Germinating in Containers
If you start mustard seeds in containers, use quality potting mix as the growing medium. Scatter a handful of seeds over the surface and cover lightly with additional potting mix and just enough water to evenly moisten soil. Place containers in a warm place indoors or in a greenhouse and keep evenly moist until germination occurs in four to seven days.
You can sow mustard seeds directly into the garden four to six weeks before your last frost date in spring. Soil should be organically rich and well-drained** with a pH of 5.5 to 6.8 and have a temperature of 40 degrees Fahrenheit or more, or germination will be slow. If the soil needs enhancement, work 1 to 2 pounds of granular 5-10-10 fertilizer into the top 3 inches of soil for every 100 square feet of row. Sow seed 1/8 to 1/2 inch deep and 15 inches apart. In U.S. Department of Agriculture plant hardiness zone 8 and above, mustard can be seeded in September or October as a winter crop. In USDA zone 4 and below, seed in July.
One of the most common causes of germination failure is overwatering, which results in rotting seeds. Seeds need sufficient warmth to germinate but not extreme heat or direct sunlight, which can prevent germination or kill seedlings off almost as soon as they appear. Other factors that hinder germination include planting seeds too deeply or in heavy, clay soil.
There are different varieties of mustard seed. The mildest in flavor is white mustard (Brassica alba), which is typically used in relishes and pickling seasoning. Brown mustard seed (Brassica juncea) is the most pungent and is used in Dijon style and Chinese hot mustards and is the kind germinated for greens. The moderately spicy black mustard seed (Brassica nigra) is used in Indian cuisine.
You can purchase mustard seeds from a local nursery or an online vendor, but the Mother Nature Network notes that these are no different that the whole seeds found in the spice isle at the grocery store. There’s no guarantee, though, that store-bought seeds will germinate because the expiration date on the jar doesn’t tell you how long the seeds sat in a warehouse or on the shelf at the store. Test a few to see if they sprout. If they don’t, make mustard.