How to Fill a Corten Steel Planter: Drainage, Layers and Compost Explained

Fill a large Corten steel planter in three layers: a drainage layer of gravel or crocks at the base, a bulk filler layer (upturned pots, broken polystyrene or logs) to save compost in very deep planters, then a quality peat-free compost mixed with topsoil for the top 300–400mm where roots actually live. Here's the full method.

Step 1: Check the drainage

Make sure drainage holes are clear and the planter is raised slightly on pot feet so water can escape freely. Waterlogged compost is the single biggest killer of container plants — more than frost, drought or pests.

Step 2: Add a drainage layer

Add 50–100mm of coarse gravel, crushed brick or broken crocks across the base. This keeps the drainage holes from clogging with compost over time.

Step 3: Bulk-fill deep planters

Most plants only root into the top 300–400mm of soil, so in an 800mm-deep cube or cylinder planter you can fill the lower half with upturned plastic pots, polystyrene chunks or untreated logs. This saves money on compost, reduces total weight, and improves drainage. Skip this step for deep-rooted shrubs and trees, which want the full depth.

Step 4: The growing layer

Mix roughly two-thirds quality peat-free multipurpose compost with one-third topsoil. The topsoil adds body and holds moisture and nutrients longer than compost alone, which matters in free-draining steel planters. For Mediterranean plants and grasses, add horticultural grit for sharper drainage.

Step 5: Settle and top up

Water thoroughly after filling — the level will drop as everything settles. Top up before planting so the finished soil level sits 30–50mm below the rim, which stops water and compost washing over the edge.

Frequently asked questions

Do I need a liner in a Corten planter?

Not for drainage or plant health — but a permeable liner is a reasonable extra precaution for edible crops, as covered in our guide to growing vegetables in Corten steel.

How much compost do I need?

Length × width × depth of the growing layer gives you litres (in dm). A 600mm cube with a 400mm growing layer needs roughly 145 litres of mix.

Will a filled planter be too heavy to move?

Large filled planters are effectively permanent — position them empty, then fill. See our planter buying guide for placement advice.

Should compost touch the steel sides?

Yes, that's fine — the interior weathers just like the exterior and the patina is not harmed by soil contact.

Get planting

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