What is Corten steel?

If you have just taken delivery of a Corten planter and it looks nothing like the photographs, this page is for you. Nothing has gone wrong.

It arrives the wrong colour

Corten, properly called weathering steel, leaves our workshop raw. Flat, blue-grey, faintly industrial. People do occasionally ring us about this on the day it arrives.

Leave it outside. Over roughly four months the surface oxidises into the deep orange-brown you were expecting.

Wet spells and dry spells alternating are what drive the process, so a British autumn does the job nicely. A planter delivered in October will usually look the part by early spring.

You do not have to do anything

No treatment. No sealant. No oil. No help of any kind.

Rain and air do all of it. Anything you paint or seal it with will actively prevent the process, and you will end up with a grey planter under a coat of varnish.

If you are impatient, repeatedly wetting it speeds things up a little. A hose every few days through a dry spell will bring the colour on faster than leaving it in a drought. It is not necessary though.

Why the rust does not keep going

This is the part that surprises people, and it is worth understanding because it is the whole reason the material exists.

On ordinary steel, rust is porous. Water gets underneath it, reaches fresh metal, and the corrosion keeps eating inward. Given long enough there is nothing left.

Corten has copper, chromium and nickel in the alloy. Those change how the oxide layer forms. Instead of flaking, it knits tight and dense against the surface, and once formed it blocks moisture from reaching the metal beneath.

The reaction slows dramatically and then effectively stops. What you end up with is a protective skin rather than an ongoing problem.

It is the same steel used for bridges, sculpture and building facades, chosen precisely because it looks after itself for decades without anyone painting it.

The one mistake worth avoiding

While it is weathering, rain running off the planter carries rust particles with it.

On pale limestone, new decking, block paving or light render, that staining is difficult to remove and sometimes permanent. It is the single thing people write to us about.

For the first few months, stand it on soil, gravel, grass or dark paving. Once the patina has stabilised the run-off largely stops and you can put it wherever you like.

If it has to go on pale stone from day one, tell us when you order and we will talk through options.

How it ages after that

The colour deepens and evens out over the first year or two, then settles.

It darkens in wet weather and lightens as it dries. That shift through the seasons is normal and is part of why people choose it. No two pieces weather identically, which is rather the point.

Ours carry a three year guarantee, but a Corten planter left alone should comfortably outlast the garden around it.

Does it need drainage?

Yes, like any planter. Standing water sitting in the base is the one thing that will shorten its life, whatever the steel is made of.

Put a layer of crocks or gravel in the bottom and use a decent free-draining compost. If you are planting something that hates wet feet, raise the rootball slightly.

Would painted steel suit you better?

Corten is not right for everyone, and we would rather say so.

Choose painted instead if you want a colour that stays exactly as delivered, if the planter has to sit on pale stone, or if you want it to disappear against a dark wall rather than draw the eye.

Our painted range is galvanised underneath, so it cannot rust through from beneath, and etch primed before the finish coat. Anthracite grey, black or white as standard, any RAL shade on request at no extra cost.

See the full planter range, or call us on 020 8539 0611 and describe where it is going. We will tell you straight which one suits.