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

2010 Annual 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

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


Sunn hemp can help to control weeds and nematodes as well as add nutrients to the soil system. Sunn hemp reduces weed densities as compared to a bare ground fallow. A sunn hemp benefits soil health by reducing plant-parasitic nematodes, increasing bacteria-feeding nematodes, and aiding the recovery of predatory nematodes. 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 collaboration with sunn hemp in their individual production systems.

Objectives/Performance Targets

  • We will establish field day demonstrations for maximizing sunn hemp establishment and biomass productivity by manipulating seeding rates, and planting times.

    We will demonstrate the qualities of sunn hemp as a surface residue in a no-tillage practice and in a strip tillage practice with residue incorporation in the row and mowing and windrowing of the between rows.


Several demonstrations and test have been conducted over the past year (Table 1). The objectives of this project have complimented the objectives of several other projects and we have joined the project to leverage and compliment available funding as well as providing synergy for the adoption of sunn hemp as a cover crop.

The multiple demonstrations that have been installed have demonstrated the utility of sunn hemp in controlling weeds (Tables 2 and 3). In the early establishment of plants, the sunn hemp has less weeds than a bare ground options. This weed control effect can last for several weeks (Fig. 1). After cash crop establishment, the weed control from sunn hemp is minimal and there are not clear differences between treatments.

Sunn hemp cover crops also have benefits in controlling plant-parasitic nematodes and enhancing the soil food web (Tables 4 and 5). While sunn hemp cannot eliminate plant-parasitic nematodes, the cover crop does reduce the number of plant-parasitic nematodes. 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. We find greater sunn hemp biomass with irrigation. The Structure index (SI) associated with sunn hemp indicates soil disturbance but the growth of sunn hemp aids the recovery of desirablepredatory nematodes after the tillage operations are complete. Overall, the sunn hemp fields exhibit a more robust and complex soil nematode community which suggests a healthier soil ecosystem. Bare ground management practices have no free-living nematodes but only plant-parasitic nematodes that have adapted to surviving stress conditions, indicating poor soil health and greater cash crop damage from nematodes.

In the remaining months of the project we will complete our publication/dissemination activities. We plan on developing web material to guide growers in using sunn hemp for ecological based approaches to weed control, nematode suppression and nutrient management in their crops.

Impacts and Contributions/Outcomes

The impacts of the project are dramatic. The last field demonstration was conducted at the bequest of a grower (Fig. 2). This demonstrates that growers are cognizant of sunn hemp, understand the benefits associated with sunn hemp cover crops, and are interested in using sunn hemp in their cropping systems.

Through the activities of the project, we have convinced influential grower of the benefits of using sunn hemp as a cover crop. As the better growers adopt and implement new production techniques, other more risk adverse growers are also likely to adopt the practice. It was an influential, respected grower that request the most recent field demonstration.

One challenging aspect demonstrated by our recent activities, is the limited amount of sunn hemp seed available to growers. The year-round availability of seed for planting is important for growers in Hawaii and other tropical areas of the Western region. Growing sunn hemp seed could be an economic opportunity for a Western region farmer. The limited availability of sunn hemp seed might serve as a discouraging force in the wider adoption of sunn hemp cover cropping.


Dr. Koon-Hui Wang

Assistant Professor
University of Hawaii
3090 Maile Way
Honolulu, HI 96822
Dr. Theodore Radovich

Assistant Professor
University of Hawaii
3190 Maile Way
Honolulu, HI 96822
Jari Sugano

Extension Agent
University of Hawaii
3090 Maile Way
Honolulu, HI 96822