I've heard of people using cardboard as an effective weed barrier for, for example, making a raised bed on top of a lawn - laying cardboard down on lawn, wetting it and planting over this.
I've been landscaping an area of the garden which was previously gravel and shrubs which I now want to turn in to a lawn. All the shrubs have now been removed but there are still some residual roots.
Would it be a good idea to lay down and wet cardboard before adding the topsoil and seeding it?
Laying down cardboard is almost as bad as laying down plastic. The cardboard will totally stop the exchange of oxygen from the air to the soil, effectively killing it in about two weeks. In addition, many cardboard boxes have been treated with a water repellent so that the boxes stay structurally sound when wet - this will prevent rain from penetrating the cardboard. There is scientific evidence to supp.rt this.
See The Case Against Cardboard, The Cardboard Controversy (and read all comments and responses) and Termites Love Cardboardfor more information
Cardboard belongs in the Recycling bin, NOT your landscape.
So what to put under the lawn? Nothing. Just seed the lawn on the new topsoil and follow the usual "new lawn" practices.
