Pest control in vegetable crop is a challenge in tropical climate such as Hawaii. Plastic mulches are often used but it is bad for the environment and difficult to remove. Living mulches provide a reasonable alternative. Mint, a living mulch interplanting with a vegetable crop, provides a high
return. In addition, mint reduces soil erosion, retains moisture, and if left to flower acts as an insectary. This farmer research grant is geared to assist farmers in understanding the benefits of a mint living mulch and making decisions on water and fertilizer inputs needed for successful adoption of mint living mulch in their vegetable production. Eggplant (Solanum melongena) serves as the vegetable crop. Demonstration plots were established and eggplant yield compared among mint vs plastic mulch, and three irrigation levels. In general, eggplant yield was greatest under the plastic mulch. Irrigation level had little effect because rainfall was sufficient. Soil quality, as measured by nematode community analysis, was good in the plots regardless of the ground cover. Plant-parasitic nematode populations were low. The profitability of mint living mulch has not yet been accounted for using partial budgeting. The sustainability of the mint living mulch vegetable cropping systems was promoted through workshops and site visits by county agents. A farm workshop, rescheduled and held on the university campus due to weather, garnered attendees who learned more about mint living mulch benefits. Evaluation and feedback from the attendees demonstrated interest in mint and conceptualization that mint may be better viewed as the cash crop with eggplant providing supplemental income to the mint crop.
This farmer research grant is geared to assist farmers in understanding the benefits of a mint living mulch and making decisions on water and fertilizer inputs needed for successful adoption of mint living mulch in their vegetable production. The specific objectives of this proposal are
(1) to determine optimal irrigation and fertilizer for a mint living mulch vegetable
(2) document pest and disease pressure in a mint living mulch vegetable cropping systems, and
(3) demonstrate and promote the merits of the mint living mulch vegetable cropping system.
Objectives 1 and 2 are expected to be achieved as they have been demonstrated at the small experimental plots at the University of Hawaii experiment station. The project will scale up to two-thirds of an acre with irrigation, a manageable size for a small farm vegetable intensification production approach. On-farm field demonstration days and workshops will be conducted to ensure interested farmers can attend. Each workshop will last 4 hours. The establishment of the eggplant-mint living mulch will take 6-7 months from seed to the first harvests. The field days will be held beginning nine months into the project to demonstrate the experiments and present analyzed results of yield, profits, pest management, post-harvest handling and value-added mint when processed to oil.
The mint was established and plastic mulch laid over plots. A split-plot design was employed to establish irrigation regimes with the mulch treatments randomized within the irrigation regimes. Weeds were removed by hand as needed. An organic turkey manure was applied irregularly. Irrigation regimes consisted of a 1-hour, 30-minute, or 15-minute run each day. Initially, Burpee ‘Meatball’ eggplant seedlings were planted. This particular cultivar was selected as an approach to whole-farm value-added products such as baba ganoush, however, this cultivar did not grow well and suffered substantial injury from mites. These plants were removed and replaced with the locally adapted cultivar ‘Poamoho Purple.’
Harvests in both eggplant cultivars were made in the late fall in 2018 and 2019 (Figs 1 and 2). The eggplants growing in the plastic mulch have provided the most fruit by number (66 vs 13) in the first planting. The second planting with data collected over two months from older plants showed similar behavior – eggplant yield was greater from plants in the plastic mulch treatment (439 vs 181 fruit). The quality of the fruit was similar among the treatments (Fig. 3). Irrigation level had little effect on yield. In the first planting, the average number of fruits was 26 across the irrigation regimes (range = 30-23). In the second planting, the average was also not different among irrigation regimes with an average of 206 fruit (range 203-213). The mint and plastic mulch provided good weed control. Less than 1 hour per week was dedicated to hand weeding in the mint and plastic mulch treatments. In the plastic mulch, weeds emerged in the eggplant planting holes, necessitating care in removing the unwanted plants. This was less of an issue in the mint living mulch.
Eggplant does best at very warm temperatures. In the winter months, eggplant is typically slow-growing and yields less. The black plastic mulch clearly benefited early eggplant growth and resulted in more yield. The mulch may have increased the soil temperature which was more amenable to the eggplant resulting in greater growth and yield. Irrigation did not have as great of an effect. The 2019 growing season was wetter with more natural precipitation than 2018. Rainfall provided most of the water needs to the eggplant.
Soil health measurements highlighted a field with good structure and low levels of plant-parasitic nematodes. The field contains a diverse nematode community consisting of fungal feeders, bacterial feeders, omnivores, plant parasites, and predatory nematodes. A significant change in nematode community over time has yet to be observed. We expect the mint to retain structure and increase in diversity and richness compared to the plastic mulch.
Educational & Outreach Activities
The education and outreach activities consisted of individual farm tours/consultations and a workshop. Local farmers in the area would be invited or ask to visit the farm and demonstration plots. During the farm visits, the growers would be shown the plots and engaged in a discussion of the benefits and challenges of the living mulch. Fellow farmers showed interest and enjoyed the smell of the mint in the field.
A field day and workshop was initially planned but adverse weather required some adjustment. The workshop was rescheduled and held at the University of Hawaii at Manoa campus with a farm tour immediately thereafter (Fig. 4). A mix of 15 farmers and researchers attended the morning workshop. B. Sipes presented to the group and introduced the benefits of mint and the effects of a mint mulch on eggplant yield and soil health. K. Chan fielded questions from the group following the presentation. Interested people met at the farm in the afternoon to view the demonstration plots.
Soil health, soil improvement, mulching, weed control
Mint brings multiple benefits to the farm. Mint can serve as a living mulch that suppresses weeds. Mint does require irrigation and fertilization. If the fertilization is insufficient, the vegetable crop and the mint both grow poorly. We have learned that is is probably better to view the mint as the cash crop and the vegetable, such as eggplant, as the intercrop that adds value. Some researchers have tested mint as a living mulch with cucumber. Cucumber was not well suited to use mint as a living mulch. We are considering mint as a living mulch for use in cacoa orchard plantings as ground cover to prevent soil loss, enhance health, and serve as weed control.