The practice of lining the base of a raised garden bed with cardboard is a popular method for preparing new planting areas. Raised beds offer gardeners better control over soil composition and drainage, but they are often placed directly over existing turf or weed-filled ground. The addition of corrugated cardboard provides a sustainable, low-effort solution to common gardening challenges by leveraging the material’s structural and biological properties.
Primary Role as a Weed and Grass Barrier
The primary benefit of using cardboard in a raised bed is its function as a physical barrier against unwanted vegetation. When placed directly on the ground before adding soil, the dense paper material effectively smothers existing grass and weeds by completely blocking the sunlight necessary for photosynthesis.
This method is effective against tough, perennial weeds like Bermuda grass, preventing them from growing up into the newly added soil. The cardboard also prevents weed seeds already present in the ground from germinating because they lack light and air exchange. This obstruction significantly reduces the need for manual weeding or the use of chemical herbicides.
Contribution to Soil Health and Moisture Retention
The cardboard layer contributes positively to soil health as it naturally decomposes. Cardboard is primarily composed of cellulose, a rich source of organic carbon that microorganisms consume, slowly incorporating beneficial organic material into the underlying native soil.
As the cardboard breaks down, it attracts beneficial organisms like earthworms, which tunnel through the material and aerate the soil beneath the bed. The fibrous structure also acts like a sponge, helping the soil retain moisture by slowing down evaporation from the ground surface. This moisture-retentive quality is helpful in new raised beds, and the decomposing carbon aids in balancing the carbon-to-nitrogen ratio.
Practical Steps for Safe Placement
Proper preparation of the cardboard is necessary to maximize its effectiveness and ensure garden safety. Use plain, brown, corrugated cardboard and completely remove any non-paper materials beforehand, including:
- Plastic packing tape.
- Staples.
- Glossy or heavily colored labels.
- Stickers.
The flattened sheets should be laid directly onto the ground, covering the entire footprint of the raised bed. Overlap the edges of the pieces by several inches, similar to laying shingles, to prevent weeds from finding gaps. Once the layer is in place, the cardboard must be thoroughly saturated with water to initiate decomposition and help it conform to the ground. Wetting is important because dry cardboard can repel water, hindering moisture from reaching the native soil underneath.
Addressing Common Concerns
A common concern is the potential for harmful chemicals in the cardboard. Modern corrugated cardboard typically uses starch-based glues, derived from plants like corn or potato, which are safe and decompose readily. Most black printing inks are now soy-based, but it is best to avoid cardboard with heavy, glossy, or brightly colored printing, which may contain heavy metals.
Another concern is that the warm, damp environment may attract pests such as slugs or termites. While slugs and earwigs may temporarily shelter under the material, mitigation involves ensuring the cardboard layer does not extend beyond the edges of the raised bed. Termites are only a concern where they are already established, and the risk is reduced by keeping the cardboard away from wooden structures.