Use landscape fabric it does not degrade; cardboard only lasts one year. you do not need to turn over the turf if use the landscape fabric. I might help if read the book.

Turf is a strong, persistent garden weed. Eventually it will grow right through the weed barrier and get its roots so tangled up in it that it becomes very difficult to pull out. You need to dig the grass out and take it away. Also, it would be a good idea (mine, not Mel's) to make the walkways between your boxes grass-free. Otherwise, the underground runners of the grass will find a way under the sides and establish themselves in your garden.