Kohlrabi is a wonderful cool-weather crop because it tastes great (like broccoli some say, or a tender turnip) and it grows really fast. It's also easily grown from seed, which makes it an all-around wonderful and satisfying plant to cultivate. The slow growth of most winter veggies is frustrating, but kohlrabi is one of those that busts that rule.
Many people don't know that kohlrabi is a member of the cabbage family. Its name being derived from the German "kohl" for cabbage and "rube" for turnip. It's sometimes mistaken for a root vegetable, but the bulbous part of the plant grows above soil, with a thin tap root below ground. I grow both a white and a purple variety for Manresa, and they can use it either raw or cooked. It's best picked when the bulb is small, tennis ball sized or less, as it tends to get woody and tough when larger.
The culture of kohlrabi is much like that of cabbage. It prefers temps under 70 degrees; if your temps get above that when the plant is small, put a shade panel in front of it to reduce heat stress (more on my home-made shade panels later). They prefer an elevated soil Ph, so add lime if your bed Ph is below 6.3. I use oyster shell lime.
You can sow kohlrabi seeds directly in the ground in early spring (it can withstand minor frosts) and thin to three or four inches apart. I would use the tiny thinnings as salad additions or in stir fries. Since it grows so quickly, if you want a continuous harvest of it, make plantings every two or three weeks until the hottest part of the summer. You can begin sowing again in the fall. I like to start my kohlrabi seeds in flats in the greenhouse, prick them out into 2-1/4" pots, and then transplant out into the garden. We plant them closely together, as they do not need much room.
In this photo, you can see this young purple variety growing in our hoophouse. Kohlrabi loves the hoophouse and grows extra fast in there, but we also have plenty of it growing outside as well. The purple type of kohlrabi, although beautiful, does not maintain its color all the way through, so be aware of that. Don't just throw away those leaves; they're great used like turnip or beet greens. Now go kick some ass and plant some kohlrabi!