I grow (try to grow) celeriac here in my garden in France, but usually only golfball-sized roots. I've read articles in French on the Rustica site which say you should cut off the central root of the plants at the moment of the third transplantation (when you plant them in their final spot), and other articles say to cut the root at the end of summer. A local gardner told me you should clip the central root and some of the exterior roots as well as the lower leaves to favor forming of the round root ... but I always thought it was the central root which developed into the large ball. Can you advise me on these points? We use celeriac here a lot in autumn and winter slow cooking and I can buy big ones from local garden markets, but I'd rather grow my own.
This article should help: