I've never grown celery before, but apparently this is how you do it. It needs support, so you sow it in three-feet-square blocks with a cane at each corner. You wind string around in this pattern. Then you can plant nine celery plants and they will all get support from the string. You can add more string higher up as it grows.
I've searched the internet, and I can't find anywhere else it recommends doing this. But this is the method told to me by my allotment gardening guru, so I'm going to stick with it and see how it turns out.