I'm drilling the edges Jennie becuase last year I found some squash standing in pots of water they were not draining well when watered. When I looked at the holes underneath the pots they were clogged with roots. I drilled holes in the sides (at the bottom) and this solved the problem.
Also, the dustbin - there was just no way to guage if water was getting to the bottom - so drill the holes down there and see if water runs out. I think I'll do it with all my pots now.

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From each according to his ability, to each according to his needs.