Mustard, generally referring to Brassica species like Brassica juncea, is a versatile cool-season crop cultivated for its pungent leaves, seeds, and ability to improve soil health. Optimal flavor and leaf production occur when temperatures are mild, making timing the single most important factor for successful growth. The best time to plant mustard shifts significantly depending on whether the goal is an early spring harvest, a prolonged fall yield, or maximizing its benefits as a soil cover.
Timing for Spring and Early Summer Greens
Planting in early spring allows mustard to mature during its preferred cool-weather window of 45°F to 75°F. Seeds should be sown directly into the garden as soon as the soil is workable and reaches at least 40°F, typically four to six weeks before the last expected frost date. Direct seeding is preferred because mustard germinates quickly and dislikes root disturbance. The plant is relatively frost-tolerant and can handle brief dips into the 20s, which often results in sweeter-tasting leaves. However, this spring window is narrow; consistent air temperatures above 75°F trigger bolting, causing the plant to prematurely produce seeds and making the foliage tough and bitter.
Timing for Fall and Winter Production
Planting mustard for a fall and winter harvest is often more successful because the cooling weather naturally prolongs the plant’s vegetative growth phase. To time the planting correctly, sow seeds approximately six to eight weeks before the first anticipated hard frost date. This schedule allows plants to reach a harvestable size before short days and cold temperatures slow growth. For a continuous supply, employ succession planting by sowing small batches every two to three weeks until about one month before the ground freezes. In mild climates, the harvest can be extended throughout the cold season using simple protection like cold frames or row covers to shield plants from hard freezes.
Planting Mustard as a Cover Crop
When the goal is improving soil structure and health rather than consumption, planting timing shifts to maximize biomass production. Biofumigant mustards are planted to suppress soil-borne pests and diseases like nematodes. For effectiveness, planting must be timed so the mustard reaches peak bloom—about 60 to 70 days after seeding—before being incorporated into the soil. For fall biofumigation, planting should occur in late summer (mid-August to early September) to ensure sufficient growth before soil temperatures drop below 50°F. Alternatively, planting in late September or early October can be timed for “winter kill,” where severe cold naturally terminates the plant, leaving protective mulch that improves the soil for the following spring crop.