80 to 90 Days to maturity. Raphanus sativus. Sakurajima Mammoth Heirloom Seeds. Cool Season Annual. Known as the “Largest Radish in the World” in Japan, Sakurajima Mammoth radishes can grow as big as 100 pounds! More often though it reaches about 15 pounds and can be harvested when 10 inches in diameter for the best flavor. Despite its size, this variety is sweet, mild, and crisp. Plant this round white variety in early fall and harvest in the winter. Then, use it for cooking and pickling. ~ 1,600 seeds / oz.