Posted: 30/05/2013 at 08:53
As you've used weedkiller that's almost certainly the cause.
Some weedkillers are supposed to become inactive once they make contact with the soil - others are designed to persist for months so you'll need to carefully read the instructions to know when it should be safe to put plants in the area you've sprayed.
The term 'weedkiller' is rather misleading. I feel they should be sold waith either 'herbicide' or 'plant killer' on the label instead.