Should I line a wooden planter, and how?

Wooden planters fail from the inside. Compost holds moisture permanently against the inner face and the end grain of the base boards, and untreated or thinly treated timber has no defence against that. A liner is the fix — but only if it drains. The two sections in the diagram show the sealed liner that people fit by instinct and the drained one that actually works.

What you'll need

• Butyl pond liner offcut or heavy polythene, true
• Staple gun and stainless or galvanised staples, true
• Drill and a 10mm wood bit
• Fine mesh or weed membrane offcuts
• Timber battens or planter feet

Step by step

1. Drill or check the base drainage before anything else.You want several 10mm holes spread across the base, or gaps between base boards. A planter with one small hole in the middle will not drain a wet fortnight.
2. Raise the planter clear of the ground.Battens, proprietary feet, or two treated bearers underneath. Air must move under the base and the end grain must never stand in water. This single step matters more than the liner does.
3. Cut the liner to cover the sides and the corners only.Take it from about 25mm below the top of the rim capping down the inside faces, and let it lap into the corners with a fold rather than a cut so there is no gap at the joint.
4. Stop the liner above the base, or slit it.Either finish it 50mm above the base so water leaves through the bottom freely, or take it across the base and cut generous slits over every drainage hole. Never leave an intact sheet across the base.
5. Staple under the capping, not on the face.Fix the top edge on the underside of the rim capping or the inner face just below it, so no liner edge is visible and no water can run down behind it. Use stainless or galvanised staples — plain steel bleeds rust stains through the timber.
6. Cover the drainage holes with mesh.A square of fine mesh or weed membrane over each hole keeps compost in and slugs and vine weevil adults out, without blocking flow the way gravel eventually does.
7. Fill with compost in one go, with no layer boundary.Compost right through the root zone. If the planter is far deeper than the plant needs, bulk-fill the lower part with inert material first, but only if there is still full working depth of compost above it.

Tips

• Butyl pond liner offcuts are the best material and last decades. Heavy damp-proof membrane is the cheap alternative and is fine for five years or so. Thin polythene sheet tears at the staples within a season.
• Treat the inside faces of the timber with a plant-safe preservative and let it dry fully before lining, so the timber has two defences rather than one.
• On a planter you have built yourself, leave a 3mm gap between base boards rather than butting them tight. It drains better than any number of drilled holes.

Common mistakes

• Lining the planter like a pond — It creates a permanently saturated reservoir at the base. Roots rot, the compost turns anaerobic and sour, and the timber stays wet against the outer face of the liner and rots anyway.
• Standing the planter flat on paving or decking — The base can never dry. Most rotten wooden planters have failed at the bottom 50mm and nowhere else, and that is why.
• Using ordinary steel staples — They rust within a year, the liner drops away from the rim, and you get black-brown streaks down the inside of the timber.

Keeping on top of it

• Check the base boards and battens for softness with a screwdriver point (Annually, in autumn)
• Re-treat exposed outer timber (Every two to three years)
• Clear the drainage holes of root and compost (When you replant)

Questions people ask