[AlaninCarlisle]

I get great results in my polytunnel with veggies about 7 or 8 inches in diameter but ones grown outside are barely bigger than a cricket ball despite plenty of manure and Growmore and a good long summer.

I start them off in modules and transplant late April into good open loamy soil. I strip the outer stalks as soon as they start to swell.

Am I just too far north to get a good unprotected crop?

[compostqueen]

I would have thought you would have done all right up there given the high rainfall you get. They do like a lot of water when they've just been planted.

I plant mine in lots of manure, just bunged on top of the bed and then plant through into the soil below.

[devonbarmygardener]

My celeriac is about 5 inches in diameter and I thought that was good!
Better than I've ever grown before

[Trillium]

You have the manure rich soil, but when are you planting them outside? Like tomatoes and other tender crops, celeriac likes to start out in warm weather like May. Once it gets growing, then it prefers cooler weather. I also watered mine regularly but not heavily, and got very good crops this year.