The short answer
On open ground, put nothing between the bed and the soil below unless you have a specific problem. The University of Minnesota Extension says a barrier there is usually unnecessary and likely to stunt roots. Do not add a layer of gravel or rocks for drainage: it holds water in the soil above instead of helping it drain.
What each material does
| Material | What it does | When to use it |
|---|---|---|
| Nothing (loosened soil) | Lets roots and water move into the ground below | Most beds on open ground |
| Gravel or rocks | Holds water in the soil above it (a perched water table) | Not recommended in beds or containers |
| Plain cardboard or newspaper | Smothers grass and weeds under the bed | Over lawn; see below for whether to remove it |
| Hardware cloth | Keeps burrowing animals out | Where gophers or voles dig |
| Landscape fabric | Holds soil in while letting water through | Beds on paving or a patio |
| Plastic sheet | Stops water | Not under a bed; only as a lining on the walls or over contaminated ground with drainage holes |
Why not gravel
The University of Maryland Extension says rocks or gravel at the bottom of a container do not improve drainage and can raise the water table in the mix, waterlogging it. An Ask Extension answer gives the same advice for planter beds on a patio: gravel creates a perched water table, so water stays in the soil instead of draining through.
Cardboard over grass
The University of Maryland Extension suggests smothering turf and weeds with plain cardboard, tape removed, or overlapping newspaper, held down with 2 to 4 inches of compost. The University of Minnesota Extension takes a different view for beds: remove the cardboard or tarp before setting the bed down, because cardboard can stop water soaking in until it breaks down. If you leave it, water well and loosen the ground where you can.
Hardware cloth against gophers and voles
UC IPM advises laying hardware cloth or ¾-inch mesh poultry wire under raised beds before planting where pocket gophers are a problem. Colorado State University Extension gives ¼-inch hardware cloth to keep out voles and pocket gophers. Fasten it across the bottom of the frame before the bed goes into place.
Mix the first layer into the ground
Colorado State University Extension warns against a sharp line between one soil and another, which slows water and air and can stop roots. Loosen the ground and mix the first few inches of fill into it rather than stacking a distinct layer on top.
Paving and contaminated ground
On a patio or concrete, the University of Minnesota Extension suggests a material that lets water through, such as landscape fabric, rather than plastic; the hard-surface guide covers depth and drainage. Over ground that may be contaminated, NC State Extension describes the opposite: seal the bottom with an impermeable barrier so roots do not reach the soil, and drill drainage holes near the bottom. The lead guide explains when that applies.
To fill a deep bed with less bought soil, see how to fill a raised garden bed cheaply.
References
- Raised bed gardens, University of Minnesota Extension
- Growing Vegetables in Containers and Salad Tables, University of Maryland Extension
- Planter beds on top of a patio (question 926805), Ask Extension (Cooperative Extension)
- Growing Vegetables in Raised Beds, University of Maryland Extension
- Pocket Gophers (Pest Notes), UC Statewide IPM Program
- Vegetable Gardening in the Mountains, Colorado State University Extension
- Block Style Layout in Raised Bed Vegetable Gardens (GardenNotes #713), Colorado State University Extension
- 16. Vegetable Gardening (Extension Gardener Handbook, AG-831), NC State Extension