Hi clare,if the plot has been weed killer treated think you will have to wait about three weeks before you can plant anything,unless the plot is in a really bad way i think it would be best to avoid rotavating and do it the hard way,good fork and muscle power,as rotavating just chops up weed roots and spreads them around,and a lot of weed killers only kill the top growth,whatever you decide to do,enjoy it and welcome to the world of lotties.