Start seeds 6 to 8 weeks before the last spring frost. Sow your seeds ¼ inches deep and 2 inches apart. Place them in a spot where they'll get plenty of sunlight, or set them under grow lamps with temperatures between 60 and 70 °F.
Kohlrabi will do well with at least two inches of good compost or aged manure turned into the planting bed.
Start your seeds indoors about 4-5 weeks before you want to transplant them to your garden. Kohlrabi, and all of your plants in the Brassica family need to be hardened off before planting in the garden. Set your seedlings outside in a sheltered area for an increasing number of hours each day for about a week. Choose an area at first where they won’t be exposed to harsh wind or direct sunlight. Leave them out for 1-2 hours at a time to start, then gradually work your way up to 7 or 8 hours by the end of the week. This gradual hardening off will allow your baby plants to become acclimated to the cold, and you can plant out earlier in the spring because of it.
Thin the seedlings so that there is only 1 seedling per cell or pot that you transplant. You can wait until the seeds have germinated, then pick the strongest seedling in each tray.
Once they’re hardened, set your plants so that 1 to 2 inches of the main stem is buried in the soil. Seedlings should have at least 3 or 4 adult leaves before you transplant them.
Arrange your kohlrabi in rows with plenty of sun exposure. Kohlrabi needs at least 6 hours of sunshine a day. The more sun your kohlrabi gets, the larger and faster the heads will grow. Set the seedlings in rows 12 to 24 inches apart.
Kohlrabi need moist soil, at least an inch a week. Unless you've had frequent rain, water the soil around the kohlrabi at least once a week. Keep about 2” of mulch around your kohlrabi to keep the moisture in the soil.