I planted 3 courgette plants in the ground, and two in dpm lined tyre stacks a while back (which means I forgot to make a note of when exactly, assuming I'd remember and I have no idea - but possible May?!)
The plants in tyre stacks are bigger and have tiny courgettes just visible - the three on the ground are about a third smaller with no sign of fruit yet.
Presumably the warmth from the tyre protection is making the difference - and the courgettes take up far less room in the tyres than they do on the ground, so I think I'll be repeating this experiment.