Kohlrabi is the same species of plant as cabbage and broccoli and Brussels sprouts. They are all cultivars of the wild cabbage plant. This particular variety was bred for their large bulging stems, which is the part you eat.
I live in hardiness zone 5A, and I planted these as small plants right before our last snow in late April. They are good at surviving cold temperatures and need to be planted around the same time as lettuce and other cold weather plants. With the recent consistent warm weather (temps in the 90s and high 80s most of the day) my kohlrabi was starting to wilt a little. The bulbous stems are around 3.5 inches, so it's time to harvest.
I like to eat them raw, so this is how I serve them.
Pull off all the leaves and stems and chop off the root area.
The part of the bulb closest to the roots is the most woody, so also chop off a little into the bulb from the root end.
Then use your knife to peel off the outer skin.