Do Raised Beds Need a Liner?

Most raised beds do not need a liner. A bed drains downward into the ground beneath it, and that drainage is the entire reason a raised bed outperforms heavy native soil -- so anything that seals the base is working against the bed. The useful exceptions are narrow: hardware cloth across the bottom if you have gophers or voles, cardboard on the ground if you are building over turf, and a strip of landscape fabric along the inside walls of a wooden bed if you want to slow rot. Plastic sheeting across the base is the one choice to avoid outright.

Notice that none of those reasons is drainage, and three of the five are better solved with something other than a liner. That is the honest state of it: liners are a solution looking for a problem in most beds.

1. Over grass or weeds: a double layer of plain cardboard, tape and labels removed, wetted down before you fill. It kills the turf, then decomposes within a season and lets roots straight through.
2. Over gophers or voles: half-inch galvanized hardware cloth across the whole footprint, run a few inches up the inside walls and stapled or bolted in place. Chicken wire is the wrong product -- the mesh is too wide and it rusts out fast.
3. Over a patio, deck or concrete: nothing across the base, but make sure water can escape. A bed on a hard surface drains to the edges, so leave a gap and expect staining underneath.
4. Over heavy clay: nothing. Loosening the top few inches of clay with a fork before you fill does more good than any barrier, because it lets bed and subsoil connect.
5. Over contaminated or suspect soil: a permeable geotextile barrier, and a deeper bed. This is the one case where a full base layer is genuinely indicated, and a soil test should come first.

A cedar bed fails where wet soil sits permanently against untreated timber. Lining the inside faces of the boards with landscape fabric puts a permeable layer between the soil and the wood, which slows that down without affecting drainage, because the base stays open.

It buys time rather than solving the problem. Even cedar is a wearing part in a wet climate, and if a bed you never have to re-board matters more than the look of timber, coated steel is the better long-run answer -- see metal vs wood raised garden beds for the full comparison.

Steel beds are the opposite case. A liner against the inside wall traps wet soil directly against the coating, which is the one condition that shortens the life of galvanizing. Leave them unlined; the coating is doing its job already, and the safety question is not a reason to add one.

The version of this question that really is about a liner is growing on a deck, a balcony or a concrete yard, where a bed with an open base will drain straight onto the surface and stain it. The honest answer there is not to line an ordinary bed but to use a product built for it: a planter with a slatted base and a fitted liner, or a fabric bed on a drip tray.

Fabric beds in particular behave well on hard surfaces -- they drain evenly rather than funneling water to one corner, and they air-prune roots instead of letting them circle. Raised beds for a deck, patio or balcony is the page for that situation.