I read somewhere that kohlrabi is supposed to be available starting in mid-summer, but my local grocery store just started getting it around the beginning of October. Right now they are only carrying the green kind. There's also a purple kohlrabi, and a baby kohlrabi. Kohlrabi are usually light green turnip-like bulbs, usually sold with the greens attached. (more expensive than turnips, but worth it.)

I have learned from experience, the bigger the bulb, the more fibrous and woody it is. Ideally you should avoid any bulb that is bigger than a tennis ball - but sometimes there isn't much choice at my grocery store.
you also have to make sure to peel it very well. appararently I didn't peel it far down enough, and after I cooked it, you could see the fibrous outer layer. (it also takes a long time to cook.) I would love to find some tender baby kohlrabi.
But otherwise, this is a wonderful vegetable - and it keeps a long time in the fridge as well.

My next project: scallopped kohlrabi with ham and cheese! I'm planning to experiment this weekend - wish me luck. :-) If I'm successfull i'll post it in the kitchen forum.