Sheet mulching is a gardening technique that suppresses weeds and builds fertile soil. In sheet mulching thick layers of organic matter are placed on the ground lasagna style. These layers are then left to decompose ultimately creating a rich planting medium (compost) that is terrific for vegetable gardens and ornamental planting beds.
The advantage of sheet mulching over composting in a bin is that the finished compost does not have to be hauled to the actual planting bed. It is created right on top the planting bed. The process can take three to six months so it is best to plan ahead and sheet mulch the season before you want to install your plants. However, it is possible to install some plants right after you sheet-mulch by punching a hole through the sheet mulch for each plant and popping them in.
Sheet mulching is effective at suppressing weeds because it is so thick – as much as 12 inches – and because one of the bottom layers has no holes in it for air and light to sneak in. The weeds and anything else underneath are snuffed out. You can even sheet mulch right over the top of sod. There is no need to remove the lawn ahead of time.
Sheet Mulch-Lasagna Composting, OSU Extension article
Prepare Garden Beds Now, article on sheet mulching from Northwest Garden News