Increasing Drought Resilience on Maui Farms with Cover Crops
Posted on Jul. 1, 2025 / Best Practices, Fertilizer , Irrigation , Soil, Sustainability, Edible Landscapes / Subscribe 0
By: Jackie Jamison, Rosemary Gutierrez-Coarite and Hannah Lutgen
Reduced water availability threatens agricultural viability across the state. Management of soil moisture on farms is key to increasing drought resilience and critical to the future of agriculture in Hawaii. Cover crops are increasingly being used for their ability to improve soil health, increase soil organic matter and nutrient availability, and increase soil water holding capacity. When used in combination with other conservation management strategies like no-till, intercropping and crop rotation, the benefits can compound even more.
Figure 1. Variety trial and blends trial at the Kula Agricultural Park in Maui, Hawaii. The top-performing species from each functional group in the variety trials were selected and combined into all possible 3-species combinations in the blends trial.
This project aims to increase on-farm water use efficiency on agricultural lands through the combined use of cover crops and conservation management. The first phase of the project consisted of seasonal variety trials where fifteen individual cover crop species and 9 cover crop blends were tested for drought tolerance and biomass production at the Kula Agricultural Park. Cover crop species were selected based on drought and heat tolerance and classified into one of three functional groups: grasses, legumes, and non-legume broadleaf plants. Blends consisted of one plant from each of the three functional groups. The second phase, currently underway, assesses the viability and benefits of cover crops on farms in upcountry Maui.
Figure 2. Biomass graphs showing total dry biomass (lbs/acre) under drought conditions in both summer (red) and winter (blue) variety trials. Asterisks show species that had significantly higher biomass than others in the same functional group.
Figure 3. After harvest, samples were sorted and dried before being weighed for dry biomass.
During the summer variety trial, sorghum (Sorghum bicolor, var. ‘Sorgrow FS105’) and pearl millet (Pennisetum glaucum, var. ‘Exceed BMR’) produced the highest biomass of the grasses. Brown mustard (Brassica juncea, var. ‘Kodiak’) produced the greatest biomass of the broadleaves, and sunn hemp (Crotalaria juncea) produced the highest biomass of the legumes. In the winter variety trial, there were no significant differences in biomass between the grasses. Brown mustard and daikon (Raphanus sativus, var. ‘Smart’) produced the greatest biomass of the broadleaves, and sunn hemp and cowpea (Vigna unguiculata, var. ‘Iron & Clay’) produced the greatest biomass of the legumes.
Figure 4. Images of top-performing species during the variety trials, from left to right: sorghum, pearl millet, brown mustard, daikon, sunn hemp, cowpea.
Germination and canopy cover at 4 weeks were also measured in all trials, and weed biomass was quantified in both the summer variety trial and the blends trial. Higher germination and canopy ratings typically corresponded with lower weed pressure and biomass at termination. Germination and canopy cover were high in all grasses except sorghum and triticale. Brown mustard and sesame had the highest ratings of the broadleaves in the summer, and brown mustard had the highest in the winter variety trial. Sunn hemp had the highest ratings of the legumes during both trials.
Figure 5. PCD – Pearl millet, cowpea, daikon, PShD – Pearl millet, sunn hemp, daikon, PCB – Pearl millet, cowpea, brown mustard, PShB – Pearl millet, sunn hemp, brown mustard, SCD – Sorghum, cowpea, daikon, SShD – Sorghum, sunn hemp, daikon, SCB – Sorghum, cowpea, brown mustard, SShB – Sorghum, sunn hemp, brown mustard.
Blends containing pearl millet as the grass and brown mustard as the broadleaf species had the highest germination, canopy cover ratings and biomass. The blends producing the greatest biomass also included: pearl millet / sunn hemp / daikon and sorghum / sunn hemp / brown mustard.
Ultimately, the best cover crops for a given farm will depend on multiple factors, including timing of planting and the availability of water, as well as farm goals, including cover crops in the rotation. For example, sorghum produces the highest biomass, making it very suitable for farms looking to increase the carbon input into their soil. However, it may not be the best choice for growers looking to increase weed suppression, due to its lower canopy cover early in growth.
In general, utilizing multi-species blends is beneficial, since different species provide different benefits. Grasses and fast-growing brassicas typically germinate quickly and provide rapid canopy cover that shades out weeds. Many broadleaf plants produce biomass that is readily decomposed by microbes and turned into organic matter in the soil, and legumes can provide valuable nitrogen to subsequent crops and potentially reducing fertilizer needs. Based on our trials, blends with both pearl millet and brown mustard would be most suitable for growers looking to reduce weed pressure while also optimizing organic matter inputs. Blends containing sunn hemp as the legume in combination with either pearl millet as the grass or brown mustard as the broadleaf would be good choices for biomass optimization.
Jackie Jamison, Junior Researcher, College of Tropical Agriculture and Human Resources, [email protected]
Rosemary Gutierrez-Coarite, Associate Extension Agent, Edible Crops, Department of Tropical Plant and Soil Science, UH CTAHR Maui Cooperative Extension Service, [email protected]
Hannah Lutgen, Extension Faculty, Landscape and Floriculture, University of Hawaii at Manoa, College of Tropical Agriculture & Human Resilience (CTAHR), Maui Cooperative Extension, [email protected]
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