The Critical Rule: Drainage
The single most important factor is providing excellent drainage. The inside of the flower box must be designed so that water cannot pool and remain in constant contact with the steel.
Why? Weathering steel requires cyclic wetting and drying to form its protective patina. If the soil is constantly wet and the interior is always moist, the steel cannot dry. This prevents the stable patina from forming and leads to continuous, faster corrosion that could eventually perforate the planter.
How to Ensure It:
Drainage Holes: Provide several large drainage holes at the bottom. These are non-negotiable.
Drainage Layer: Use a deep layer of gravel, hydro granules, or broken pottery at the bottom of the planter above the drainage holes to prevent them from clogging with soil.
Avoid Waterlogging: Use well-draining soil and avoid overwatering.
Potential Reasons You Might Consider a Liner (Optional)
While not necessary for the structural integrity of the box itself, you might consider a separate liner for horticultural reasons:
Soil pH: The initial runoff from weathering steel is slightly acidic. Some plants that prefer alkaline soil might be sensitive to this in the first few years as the patina stabilizes. A liner eliminates this concern.
Containment: A plastic or composite liner makes it easier to remove and replace plants or soil.
Heat: Steel planters can get hot in the sun, which can warm the soil and affect moisture levels. A liner can provide a small amount of insulation.
Important: If you use a liner, it must be removable to allow the interior of the steel to drain and breathe. Do not permanently attach a waterproof membrane or coating to the inside, as this would trap moisture between the liner and the steel, guaranteeing hidden corrosion.
Summary:
Necessary: Excellent drainage is the mandatory form of "internal protection."
Not Necessary: Painting, tar coatings, or permanent waterproof liners on the interior. These defeat the purpose of using a maintenance-free material.
Optional: A removable plastic or composite liner can be used for plant health and practical gardening reasons, but it is not required to protect the steel itself.
By simply ensuring water can drain freely, the interior of the Q355NH planter will develop a protective patina just like the exterior, resulting in a long-lasting, maintenance-free structure.
