[vanattab]

I could see this being the reason your carrots didn't grow well though. If the carrots tap root struggles a bit through the cardboard it could mess up devlopment. I think this is why they say not to transplant carrots. The tap root bottoms out quickly and struggles to recover.

[bgnn]

Oh carrots were in a deep compost bed (40cm) first. They only grew like 2cm long roots, nowhere near the box.