Corten Steel: Is It Safe For Gardens?
Corten steel looks fantastic, but is it safe for garden use? Horticultural expert Sarah Jay explores all elements of using corten steel in the garden!

Contents
Corten steel has become an increasingly popular material for home gardening and commercial landscaping. As a type of weathering steel, it develops a protective layer of corrosion-resistant patina, giving it both durability and a distinctive aesthetic appeal.
Naturally, some concerns arise regarding corten steel and weathering steel in general. While these concerns aren’t unfounded, unless significant atmospheric corrosion is present—which we’ll discuss later—the mechanical properties of COR-TEN steel alloys make it an excellent choice for raised beds and planters in most weather conditions.
In this piece, we’ll explore this topic in detail. So if you’re wondering whether corten steel is right for your garden, keep reading!
What is Corten Steel?

Corten steel is a type of chromium-copper alloy weathering steel that relies on a wet/dry cycle to form a protective layer of rust. As the surface develops, it changes color over time, starting with a red-orange hue and eventually forming a purplish patina.
Rust is often seen as a sign of deterioration. In this case, it is essential for creating the desired appearance and sealing the material, preventing further corrosion.
In fact, corten steel is highly resistant to corrosion. It has been used in notable architectural projects, such as the Broadcasting Tower in Leeds, United Kingdom.
Corten ASTM Designation

The original COR-TEN A received a standard designation for low-alloy, high-strength, and high-corrosion resistance from the American Society for Testing and Materials (ASTM). Newer ASTM grades for Corten B steel share these properties but have received standard designations indicating they can be manufactured and used in thin sheets.
Corten steel is composed of copper, chromium, manganese, and nickel.
Difference Between Corten and Redcor

One distinction worth explaining is the difference between Corten steel and Redcor. COR-TEN refers to a hot-rolled steel alloy commonly used in the railroad and shipping industries. Redcor, a type of Corten steel, is cold-rolled rather than hot-rolled. This cold-rolling process helps stabilize the chemical composition during sheet formation, ensuring greater uniformity from product to product.
Difference Between Corten A and Corten B

Let’s also discuss the differences between Corten A and Corten B steel. While they are largely the same material, Corten A—the original COR-TEN—includes phosphorus, making it more suitable for building facades and gas flues. Corten B, which lacks this addition, is better suited for large structures.
An interesting aspect of Corent B planters is that they are completely safe for growing food. The small amount of iron oxide released into the soil during the rusting process is non-toxic and does not negatively affect plants.
The Rust Process

Let’s take a closer look at rust in Corten steel. The protective rust layer that forms on weathering steels is safe for plants—not only because the trace amounts of iron, manganese, copper, and nickel are non-toxic, but also because these micronutrients play a role in promoting healthy plant growth. The protective patina that develops on the steel adds to its durability while remaining plant-safe.
However, some architectural studies suggest that in large quantities, under conditions that accelerate corrosion, Corten A steel could act as an environmental contaminant. It’s important to note that these studies are not relevant to Corten B steel or Redcor. Additionally, the study in question focused on the facade of a large building over several years of exposure. While large amounts of Corten A may pose environmental concerns under certain conditions, smaller amounts of Corten steel, when used in appropriate environments, remain non-toxic.
As the rust layer develops, the tensile strength of Corten B steel increases, making it more corrosion-resistant. For those living in areas where atmospheric corrosion is more likely, the steel can be allowed to reach the desired rust color before applying a sealant. This helps maintain both its appearance and structural integrity.
Corrosion

The main concern most people have about Corten steel is its durability. In raised bed applications and the right environment, COR-TEN steel is highly resistant to corrosion. This is why it was initially chosen for structures in architecture and construction (such as the Broadcasting Tower in the United Kingdom) before being adopted for gardening projects.
However, its resistance largely depends on the climate and weather conditions in a given area. Proper oxidation occurs in optimal wet/dry cycles, but regions with high humidity present challenges to Corten steel’s durability. Additionally, resistance to corrosion is lower in areas with salt spray, particularly in coastal regions. People living near the ocean may experience higher corrosion rates with Corten steel beds.
For those in such atmospheric conditions, Aluzinc-coated galvanized steel beds—like the Birdies Original metal raised bed—are a better alternative. Fortunately, galvanized steel is also safe for gardening!
That said, Corten steel beds exposed to fog, snow, rain, or other moisture are protected by the rust layer that forms on the surface, shielding the underlying steel. They also offer a distinctive aesthetic, with the patina developing a rich, weathered hue over time.
How Long Does Corten Steel Last?

As long as Corten steel beds are not exposed to the intense atmospheric corrosion discussed earlier, they will last for years. The rust that develops on the surface forms a protective layer, enhancing corrosion resistance and tensile strength. This durability is why Corten steel is widely used in landscaping and home gardening.
The steel’s rust layer increases its resistance to degradation, making it the premier weathering steel material in construction and architecture for years.
Corten Steel Raised Beds

While we’ve covered the general properties of Corten steel and its use in architectural projects, let’s now discuss best practices for growing plants in Corten steel raised beds. In most environments, the atmospheric corrosion resistance that develops on Corten steel actually makes it more durable than other materials.
However, understanding how COR-TEN forms its protective patina can help you determine the best placement and use for your raised beds. Proper positioning and maintenance will ensure both longevity and optimal growing conditions for your plants.
Design

Beyond large-scale construction and architecture, Corten steel has a wide range of uses. One of the most exciting developments in weathering steel production is the creation of stylish and durable garden beds. These beds come in various forms, offering consumers options such as traditional raised beds or compact planters designed to sit on railings or countertops. There are even round planters, perfect for urban gardening.
As these beds rust, the yield strength of the steel alloy continuously regenerates, enhancing both the appearance and corrosion resistance of the exterior as it is exposed to the elements.
Placement

Because Corten steel beds rust and their surface materials can run off, it’s best to place them directly on the ground or in an area where runoff won’t be an issue. In business complexes where planters may sit on the pavement, weathering steel can leach onto surfaces, especially after rain.
While this isn’t a structural issue (the alloy continuously regenerates as it rusts) any surface beneath the beds will accumulate runoff material. If you need tips on removing stains, check out the final section of this piece.
If placing your bed on the ground, lay down 1/4 inch to 1/2 inch of gravel before filling it. This improves drainage, protects the bottom of the bed, and helps suppress weeds throughout the season.
Runoff is not harmful to the environment or the plants you’re growing, as previously mentioned. The metal’s yield strength remains unchanged whether placed on soil or another surface. However, staining can be an aesthetic concern, as untreated rust runoff may discolor concrete without regular cleaning and maintenance.
To prevent staining, clean any runoff promptly with routine washing or power washing. Alternatively, place your Corten steel beds on rust-colored gravel, cardboard, or bare soil to minimize the impact.
Accelerating Corrosion

Another interesting aspect of Corten steel beds is that consumers can accelerate the corrosion process to achieve their desired look. Beds are shipped from the factory with a protective coating to prevent corrosion during transport. Once exposed to the elements, this layer wears off, allowing the natural rusting process to begin. However, you can speed up this process at home to achieve the perfect weathered patina.
To accelerate rusting, mix 2 ounces of vinegar, ½ teaspoon of salt, and 16 ounces of hydrogen peroxide in a spray bottle. Shake vigorously to combine the ingredients. Wear gloves and goggles for protection. Spray the entire metal planter box, wiping it with a towel if needed to smooth the texture. This process speeds up patina development, forming a protective coating on the oxidized metal. Repeat the application, allowing the planter to dry between treatments, until you achieve the desired look.
Accelerating corrosion on a Corten steel bed is a simple process and can be done over multiple applications. This is yet another benefit of using Corten steel in the garden.
Sealing

Once your Corten steel has oxidized to your desired level, either naturally or through acceleration, you can seal the metal to prevent further rusting. There are many suitable sealants on the market, but polyurethane-based sealants work best.
Keep in mind that sealing will darken the bed’s exterior, so it’s important to test before applying it fully. To do this, select a small area of the bed and apply the sealant. Allow it to dry completely, then check the color to ensure it matches your desired look. If you’re satisfied, apply the sealant over the entire exterior of the bed.
Cleaning Corten Stains

If you’ve already placed your Corten steel bed on a concrete surface and stains have started to develop, don’t worry! Before cleaning, test this solution on a small area of the pavement to ensure it produces the desired result.
Pour vinegar, lemon juice, or a mixture of both onto the stain and let it sit for several minutes. Then, scrub the area with a wire brush and rinse away the cleaner. Reapply the solution and repeat as needed until the stain is removed.