What you put under raised garden bed frames depends on your ground and what problems you want to solve. Most gardeners do fine with plain cardboard over grass or bare soil. Some spots need hardware cloth for pests or fabric for bad ground.
I set up beds on four different surfaces over the years to see what works best. Bare soil needed nothing at all. Grass died off with cardboard in one season. Gravel paths worked fine with fabric to keep stones from mixing in. My concrete patio bed needed drainage holes drilled into the slab below.
Good raised bed base preparation starts with knowing what sits below your planned spot. Clean native soil that drains well needs no bottom layer at all. Your plants can send roots down past the bed frame into the ground for extra water and nutrients. This connection makes for healthier plants.
Cardboard works great as a simple bottom layer for most home gardens. It blocks light from reaching grass and weeds so they die off during your first season. Over 6-12 months the cardboard breaks down into organic matter. Worms love eating through wet cardboard and improve the soil beneath as they go.
The ground under raised bed frames matters more than most new gardeners realize. Grass or weeds can push up through soil if you give them light. Aggressive types like bermuda grass laugh at thick mulch and grow right through. Cardboard stops them long enough for you to get your garden going.
Hardware cloth becomes key in areas with burrowing pests. Gophers and moles can destroy a garden from below in just days. This metal mesh has openings small enough to block rodents while letting water drain and roots grow. Staple it to your bed frame before you add any soil on top.
Landscape fabric or pond liner works best over bad or toxic soil. You want full separation when lead or other toxins show up in soil tests. These barriers keep your plant roots in the clean soil you add and block any contact with what sits below. Urban lots near old houses often need this level of care.
To prepare ground raised bed sites on grass, just lay cardboard flat and wet it down. Overlap sheets by 4-6 inches so grass cannot find gaps. Skip mowing first since the cardboard handles anything green below. Add your soil right on top and plant away.
My best results came from the simplest approach. Lay cardboard, add soil, start planting. No digging, no fabric, no extra steps. The cardboard holds down weeds that first year while your plants get going. By year two it has broken down and your bed soil connects with the ground below.
Match your bottom layer to what your yard needs rather than adding stuff you don't need. Cardboard handles most grass and weed issues. Hardware cloth stops digging pests. Fabric seals off bad soil. Clean ground needs nothing at all. Pick the right tool for your specific situation and skip the rest.
Read the full article: Raised Garden Beds: From Setup to First Harvest
