- Add compost and mulch on top of the cardboard. Spread compost directly over the cardboard and then cover it with bulky materials like wood chips to optimize water conservation and weed control. Adding 1-2 inches of compost will help build soil and provide a planting medium for 4-inch or smaller plants. However, if your main goal is weed suppression just add the mulch.
- The compost/mulch layer should be a total of 3 to 5 inches deep. The top layer of mulch mimics the newly fallen organic matter of a forest. Good materials for the top layer include chipped plant debris, tree prunings, leaves or even straw. You will need a lot of mulch. Use the materials calculator to determine how much mulch and/or compost you need.
Tip: Cover the wet cardboard with compost and use it as a pathway to the middle of the bed; that way you don't have to walk on, and potentially rip holes through, the wet cardboard.