It’s probably being used as sheet mulch.
What is sheet mulch?
Any place where a gardener might want to put some new plants is probably already occupied by other plants. There are several ways to get rid of old plants in order to make room for new ones. For example:
Digging. Old plants can be dug up, creating a space of bare soil. This is a lot of work!
Poisoning. Old plants can be killed by spraying herbicide. This exposes the gardener to toxic chemicals.
Smothering. Also known as sheet mulching, this involves placing an opaque cover over the old plants, depriving them of the sunlight they need to survive. Though this method can take a long time to kill the plants underneath, it is very easy to do.
How is sheet mulching done?
While sheet mulching can be accomplished using landscape fabric or black plastic, natural gardeners usually prefer to use organic materials like cardboard, newspaper, carpet, or even old pairs of blue jeans. These materials are laid over the planting area, making sure to fully cover the plants underneath, and overlapping the edges of the materials to prevent plants from growing up through the cracks.
Soil or a loose mulch is then placed over the top, as a medium for the new plants to grow in. Organic sheet mulch will gradually decompose, along with the buried plants, creating more soil for the new plants to use. Once the new plants are established, it’s usually hard to tell there’s cardboard underneath them!