What is Cor-Ten steel? Cor-Ten steel is the trademarked name for a weathering steel that is known for its distinct, rusty finish and is used on building exteriors and in sculpture and integrated into landscape design. While the U.S. Steel Corp. holds the trademark to the name Cor-Ten, the term is commonly applied to all weathering steel, a group of steel alloys that will gain a rust-like appearance over time. “When you buy weathering steel today, it may or may not be Cor-Ten,” says designer and fabricator Branden Adams of BaDesign in Oakland, California.
Originally designed to eliminate the need for painting or any other protective coating, Cor-Ten steel develops a naturally oxidized finish over the course of a few years that not only protects it from further corrosion, but also makes it a desirable design material. “In this case, rust is ‘good’ in that it not only protects the underlying metal, but it also displays beautiful, earth-toned colorization,” says Montana-based metal artist Pete Christensen.
Watch now: Learn more about using Cor-Ten steel
Landscape designer Andrew Beck used Cor-Ten steel for the round raised beds in the garden in Perth, Australia, shown here. The material creates a colorful contrast to the green foliage, and its thin profile allowed him to pack the planters in tightly for this artistic arrangement. “When we use mild steel, we have to anticipate greater levels of corrosion and therefore use a heavier gauge of metal, meaning it has a much greater weight and is more awkward to work with in the larger-size planters,” he says.
Regardless of what’s growing inside them, Cor-Ten raised beds are striking design features that will add beauty to any garden.
Additionally, the material’s thin profile takes up much less space than stone or wood, making it great for tighter spaces. Cor-Ten is very heavy, so it might not be feasible on some roof gardens or other places with weight restrictions.
While Cor-Ten is more expensive upfront than wood, its longevity means maintenance and replacement costs are minimal.
Building Cor-Ten raised beds isn’t going to be a DIY project unless you have access to fabrication tools and have experience with welding, grinding and other metal prep.
Some online shops sell Cor-Ten planters ready-made or in pieces. If that is the case, you can assemble them, if needed, plop them into place, fill them with soil and get planting.
Find landscape designers | Find landscape architects
Tiffin, who made these boxes in San Francisco, estimates up to $1,500 per large handcrafted box, plus an installation fee. The bigger the box, the more likely it will need to be installed on-site, which adds to the cost for labor. “In general, there is a higher upfront cost compared to wood, but the lifespan will be at least twice as long. Cor-Ten will outlast most other materials,” Adams says.
Best time to start: Anytime can be a good time to start. “Pay attention to your local planting season, and allow a couple of months ahead of that to have your planters fabricated, delivered and ready for use,” Adams says.
Maintenance: Cor-Ten planters are known for their minimal maintenance. “It’s one of those things you should be able to plop down and walk away,” Tiffin says. Although, like anything, you’ll want to check the beds from time to time to make sure nothing is failing.
- Rusty runoff: Rusty runoff on concrete sidewalks or sides of buildings is a common sign that Cor-Ten steel is nearby. The material is going to rust, there is no avoiding that, but you can detail the design so that it doesn’t stain surrounding materials. Bring it away from the concrete or design drainage to carry runoff away from paving. The rust stains will be much less noticeable if the planters are installed on gravel or mulch. “Over time, the rust becomes more stable and is less likely to rub off,” Adams says.
- Growing edibles: It is safe to grow edibles in Cor-Ten planters without any additional treatments, although some designers will install liners or other barriers between the soil and raised bed. “We usually coat the interior of the tanks with a food-grade waterproofing coat similar to that used in rainwater tanks,” Beck says.
- Heat: You may want to be careful about touching your raised beds on a hot day, as steel gains heat more quickly than soil. The raised beds will slowly transfer heat to the soil, however, keeping it warmer overnight. For this reason, plants might even benefit from growing in Cor-Ten raised beds, Adams says, as long as they receive enough water and have enough soil volume. “A small planter may not allow enough soil volume to offset the heat gain from the Cor-Ten container,” he says.
- Climate: Cor-Ten can be used in a range of climates; you’ll see it everywhere from Seattle to Chicago. “Homeowners should keep in mind that marine environments will speed up the corrosion process,” Adams says, but it will still outlast wood. “Cor-Ten performs best when it is allowed to go through continuous wet-dry cycles. It’s when it is allowed to thoroughly dry out that its alloying metals form the best coating on its surface.” The air in coastal environments holds more moisture and keeps materials damp, preventing these cycles. Additionally, the salt in the air can cause pitting. “My personal feeling is that for things like planters, it’s fine to use it in marine environments. I would hesitate to use uncoated Cor-Ten for major architectural applications in marine environments,” Adams says.
Additionally, Cor-Ten is sometimes used in water features, but there is potential for staining. “If you like, or can accept, the staining, then go for it,” Adams says.