The bottom of raised garden bed setups vary based on your ground and what problems you face. Most gardeners do well with plain cardboard laid flat on the soil. Some spots call for hardware cloth or nothing at all. Your choice depends on weeds, pests, and soil quality in your yard.
I tested both cardboard and landscape fabric as a raised bed liner over four seasons in my backyard. The cardboard beds grew healthier plants with more earthworm activity. The fabric beds stayed weed-free but felt less alive beneath the surface. I now use cardboard for all my new beds after seeing these results.
Cardboard works through a simple process that helps your garden long term. It smothers grass and weeds for the first season while blocking light from seeds below. Over 6-12 months, the cardboard breaks down and adds organic matter to the soil. This happens right where your plant roots meet the native ground beneath. Worms love to feed on the decaying cardboard too.
Landscape fabric takes a different approach that lasts much longer than cardboard. It stops weeds from pushing up through your bed for many years. But it also blocks earthworms from moving between the native soil and your garden mix. This trade-off matters more in beds where you want deep roots to tap into the ground below. Tomatoes and peppers grow better with access to that deeper soil layer.
Using cardboard under raised bed works best when you build on a lawn or weedy spot. Flatten the boxes with tape and labels removed first. Overlap the edges by 4-6 inches and wet them down before adding soil. The moisture helps the cardboard mold to the ground and stay in place while you fill the bed. I learned this trick after my first cardboard layer shifted and left gaps.
Hardware cloth becomes key in areas where gophers or moles tunnel through gardens. This metal mesh has half-inch openings that let water drain and roots grow down. At the same time, it keeps burrowing pests out of your growing space. Staple the mesh to the inside of your bed frame before you add any soil on top. The mesh should curve up the sides by at least 3-4 inches for a tight seal.
Some spots call for no bottom layer at all. Good native soil that drains well needs nothing extra. Direct contact between your bed mix and the ground below creates the best growing setup. Plant roots can reach deeper and soil life moves freely between layers. This approach costs nothing and works great over sandy or loamy ground.
A weed barrier raised bed setup works best over bad or toxic soil. You want full separation in these cases. Use heavy landscape fabric or pond liner to keep plant roots in the clean soil you add on top. This protects your crops from anything harmful below. Old industrial sites or areas near treated wood structures often need this level of protection.
Match your bottom layer to your specific yard for the best results. Start with cardboard if you have grass or weeds below. Add hardware cloth if critters dig in your area. Use thick fabric over bad soil. Skip the layer over healthy ground. Your plants will show you if you made the right choice by how well they grow that first season. Most gardeners find cardboard gives them the best balance of weed control and soil health over time.
Read the full article: Raised Garden Beds: From Setup to First Harvest
