All the brassicas (cabbage family) I planted in East Texas--Hot, Dry, VERY late into the ground (all strikes against success)--did fabulously well in old-style square-foot soil (50% solid red clay, 50% Mel's Mix)! I'm sure you can do well with your celeriac and kohlrabi dreams.

The season before I moved, I decided to try growing both of these veggies in East Texas, so I researched them. I planned to plant them both at 4/1 (4 per square foot), because that puts them on 6-inch centers--generous, compared to in-row spacing. (If I'd known about the Gigant Kohlrabi variety, I might have tried for 20-pounders at 1/1.)

I think I'll try them in 2x2 boxes this Fall, just to see....

Celeriac Culture:
http://www.southernexposure.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=CELE

Kohlrabi (also shows up as: kohlrabbi) Culture (and pic of 27-pounder on a scale!)
http://www.southernexposure.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=KOHL

Kohlrabbi photo: http://botit.botany.wisc.edu/images/130/Angiosperm/Brassica_oleracea/

Here is a link that might be useful: Cool Celeriac-named business with great photo.

Thank you for the information. How many weeks before last frost should they be started from seed?
Are both celeriac and kohlrabi planted 4 per square?

You're welcome, JRC.

Actually, in Texas I would plant in the fall for a winter/spring crop. I'm guessing you'd start these plants to germinate about 4-6 weeks before last frost, harden off well, and use small covers to protect the young plants in case of late freezes.

You can probably use standard brassica (brussels sprouts, broccoli, cabbage) timing, especially for the kohlrabi.

Maybe one of our northern members can give you better info--or you can ask for advice from the Vegetable forum and your area forum. Seedling advice should be good for your needs.

All the best with your new spring garden!