Planting Dates for Fall or Spring Biofumigation Using Mustard

Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu

Biofumigation is a technique for managing soil-borne pathogens. The biofumigation process involves growing specific cover crops (mustard or sorghum species), then mowing the cover crop and incorporating the green cover crop material into the soil. The chopped cover crop material releases compounds into the soil that kill certain plant pathogens and nematodes. In this region, growers have been interested in using biofumigant mustards for managing Fusarium, Phytophthora capsici and root-knot nematodes (RKN). There is ample evidence that mustard biofumigation is effective for reducing RKN. There is some evidence that biofumigation is effective for Fusarium and P. capsici control.

The standard recommendation for Delmarva has been to plant biofumigant mustards in the early spring, but the timeline for spring biofumigation does not allow for planting until late May or June. In 2022 and 2023, I measured biomass production in mustard planted in late summer and early fall to determine recommended planting dates for fall mustard biofumigation. In 2023 I also had September and October mustard plantings that overwintered. In 2022 ‘Caliente Rojo’ mustard that was planted on August 8, 15 and 22 produced sufficient biomass to complete the biofumigation process in September or October. A planting made on September 5 did not produce adequate biomass in time for effective biofumigation (Figure 1). In 2023, plantings of ‘Caliente 199’ and ‘Caliente Rojo’ made on August 21 were ready for incorporation in late October. Plantings on September 21 and October 12 overwintered and were ready for incorporation in the early spring of the following year. We had a mild winter and biofumigant mustard may not overwinter in Delaware every year, however this planting timing could have a fit in some situations.

Figure 1. Fall 2022 biofumigant mustard planting on September 26. August 8 planting is at early flowering and ready to mow and incorporate. The September 5 planting on the far left did not produce sufficient biomass in time for effective fall biofumigation.

Another constraint on fall biofumigation is soil temperature. Recommendations from other regions suggest that biofumigation is not as effective at soil temperatures below 50 °F. In southern Delaware, average daily soil temperature falls below 50 °F in mid-November (Figure 2).

Figure 2. Average daily soil temperatures for September through December at Georgetown, Delaware in 2019-2022.

Based on my experiences in 2022 and 2023, I recommend planting between August 1 and August 22 if you want to biofumigate in the fall. For overwintering, mustard should be planted in late September or early October. Avoid early September planting dates as the plants become too mature to overwinter, but are not ready for incorporation until soils are too cold for effective biofumigation. To promote production of adequate biomass, fertilize with at least 50 lbs/acre of nitrogen. Overwintered plantings will need additional nitrogen in late winter to promote spring growth. Ammonium sulfate is the recommended nitrogen source, since it also supplies sulfur, which the mustard plants need to produce the active compounds necessary for biofumigation.