Where does it all come from?

No matter how many times you fork through the peat, more and more couch grass comes up from no where. We decide that we will do this only twice as it seems almost impossible to take it all up. Notice in the background out plot 2 with the tarpaulin sheet over. An attempt to cover this with the peat and pick out grass roots separately did not work so decided to leave it there to kill off weeds.