Metal raised beds: Corten steel or aluminium, open at the base

A metal raised bed is not a planter: it has no base. You stand it on the garden soil, fill it, and the planting stays in contact with the ground. Roots go as deep as they like, rain drains away into the soil instead of sitting in the bed, and the steel or aluminium frame gives you a crisp, raised border, out of reach of the mower and of passing feet.

That principle comes with one simple rule: a bottomless frame only goes on permeable ground. On a patio, a concrete slab, a balcony or a roof, nothing would hold the soil in. There you need a planter with a base, such as Thalie on legs, raised on four corner legs: 23 sizes in Corten steel from £900, and 5 sizes in aluminium from £1,160. Our guide on how to choose a Corten steel planter helps you match the planter to the surface beneath it.

We do not sell galvanised steel: our raised beds come in Corten steel or in powder-coated aluminium only.

We took from our product pages the sizes offered in both metals, given as length × width × height in millimetres. The volume shown is the gross volume of the frame, worked out from its outside dimensions, to give you an idea of how much soil to order. Weights are the empty weights published on each product page; prices are UK prices, in the signature patina for Corten steel and in any of the eight colours for aluminium, which all cost the same.

Size for size, the aluminium bed weighs 1.7 to 2 times less than its Corten steel twin, even though its 3 mm wall is thicker than the 2 mm of the Corten: a 1200 × 1200 × 800 mm Rhéa drops from 76.2 kg to 39.5 kg. For a frame you move around, carry to the far end of the garden or put in place without help, that difference counts.

On price, the gap is narrower in the UK than you might expect: aluminium costs 14% to 47% more depending on the size. It is smallest on the little 400 mm cube (£830 against £730) and widest on the large 1400 mm square (£2,090 against £1,420). Across the three lengths of Gaïa, it stays between 34% and 38%. On the smaller sizes, weight, colour and pale paving can therefore tip the choice more than the budget does.

The real difference lies in the range of sizes. We make our bottomless frames in Corten steel in many more formats:

• Rhéa: 27 sizes in Corten steel, from 400 to 1400 mm square and from 400 to 1000 mm high; 10 sizes in aluminium, up to 800 mm high, all of them in the table above.
• Gaïa: 21 sizes in Corten steel, from 900 to 2000 mm long, 300 to 600 mm wide and 400 to 1000 mm high; 3 lengths in aluminium, all 500 mm wide and 600 mm high.
• A large, low bed of 600 to 1400 mm square and 400 mm high, a narrow 300 mm strip along a wall or a frame 1000 mm high: these sizes exist only in Corten steel.

Height decides how comfortable the bed is to work. A 400 mm frame lifts a border, keeps seedlings clear of the path and is plenty for herbs; at 600 or 800 mm you sow, weed and harvest with far less bending; the 1000 mm frame, offered in Corten steel, brings the soil closer still to your hands. Think about filling, too: the gross volume in the table gives the order of magnitude of soil to buy, and a large 1200 mm square, 800 mm high, already takes more than a cubic metre.

Width decides how easily you reach the middle. A 1000 or 1200 mm square can be worked from all four sides; beyond that, the centre becomes hard to reach. Against a wall or a fence, where you can only get at one side, the rectangular Gaïa is more practical, 500 mm wide in both metals and as narrow as 300 mm in Corten steel.

Finally, prepare the ground before delivery: a clear, level spot that drains well. If moles or voles visit the garden, lay mesh directly on the soil before you fill the frame. And set the bed in its final position before filling it, as it will not move easily afterwards.

In a bottomless frame, you grow in your own garden soil, which the frame edges and raises. The iron oxides of the patina are not a poison for plants: iron is a natural element of the soil. Our page on Corten steel sets out what is known about Corten, plants and the vegetable garden.

What we do not claim is any particular food suitability, for Corten steel or for powder-coated aluminium: our raised beds are planting frames, not containers certified for food contact. For a kitchen garden, choose the metal for how it wears, what it weighs and how it looks, and treat your soil as you would in an open bed.

Corten steel gives a vegetable patch a frame of sienna and amber, a shade that keeps changing with rain and sun. Its wall needs to be able to dry out: free-draining soil and no standing water at the foot of the frame keep it in good order. Next to a pale path, stone paving or a stone edging, rainwater can carry rust run-off. The protective varnish, offered as an option on Rhéa and Gaïa in Corten steel, greatly reduces it without ruling it out: even varnished, Corten steel can still stain the ground. Leave a strip of gravel between the bed and the paving, and read our Corten steel care guide.

Powder-coated aluminium does not rust and does not stain the ground. It comes in White RAL 9016, Grey RAL 7016, Black RAL 9005 (shown at 30% gloss), Blue RAL 5001, Yellow RAL 1005, Green RAL 6018, Red RAL 3020 and Orange RAL 2004. Beside a swimming pool, on the coast or next to very pale paving, it is the worry-free choice; our guide to Corten steel near the sea explains why. A black or anthracite grey frame sets off green foliage, while white lifts a shady garden.

Start with the ground, then the space and the style of the garden.

• A vegetable plot in the middle of the garden, worked from all four sides: the square Rhéa, 1000 or 1200 mm across.
• A growing strip along a wall, a fence or a path: the rectangular Gaïa, up to 2000 mm long.
• A large, low bed 400 mm high, a 1000 mm high frame or a narrow 300 mm strip: Corten steel, the only metal in which these sizes exist.
• A frame you can carry on your own, a particular colour, pale paving to keep clean or a seaside garden: aluminium, which in the UK costs only 14% to 47% more.
• A patio, a slab or a balcony: not a bottomless frame, but Thalie on legs or one of the planters with a base compared in both metals on our metal planters page.
• Ideas for working raised beds into a garden layout: our guide to Corten steel garden ideas.