Onions prefer clear ground until they get established but after that they will survive whatever grows around them. I had a superb crop of onions last year and you would have thought I was growing weeds in that area of the field - you honestly couldn't see the onions unless you looked really hard.
Having said that, it was particularly wet up here last year (whereas it has been particularly dry this year so far) so there was loads of moisture in the ground so not much competition for moisture - might be a different outcome if it is particularly dry - though I haven't noticed my onions suffering much so far this year - they are all just beginning to bulb up now, and once again you can only just see them through the weeds.
Weeds are not always bad - I think it was Snagder who said that he considers chickweed to be nature's mulch !!

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Rat

British by birth
Scottish by the Grace of God

Blog updated Wednesday November 13th