Celeriac

I grew celeriac this year as we quite like it as a vegetable, especially as a substitute for mash potato.

I went up my allotment this morning to collect some veg and decided it's time to try the celeriac. They are big healthy plants so my expectations were high, as I pulled the first plant a big lump of soil came free, increasing my hopes for fat vegetables. 

Unfortunately I was disappointed, all there was was a massive tangle of roots and no bulb at all. I pulled the whole row and nothing on any of them. 

That's a shame, I don't know what I did wrong. Will have to do some reading.