Should I line a raised bed?

Which surfaces of a raised bed benefit from a liner and which must be left open, plus the specific cases where a base liner is right.

Difficulty: beginner
Time: 30 min
Read: 3 min

Short answer

Line the inner faces of the timber to keep wet compost off the wood, and leave the base completely open so roots, worms and water can move between the bed and the ground. The only reasons to line a base are contaminated soil, a hard surface underneath, or invasive weeds coming up.

The confusion comes from treating a raised bed as a large container. It is not — its main advantage over a container is the soil beneath it, and a base liner throws that away.

What you'll need

Damp-proof membrane or pond liner offcut (optional)
Weed membrane (optional)
Cardboard (optional)
Staples or clout nails

What goes inside a raised bed

Step by step

Line the inner faces only.Staple a damp-proof membrane or heavy polythene to the inside of the timber, from the top of the board down to the ground. It keeps permanently wet compost off the wood and adds years to the bed.
Leave the base open in almost every case.Roots go far deeper than the bed. Worms move up into it. Water drains down out of it. An open base gives a much better growing environment than a lined one.
Put cardboard on the base over grass or weeds.Plain brown cardboard, well overlapped, kills grass and annual weeds beneath and then rots away within months, leaving the base open. This is the right answer for a bed built on lawn.
Use a permeable weed membrane over persistent perennials.Where bindweed, couch or ground elder are present, a permeable membrane on the base blocks them while still allowing drainage. Ideally clear them first — they find the edges eventually.
Line the base fully only when you must.Contaminated ground, a bed on concrete or decking, or a bed over tree roots you must not disturb. In those cases treat it as a container: waterproof liner with generous drainage holes punched in it.
Punch drainage holes if you do line the base.A sealed base makes a bath. Plenty of holes, and a coarse layer at the very bottom, because in this case it genuinely is a container.
Do not line with black plastic and expect it to last.Cheap black polythene degrades and shreds within a few seasons and then works its way through the soil in fragments. Damp-proof membrane or pond liner offcuts are the durable options.
Fill correctly once lined.Coarse woody material and upturned turf at the bottom of a deep bed, then roughly three parts topsoil to one part well-rotted compost, to about 50mm below the rim.

Tips

Line the sides, never the base — that one rule covers almost every situation.
Cardboard is the best base treatment on a lawn: it kills the grass and then disappears.
A lined base turns a raised bed into a very large, badly drained pot.

Common mistakes

Lining the base with waterproof plastic to stop weeds — It stops drainage, roots and worms as well. Use permeable membrane if you need a weed barrier, and cardboard if you only need to kill grass.
Using treated timber and no liner — Modern tanalised timber is far less of a concern than old creosoted or CCA-treated wood, but a liner on the inner faces removes the question entirely and doubles the life of the boards.

If it doesn't work

Bed waterlogged after rain

Cause: Base lined with impermeable plastic — Fix: Punch generous drainage holes, or lift the liner out of the base entirely.

Boards rotting after three years

Cause: Wet compost in direct contact with untreated timber — Fix: Empty, staple a liner to the inner faces, refill.

Weeds coming up through the bed

Cause: Perennial roots below, cardboard already rotted — Fix: Dig out the roots. A permeable membrane at build time would have delayed but not prevented this.

Plastic shredding into the soil

Cause: Thin polythene degraded by UV and frost — Fix: Remove what you can, and replace with damp-proof membrane on the sides only.

Questions people ask

Do I need to line a raised bed at all?

The sides, yes, if you want the timber to last. The base, only in specific situations — contaminated ground, a hard surface underneath, or invasive perennial weeds.

Can I put weed membrane in the bottom?

A permeable weed membrane is fine and sometimes useful. An impermeable plastic sheet is not — it prevents drainage and root movement.

Is treated timber safe for growing vegetables?

Modern tanalised timber is widely used for vegetable beds and considered acceptable. If you would rather not, line the inner faces, or use untreated larch or oak, which last well without treatment.