55 Days to maturity. Brassica juncea Chirimen Hakarashi Mustard Seeds. Non-GMO, annual heirloom. These Japanese mustard leaves are delicious. The plant has broad, curly leaves with serrated edges that grow on a round stalk. This mustard bunches loosely and is commonly grown for baby greens as well as for fully mature greens. It's a cool-weather plant, best suited for early spring or early fall planting. Harvested early when they are 3 inches tall or more, about 35 days from sprouting. The mature plant is still delicious but will be much spicier! Approx 10,000 seeds / oz.