Are metal planters bad for plants?
Whether galvanised troughs, corten steel and zinc containers really cook or poison roots, which ones actually cause problems, and the three changes that make any of them work.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
Short answer
Metal does not poison plants at normal garden pH, but a thin metal wall in full sun heats the compost at the edge far more than terracotta or plastic does, and metal containers rarely come with adequate drainage. Drill proper holes, line the sunny side, and keep them out of all-day sun.
Galvanised troughs, dolly tubs, corten planters and zinc window boxes are everywhere, and the folklore around them is mostly wrong. The zinc coating on galvanised steel is stable and does not leach meaningfully into compost. The real problems are heat, drainage and, on cheap thin steel, rust at the seams — and all three are fixable.
Step by step
- Deal with drainage first, because most metal planters have none.Decorative metal containers are frequently sold with no holes at all, or one token hole. Drill several 10 to 12mm holes across the base with a metal bit, and touch up the bare edges with a rust-inhibiting primer or paint.
- Understand the heat problem.Metal has high conductivity and low thermal mass, so a sunlit wall transfers heat straight into the compost against it. Roots at the edge can be damaged where the same plant in terracotta would be fine. The plant in the middle of a large trough is unaffected.
- Line the inside of the sunny faces.Bubble insulation, a sheet of polystyrene, or even flattened cardboard against the south and west faces breaks the heat transfer. Leave the base unlined so water still leaves freely.
- Choose the position for the container as much as the plant.A metal planter in morning sun or dappled shade behaves like any other pot. The same planter on a south-facing patio in full sun is a hard place to grow anything with fine roots.
- Water more, and mulch the surface.The heat drives evaporation from the compost surface and from the edge. A 25mm mulch of bark, gravel or slate on top makes a bigger difference to a metal container than to any other kind.
- Accept that corten is meant to rust, and plan for the runoff.Corten steel weathers to a stable oxide layer, which is the point of it. During the first year or two it runs orange, and that stains paving, decking and light-coloured render permanently. Stand it on gravel or on a plinth where the runoff does not matter.
Tips
- Galvanised steel does slowly corrode at the base where it stays wet. Pot feet or a plinth roughly doubles the working life of a galvanised trough.
- Zinc and galvanising react with strongly acidic composts over time. If you are growing ericaceous plants, line the container rather than planting straight into it.
- Old feed troughs and dolly tubs are excellent large planters and usually far cheaper than a garden centre equivalent of the same volume. Check for a plug hole before drilling.
Common mistakes
- Planting straight into a sealed metal container with no holes — There is nowhere for water to go and metal does not breathe the way terracotta does. The compost turns sour and the roots die within a season.
- Using a metal planter for shallow-rooted salads on a hot patio — It is the worst combination — small root volume, hot edges, fast drying. Metal suits large containers with substantial plants, not shallow trays of lettuce in full sun.
Questions people ask
Is galvanised steel safe for growing food?
Yes, for normal vegetable growing. The zinc coating is stable and zinc is a plant micronutrient; the quantities involved are not a concern at ordinary compost pH. Line it if you are growing in a strongly acidic mix.
Why has my corten planter stained the patio orange?
That is the first-stage runoff before the oxide layer stabilises. It is largely permanent on porous paving. Move the planter onto gravel and expect the running to reduce substantially after a couple of years.