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

2010 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 10 vegetable field trials in Hawaii during this project period to evaluate the benefits of integrating sunn hemp cover cropping (SH) with soil solarization (Sol); i.e. SH+Sol, to improve nematodes, weeds and soil health management. Nematodes, weeds and soil health data were collected at three of these sites (Whitmore Pineapple, Khamphoute Eggplant and Whitmore Cowpea trials). The remaining trials are for commercial demonstration purposes.

Summarizing all of the temperature data collected, we were able to generate a solarization temperature schematic scheme for Hawaii over a one-year period, where sufficient lethal heat can only be obtained between April and October. Although Sol alone suppressed weeds efficiently, integration of SH+Sol suppressed weed densities better than SH or Sol alone in the pineapple trial. However, integration of SH and Sol did not improve weed suppressive effect as compared to Sol alone in the eggplant and cowpea trials. Weed suppression usually only lasted one or two months after crop planting, but no difference among treatments thereafter. SH suppressed plant-parasitic nematodes most efficiently and consistently, except in Khamphoute’s eggplant trial, where cover crop treatment was imposed one cropping season ago. Integrating SH+Sol did not increase the nematode suppressive effect on plant-parasitic nematodes as compared to SH alone except when the solarization was conducted during late fall to winter. It is possible that high heat from Sol deactivate the SH allelopathic compounds against plant-parasitic nematodes. Based on nematode community analysis, Sol in the summer consistently disturbed soil health conditions. When solarization was conducted during late fall to winter, SH+Sol improved soil health condition better than SH alone. While combination of SH+Sol during the summer is beneficial for weeds suppression, it is not beneficial for nematode and soil health management as compared to SH alone.

We continue to study:

1) the distribution of nematode allelopathic compounds in sunn hemp tissues and how it could vary temporarily (based on phylogeny of sunn hemp),

2) allelopathic effects from three species of Crotalaria, and

3) impact of solarization on this allelopathic effect.

Objectives/Performance Targets


Our approach is to use sunn hemp (SH) as an interplanted cover crop and green manure (Fig. 1), solarization and a combination of SH with solarization within the same field (Fig. 2) for nematode, weeds and soil health management.

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.



Ten field trials were completed during this project period (Table 1). All of these field trials were to achieve Objectives 1 and 2. Complete data analysis were perform at the first three experimental sites (Khamphoute Farm, Whitmore cowpea and Whitmore pineapple) to examine the benefits of SH+Sol on nematode, weeds and soil health management. The papaya trials in Kualapuu and Leia were to demonstrate to farmers how to use sunn hemp for soil health management, as nematode is not a major concern for their production. We added in an additional trial in Waianae (Kanishiro Farm) to examine the benefits of solarization and sunn hemp for Fusarium wilt management on Manoa lettuce and Kai Choi, due to a request from a farmer participating in our 2009 workshop.

We are currently performing nematode assay in the lab to determine the differential lethal effect of SH on Meloidogyne incognita for Objective 3 and 4. Sunn hemp was partitioned into leaf, stem, flowe, and root tissues at two-, three- and four-months old. Three species of Crotalaria is currently under cultivation in the field to be used for nematode lethal assay. No result could be presented for Objective 3 & 4 at this time.

Results for Objective 1

Weed assays

Field trials clearly demonstrated that solarization suppressed weed population densities at early stage of cash crop growth. This effect was most obvious in the Whitmore Pineapple Trial.

( _project.pdf; http://www.ishs 17.pdf)

The combination of sunn hemp and solarization (SH+Sol) prolonged the weed suppressive effect for two months after pineapple was planted into black plastic mulch. SH and Sol alone only suppressed weed densities for one month after crop planting, as compared to the control (C). Similar results were obtained in Khamphoute Trial I (2009), where SH only suppressed broad leaf weeds at three weeks after eggplant planting, but solarization suppressed weed coverage up to three months after eggplant planting (Table 2). Similar trends of weed suppression by Sol and SH+Sol were observed in Khamphoute Trial II (2010) (Table 3, Fig. 3) and in the Whitmore Cowpea Trials (to be reported in a student dissertation).

Plant-parasitic nematode assays

Effects of SH+Sol on plant-parasitic nematodes varied. In the Whitmore Pineapple Trial (Wang et al., 2010) and Whitmore Cowpea Trial II (Table 6), SH suppressed plant-parasitic nematodes (% herbivores) most efficiently, whereas SH+Sol did not suppress plant-parasitic nematodes as compared to C or SH (P < 0.05). However, in the Whitmore Cowpea Trial I, SH+Sol suppressed % herbivores and abundance of the key plant-parasitic nematodes, reniform and root-knot nematodes, better than SH alone (Table 5). This difference between trials could be due to temperature achieved during soil solarization (Fig 4). Solarization conducted at the Whitmore Cowpea Trial I (December to January) did not generate sufficient heat to kill plant-parasitic nematode (lethal temperature of 42?C). This result could be suggesting that solarization heat might have degraded the nematode allelopathic effects of sunn hemp, deviating from our hypothesis that solar heat might enhance the release of allelopathic compounds from sunn hemp. We are planning to further test this effect in Objective 4.

Results for Objective 2

Soil health analysis

We used nematode community analysis to evaluate soil health. Soil health analysis for the Whitmore Pineapple trial was summarized in our webpage.


Beside comparing SH, Sol and SH+Sol to untreated control (C), we also compared soil health indices to a nearby pineapple plantation (PP) plot planted at the same time. The plantation practice involved multiple deep subsoiling, intensive nematicide application (preplant fumigation with Telone followed by post plant Vydate or Nemacure at 3-month interval), preemergence and post-plant selective herbicides, and etc. Nematode community analysis clearly showed that soil from PP had lowest EI, indicating that it was nutrient depleted, with zero SI, indicating that it was severely disturbed with no omnivorous and predatory nematodes (Fig. 5). These nematodes are higher in the soil food web and are sensitive to disturbance. PP also had highest CI, indicating that it was dominated by fungal decomposition and underwent a stressful environment. On the other hand, SH treatment had highest nematode richness, higher % of both bacterivorous and fungivorous nematodes and highest % omnivorous nematodes at termination of preplant treatment (Pi3), indicating soil communities with diverse groups of nutrient recycling free-living nematodes. Planting of SH increased EI and reduced CI initially (at Pi2) but resulted in higher SI throughout the experiment, indicating a less disturbed and healthier soil conditions (Fig 5). Overall, we did not find SH+Sol improved soil health conditions as compared to SH alone in this trial.

Experiment at Khamphoute Farm was slightly different from other trials where cover crop treatments were conducted in a previous cropping season as an intercrop between cash crop rows. Each cover crop treatment was split into solarized (Sol) or not solarized (non-Sol) (Table 4). Soil health analysis revealed that cover crops were not different from the bare ground control (BG). However, Sol significantly reduced soil health conditions as indicated by lower level of richness and enrichment index (EI) and greater channel index (CI) (Table 4). This is not surprising, as we anticipated Sol to be a disturbance to nematode communities due to its high heat affecting many beneficial organisms in the soil. However, no difference between Sol and non-Sol was observed toward the end of an eggplant crop (data not presented), indicating that the soil disturbance from Sol is only temporary. Despite the negative impact of Sol on soil health, cover crop and Sol treatments significantly increased eggplant plant growth (Table 7). However, integrating SH+Sol did not improve the plant growth better than SH or Sol alone.

We repeated another experiment at Whitmore to compare SH, Sol and SH+Sol to C (weedy fallow control) in 2009 (Trial I, Table 5). Solarization was conducted in late fall to winter (December-January) and did not generate sufficient lethal heat in this trial (Fig. 4). However, the impact of SH+Sol on nematode community analysis from this trial is promising. Sol alone again decreased richness and EI (Table 5). Adding sunn hemp cover cropping prior to Sol (SH+Sol) resulted in highest % bacterivores, lowest % herbivores (plant-parasitic nematodes), lowest F/B (indicating a communities dominated by bacteria decomposition pathways that will lead to higher nutrient mineralization), highest EI (indicating soil nutrient enrichment, even slightly higher than SH alone) and lowest CI and BI (indicating least stressful soil conditions). A second Whitmore Cowpea trial (Trial II) was super-imposed on Trial I treatment plots in 2010. Solarization was conducted in late summer to fall (August to October) with significant heat accumulation (Fig. 4). Solarization again disturbed nematode communities (lowest %omnivores, richness and EI) (Table 6). Unfortunately, SH+Sol in this trial did not reduced the negative impacts on nematode communities as compared to Sol alone (Table. 6). Therefore, there is a trend that SH+Sol performed in the summer/fall had generated high amount of heat that caused significant disturbance to the soil communities despite the significant improvement of soil health condition contributed by growing SH alone (Table 6).

Additional accomplishment

By accumulating the solarization temperature data over the last two years, we also generated a solarization scheme over four seasons in Hawaii. The maximum temperature and numbers of hours accumulated above 42?C (lethal temperature of plant-parasitic nematodes) are plotted in Fig 4. In many instances, SH+Sol produced higher maximum temperature and more hours above 42?C as compared to Sol alone (Fig. 4). Heat accumulated in the top soil layer (measured at 5 cm) is always higher than that in the deeper layer (measured at 10 cm), indicating that solarization is only able to heat up the upper soil layer. Based on this solarization schemetic scheme, we concluded that solarization will only be effective for suppressing plant-parasitic nematodes in Hawaii from April to October.

Impacts and Contributions/Outcomes

Conference presentation:
  1. Wang, K.-H., B. S. Sipes, and C.R.R. Hooks. 2010. Sunn hemp cover cropping and solarization as alternatives to soil fumigants for pineapple production. 2010 International Pineapple Symposium, Persada, Johor Bharu, Malaysia. July 13-15, 2010., K.-H., C.R.R. Hooks, S. P. Marahatta, R. Manandhar. Use of a strip-till cover crop system to manipulate above and below ground organisms in cucurbit plantings. 2010 Meeting of the American Phytopathological Society (APS) at Charlette, NC. August 2010.
Client Presentation:
  • Wang, K.-H. 2010. Nematode research program. 15 June 2010. CTAHR Road Show. Windward Community College, Kaneohe, HI.Wang, K.-H. 2010. Non-chemical approaches for nematode management. 7-8 June, 2010 Integrated Crop and Livestock Management Workshop, Komohana Extension and Research Center, Hilo, HI.

    Wang, K.-H. and C.R.R. Hooks. 2010. Is your cover cropping practice benefiting? A soil ecologist’s point of view. 20 February 2010. 19th Annual MOFFA (Maryland Organic Farmers Association) Winter Meeting, Annapolis, MD.

    Wang, K.-H. and C.R.R. Hooks. 2010. Does reduce pesticide use benefit soil health: A nematologist’s point of view. 25 February 2010. Harford County Mid-Winter Educational Meeting, Deer Creek Overlook, 4-H Building, Street, MD.

    Wang, K.-H. and C.R.R. Hooks. 2010. Use of nematodes and soil microarthropods as soil health bioindicators: a visit of Hawaii ecological based pest management projects. 26 February, 2010. Entomology Colloquium, Department of Entomology, University of Maryland, College Park, MD.

    Field day: Wang, K.-H., S. Fukuda, and J. Sugano. 5 Aug, 2010. Cover crop research update. University of Hawaii, Poamoho Experiment Station, Poamoho, HI.

Publication (Extension article)
  1. Wang. K.-H. and B.S. Sipes. 2009. Solarization and Cover Cropping as Alternatives to Soil Fumigants for Nematode Management in Hawai‘i’s Pineapple Fields. CTAHR Cooperative Extension Service SCM-29. 4 pp.
  1. Wang, K.-H., B.S. Sipes, and C.R.R. Hooks. 2010. Environmental friendly approaches for managing nematodes and weeds on pineapple. Pineapple News 17: 27-32. May 2010., K.-H. Soil solarization and cover cropping as alternatives to soil fumigation for pineapple growers in Hawaii. H?nai?Ai Newsletter Spring 2010. /sustainag/news/articles/V3-Wang-SHPineapple.pdf.

To date, eight commercial farms have participated in this sunn hemp projects. They include small-scale (< 10 acre), medium-scale (10 to 100 acres) and large scale (> 1,000 acres). Although only a small portion of their farm site is planted with sunn hemp, these farmers are showing initial adoption of sunn hemp cover cropping.

Introducing solarization techniques to some of these farmers provided another non-chemical management strategy for these farmers to manage nematode and weed pests. Schematic scale of solarization temperature over four seasons in Hawaii will be useful information for farmers that are interested to perform solarization in Hawaii.

We have demonstrated the effects of sunn hemp cover cropping on leguminous crops, leafy vegetables, eggplant, tomato, papaya and pineapple. While we failed to observe that integration of sunn hemp and summer solarization could improve the nematode pests suppressive effect of sunn hemp, we are looking forward to examining the stages of sunn hemp that is most suppressive to plant-parasitic nematodes. This would help the farmers to decide best time of cover crop termination. Updated information from this project is now available online at the CTAHR Sustainable and Organic Agriculture Program and CTAHR Cooperative and Extension Publication. Results were shared with other scientists at international symposium and conference and could stimulate more research in this area.


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