Peter
Out of interest, I transplanted my sweetcorn plants grown in soil blocks yesterday (put them under fleece too!) and I noticed that, far from the roots stopping growing at the soil block edges, they grew into neighbouring blocks, with the result that you have to be careful lifting them not to tear neighbouring blocks. It’s not a major problem, but if you want the roots to stop at an air barrier, I guess you need to keep soil blocks with a clear space between them and their neighbours, something I neglected to do this time.
I suspect I should have planted them out sooner, but as with any new technique and new seed supplier, you err on the side of caution (I left 4 weeks prior to transplantation – next year I will see if a timetable can be used to allow transplantation two weeks after sowing).
The scallions were transplanted out in blocks 3 weeks after sowing and have taken OK in the garden.