Hi Arturo
If the 8-10 inches spacing seems wasteful of greenhouse space, maybe you could grow small Bok Choy between the celeriac plants. Just chop the Bok Choy or eat the outer leaves before they shade the celeriac.
It's easy to find Bok Choy varieties fully mature at 35-40 days, and cutting them younger and smaller only gives you more tenderness and sweetness.
