Allotments 4 All

We have shallow beds in our school plot (6" ish).  Can we grow onions in it or do they need deeper soil?

should be ok (ish) depends what at the base of the beds such as gravel or concrete which won't help

I would top up to maximum level as you can to give them the best chance

heap it up in ridges, if you can get some well rotted muck, the stables down the road has it to take away, your onions will thank you for it  ;D

I think drainage might be an important issue: surely if the beds don't drain well, the roots will be sitting in a watery medium ideal for encouraging rot?