Nematropica 38: 115-125.

All participating farmers are adopting sunn hemp cover cropping. They learn that planting sunn hemp is a good solution to their nematode problem besides additional benefits in building up soil health. Farmers and master gardeners that attended the workshop received sunn hemp seeds for planting. A teacher at McKinley High School learns about sunn hemp through a student worked in our lab. She then requested sunn hemp seeds to demonstrate the use of cover crop in their high school agriculture project. Aloun Farm and Khamphout Farm received additional sunn hemp seeds to be planted in two additional acres of their farmland beside the demonstration plots. Aloun Farm had shown their interest to expand their sunn hemp planting after corn as soil after continuous planting of corn had shown nutrient deficiency despite regular fertilization. Alberta deJetley in Lanai is purchasing more sunn hemp seeds from Crop Care Hawaii, LCC for their farm and is participating in soil conservation program in NRCS EQIP Program. Rick Tamanaha in Molokai is convinced that planting of sunn hemp as ground cover can maintain soil moisture and will be a key to increase his organic papaya production in Molokai, a prime place for papaya production in Hawaii as it is located in a papaya ring spot virus-free zone. Many home gardeners like the idea of planting a crop to protect their vegetable crops or landscape plants from nematode damage. It is always a welcoming suggestion for them to plant sunn hemp in their garden.


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