Bman wrote:
So i'm wondering if 5cm thick bags of soil are going to do the trick?
Or should I take several of them, cut out the tops and bottoms and
stack them together, making a kind of planter? If I had three of them
on top of each other, that would provide a really deep bed of earth to
root into.

Sounds very tricky with sweet corns. I never grow less than 10 plants -
and like you I tried with grow bag and it wasn't successful at all as
the corns were heavy and didnt' get enough root space. They grew wobbly
and some fell and I had to prop them, attach them etc. and by late
August the whole thing looked more like a conceptual art installation
than a bed of sweet corns ) The idea to stack the bags for the sweet
corns is a good idea but how many sweet corn plants can you do in this
way? They need to get pollinated together - they need plenty of air
around them too. Last year I lost half my crop to the badgers - you
need to protect these well because all creetures just adore sweet corns!