Improving and extending the superhero status of the sunn hemp to other growers in need of help

Final Report for SW07-604

Project Type: Research and Education
Funds awarded in 2007: $10,000.00
Projected End Date: 12/31/2009
Region: Western
State: Hawaii
Principal Investigator:
Dr. Cerruti R. R. Hooks
University of Maryland
Dr. Koon-Hui Wang
University of Hawaii
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Project Information


We completed 6 demonstration trials in commercial vegetable farms in Hawaii to demonstrate the use of sunn hemp in conventional cover cropping system as well as in an improved cover cropping system (strip-till living mulch system). Extensive data were collected to demonstrate that strip-till sunn hemp 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 the conventional sunn hemp cover cropping system. Thus, we were able to enhance the sustainability of sunn hemp cover cropping beyond its conventional system. In addition, we also occasionally observed a suppression of insect pests in sunn hemp living mulch system. Unfortunately, we were not able to see positive outcomes from practicing sunn hemp row switching technique (switching rows between cash crop and cover crop rows between cycles). This could be due to technical difficulties in tracking planting rows in previos cropping cycle. Continuous efforts are underway to overcome this problem by practicing no-till planting. Multiple outreach routes were generated during this project period to promote the use of sunn hemp for Hawaiian vegetable growers as well as home gardeners. A bonus of this project is that we also convinced some fruit crop farmers (papaya and pineapple) to try out sunn hemp cover cropping. We are encouraged to documented that two cycles of sunn hemp cover cropping consistently increased crop growth or yield in this sunn hemp strip-till cover cropping system.

Project Objectives:

The overall goal of this project is to 1) enhance the sustainability of using sunn hemp cover crop, and 2) further advance the benefits of sunn hemp for suppressing nematode pests and enhancing beneficial soil microorganisms (free-living nematodes and soil microarthropods) demonstrated previously. To achieve these objectives, sunn hemp (SH) was planted for three months and compared to a fallow (F) with weeds treatments. Subsequently, alternate rows of sunn hemp were stripped-tilled. Then, cash crops will be planted over the strip-tilled rows. The remaining sunn hemp rows will be cut and residues left on the soil surface as organic mulch. The novelty of this project is the follow up of sunn hemp cover cropping system after the first cycle of cash crop. After the initial crop has been harvested, cash crop rows and organic mulch rows will be switched. Nematode community analysis was used to analyze soil health conditions.


Because of limited land, many small- or intermediate-scale farmers in Hawaii tend to grow similar crops continuously in the same fields within the same rows. Growing cover crops with cash crops offers one alternative to reduce below ground (e.g., fungal diseases, plant-parasitic nematodes) and above ground pest populations. During a recently completed Ag Professional + Producer Western SARE project entitled “A superhero without a cape: using the cover crop sunn hemp to feed the soil, suppress nematodes, and smother weeds”, PIs of this project found that sunn hemp cover cropping effects for suppressing plant-parasitic nematodes and soil-borne fungi wear off in the second cycle of a double cropping system. Thus, in this project, we proposed a more sustainable long-term approach by conducting strip-till living mulch cover cropping, followed by alternating rows of sunn hemp and vegetables during each planting. Thus, instead of planting the follow up crop into the same row as the previous crop; the sunn hemp rows that remained during the initial crop planting would be strip- tilled under (green manure) and the second cash crop would be planted into the tilled strips. A similar approach for nematode management was successfully used in cotton. A second cotton crop was planted in the alley between previous cotton rows. By season end, reniform nematode (Rotylenchulus reniformis) densities were lower in the new cotton rows than the initial in-row planting areas. In a sunn hemp row switching method, sunn hemp that tilled under supplies nutrient to the soil and releases compounds that are toxic to nematodes. In addition, continuous clipping of sunn hemp (monthly clipping) will add surface mulch to the soil, thus to provide slow release of nutrients to the soil over a longer period of time. We anticipate using this modified strategy will help prolong sunn hemps’ benefits beyond the initial crop by significantly reducing nematode pests and increasing soil health conditions.


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  • Khamphout Chandara
  • Alberta de Jetley


Materials and methods:

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 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 switch the cash crop planting rows with the cover crop living mulch rows and replanted with eggplant. Cash crops were planted in the same planting rows in cycle I and II for the bareground treatment. Trial II was conducted from October 2008 to May 2009 at Benny’s Farm in Lanai to demonstrate the use of sunn hemp cover crop for nematodes, weeds and soil health management followed by tomato production. Trial III was conducted on May 2007 to Aug 2009 at the University of Hawaii Poamoho Experiment Station where sunn hemp was planted for 3 months, strip-tilled and alternate rows of sunn hemp were served as living mulch. Two consecutive cucurbit crops (sweet-slice cucumber and winter gourd) were planted in this system and population densities of nematodes, weeds, insects and microarthropods were examined. In addition to these three trials that were set up according to description in the proposal, new farmers requested to participate in our “Sunn hemp Superhero” 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 IV was conducted at Otsuji’s Farm from April to July to demonstrate the use of sunn hemp to manage root-knot and 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 followed up using soil from Otsuji Farm. Trial V 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 VI 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.

Research results and discussion:

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: Sunn hemp (SH) cover crops were strip-tilled in Cycle I followed by intercropping of bitter melon with SH living mulch (Fig. 1). 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 the bitter melon crop. After 2 months of fallow period, eggplants were planted in cycle II by switching the cash crop rows with the cover crop rows. Cover crops were intercropped with eggplants. Cash crop planting rows remained the same for the bareground (BG) treatment (Control). Unfortunately, no differences in nematode community indices and root-knot nematodes count even at 3 months after eggplant planting. However, marketable yield of eggplant were higher when intercropped with SH as compared to the BG control. This could be due to wind protection and reduction of mite infestation on the eggplants when eggplants were interplanted with sunn hemp.

Trial II: Planting of SH significantly 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 increased tomato plant growth (measured by stem diameter), and resulted in significantly higher percentage of healthier plants at the end of the experiment. Soil nutrient analysis indicated that planting of SH consistently increased C, N, P and Mg content in the soil prior to tomato planting. Due to the location of Lanai City in another island, less frequent soil sampling was taken. 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 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 III: Sunn hemp in a strip-till living mulch system was evaluated in two consecutive crops of cucumber and winter gourd (Fig. 2). Data on above and below ground insect and nematode beneficial and pests were collected and summarized in a handout to participants during “Sunn hemp Superhero Workshop” at Waimanalo, Oahu on September 29, 2009. ( This trial demonstrated that at termination of cover crop, sunn hemp generated great amount of biomass with high amount of organic matter and nitrogen. However, majority of the N is in organic form and is not readily available plants to uptake. Additional sunn hemp clipping generated more organic biomass that can serve as good surface mulch for weed control (reduced weed biomass by 4 fold at cucumber planting) and maintain a source of organic matter over a longer period of time. Weed coverage was significantly lower in SH plots as compared to BG treatment during early stage of cash crop growth. Cucumber leaves had lower numbers of thrips and whiteflies in SH plots than the BG treatment. Great amount of organic matters from sunn hemp mulch also enhanced deteriovores such as isopods that will enhance decomposition. SH significantly suppressed plant-parasitic nematodes and increased free-living nematodes that are involved in soil nutrient cycling. However, cucumber yield was lower in SH plots vs the BG control partly due to more severe fruitflies damage and shading effect of the cover crops in Cycle I. In Cycle II, winter gourd was planted immediately after the cucumber crop was plowed down. An outbreak of charcoal rot due to a seed-borne fungus, Microphominia occurred. Plants in SH plots were more vigorous, thus more seedlings survived from charcoal rot disease in SH plots than the BG. Sunn hemp continued to suppress reniform nematodes until mid season of the winter gourd crop. SH continued to maintain higher population of beneficial nematodes, EI (Enrichment index), nematode richness and diversity as compared to BG control, indicating a healthier soil conditions. It is especially encouraging to see higher numbers of omnivorous and predatory nematodes in Cyle II than that in Cycle I, another indication of healthier soil environment due to strip-till cover cropping that imposed less soil disturbance. In addition, SH also provides continuous source of leaf litters or organic matter to sustain high number of amphipods, a microarthropods involved in soil nutrient cycling. However, in this cycle, SH did not suppressed thrips and pickle worm damage on winter gourd. None the less, SH increased winter gourd yield at the end of the experiment. Although this is partially due to better survival of winter gourd seedlings in SH plots during the outbreak of charcoal rot, it is also due to better plant growth in the SH plots. This is consistent with many other studies that organic farming approaches might take times to show positive output. Time required to build up beneficial organisms in an agroecosystem might be paid off in a long run.

Trial IV, V and VI are demonstration trial without extensive data collection. Similar trends to Trial I, II and III were observed by the participating farmers directly. Otsuji Farm want to compare sunn hemp to yard waste compost treatment, Aloun Farm requested (and received) 80 lb of sunn hemp seeds to plant in additional 2 acres of farm land, Kaleikoa Farm is now expanding to another plot of papaya-sunn hemp intercropping system.

Research conclusions:

Outcome: 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 an Associated Extension 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, 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 website for the PI was created ( A newsletter article ( related to this project were published. A video “Sunn hemp for soil health management” was posted on YouTube (

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.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

1. A workshop was held on 29 September 2009 at Waimanalo Experiment Station.
2. A comprehensive handout was prepared for this workshop, posted on CTAHR Sustainable & Organic Program website: (
3. Video:
4. A newsletter article ( related to this project were published in University of Hawaii CTAHR Sustainable and Organic Program Newsletter.

Education and Outreach Outcomes

Recommendations for education and outreach:

Areas needing additional study

Row switching between cover crop and cash crop rows for subsequent planting cycle needs to be evaluated further. It is possible that our row switching technique is not precise and it is easy to miss the exact planting rows from one cropping cycle to another. We are planning on conducting row swatching technique in a no-till farming system where crop residues from previous crop is obvious and planting rows are easy to be tracked. This experiment is underway in our Poamoho Trial.

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.