Celeriac - starting late

My Celeriac have been very late starting this year - they are still about 2mm high (yes, that's millimetres!) and only just past the primary leaf stage.

What do you reckon - have I shot it for this year?

According to the RHS website latest sowing time is April.

I would feed what you have, keep them watered and see if they improve!

Celeriac needs a long growing time but still doesn't bulk up til quite late.

Very late sowings will only make small roots but could be good for soups

I am doing well this year with celeriac. I found that you need to give them a rich diet in order to help their roots swell. Though not too much nitrogen aparently this just encourages leaf growth and no benefit to the roots.
I have experimented this year being the organic gardener I am (not) with a pinch of super phosphate in each hole when I planted them on. I thought that it would promote healthy roots.