Mustard Growing Information Direct Seed: Can be sown early spring through fall in the open field, and fall through winter in a tunnel or greenhouse in most regions. Sow in the fall in warmer regions. Soil temperature for germination is 40-70° F. Spacing: Can be sown densely for baby leaf harvest in rows that are 4-12" apart, or in 2-4” bands that are 12-18" apart, at ¼” deep. Decrease density to harvest full size plants for bunching. Harvest: At desired size for baby leaf or bunching. For multiple harvests, cut above growing tip.