Direct sow seeds outside 4/15 through 7/15. Sow seeds 1/4" deep, 1" apart, in rows 15" apart. Or seeds can be started inside in containers mid March and planted outside mid April.
For tender roots, harvest when roots are 2-3" in diameter before they are overmature and become tough.
Kohlrabi plants are difficult to save seed from because they must be grown over two years to produce seed which means, in the north, they will need to be dug up in the fall, stored over winter, and replanted in spring. In the Brassica family so it is insect pollinated and will cross with all Brassica oleraceas including broccoli, cauliflower, cabbage, broccoli, etc. Isolation distance: ½ mile. Minimum population size: 20-50 plants. To harvest seed, allow plants to flower and collect seed from mature pods.