The short answer
A bed on open ground usually needs nothing underneath: the University of Minnesota Extension says a barrier between the bed and the soil below is likely to stunt root growth. Add hardware cloth if gophers or voles dig, landscape fabric if the bed sits on paving, and a lining on the walls only for a reason such as treated wood or contaminated ground.
Which liner does what
| Situation | What to use | Source |
|---|---|---|
| Bed on open ground | Nothing under the soil | University of Minnesota Extension |
| Pocket gophers | Hardware cloth or ¾-inch mesh poultry wire under the bed | UC IPM |
| Voles and pocket gophers | ¼-inch hardware cloth under the bed | Colorado State University Extension |
| Bed on a patio or concrete | A material that lets water through, such as landscape fabric, not plastic | University of Minnesota Extension |
| Treated wood walls, if you want less contact | Plastic on the inside of the walls, or lining the edges | Washington State University Extension; University of Minnesota Extension |
| Galvanized walls, if zinc is a concern | A liner, or a stainless steel bed | University of Minnesota Extension |
| Over contaminated ground | An impermeable bottom with drainage holes near it, or garden fabric under 16 inches of clean soil | NC State Extension; EPA |
Weed barrier under the bed
The University of Minnesota Extension says a barrier under a bed is usually unnecessary and likely to stunt roots. If you use cardboard to kill grass first, the University of Maryland Extension holds it down with 2 to 4 inches of compost; the University of Minnesota Extension suggests removing it before the bed goes on, because it can stop water soaking in until it breaks down.
Fitting hardware cloth
Cut the mesh to the outside size of the frame and fasten it across the bottom before you set the bed down. The 4 × 8 bed plan shows where it goes, and the bottom-of-the-bed guide covers gravel and cardboard.
References
- Raised bed gardens, University of Minnesota Extension
- Pocket Gophers (Pest Notes), UC Statewide IPM Program
- Vegetable Gardening in the Mountains, Colorado State University Extension
- Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension
- 16. Vegetable Gardening (Extension Gardener Handbook, AG-831), NC State Extension
- Healthy Gardening Practices to Reduce Exposure to Lead-Contaminated Soil, US Environmental Protection Agency
- Growing Vegetables in Raised Beds, University of Maryland Extension