I have left a couple of Kohlrabi's in the ground overwinter.Will they produce any seed this year that I could save?
Yes they will flower this year. They belong to the same family (Brassica oleracea) as cauliflower, curly kale, broccoli, sprouts and a few others which they can cross pollinate with. Pollination is by insects, individual plants are self-incompatible, but each plants pollen will fertilize flowers on another plant. I just checked and 6 plants are suggested as the minimum to use for seed producing, although more would be preferable which would allow for any losses and to be able to choose the best plants etc. Good luck :)