In article , M K Rogerson
writes
Alan, I've tried several times and cannot produce supermarket sized roots.
Best I've managed was about half the shop size. I gave up and just buy them
now. If you haven't tried Celeriac chips you need to do it now!

Celeriac need a very long growing season. Get them started early
(Feb./Mar.) and keep them growing on [without pushing them] through
until late autumn. Once they are mature, they can stand in situ to ripen
if the weather is not too cold and wet.
--
Alan & Joan Gould - North Lincs.