Mustard is an ancient plant which is  full of appeal for contemporary gardeners. The plants are easy to grow and produce seed in as few as 60 days. Sow mustard seeds 1/4 to 1/3 inch deep, and 3″ apart. Thin seedlings to 5″ – 9″ apart. Separate the rows, 1 foot apart. ow seeds early in the spring and a second crop in the early fall. They prefer cool weather, so leave the middle of the summer for the heat loving vegetables. Maturity: 45-50 days