How to Make a Concrete Planter Box | Molds, Mix, & Cure Tips

A concrete planter box is made by pouring stiff concrete between two nested molds with a 2-inch gap, then curing for four days before demolding and sealing for outdoor use.

A concrete planter box lasts for decades and doesn’t blow over in wind. The trick is getting the mold right and the mix stiff enough to hold its shape without cracking. Here’s the exact process for building one that won’t fall apart after a season of rain and frost.

Building the Two Molds: Outer Box and Inner Box

The outer mold determines the final size of your planter. The inner mold creates the hollow center where soil goes. Build the inner box 2 inches shorter in both length and width than the outer box — that 2-inch gap on each side becomes the wall thickness of the concrete.

Plywood works best for rigid forms; heavy cardboard is a cheaper option that tears away easily at demold time. Whatever material you choose, seal every seam and corner with 100% silicone caulk. Non-silicone caulk won’t flex and will let water in, ruining the mold’s shape. For drainage, cut a section of 2-inch PVC pipe and install it through the bottom of the outer mold, positioned 2 inches above the base, with the top capped to keep concrete out of the pipe during pouring.

Wear a safety mask and gloves when cutting dry concrete or sanding — the fine silica dust is dangerous to inhale.

Mixing Concrete and Preparing the Molds

Mix your concrete to a stiff, workable consistency — think of wet sand or peanut butter. Too much water weakens the mix and causes cracking during the drying phase. Mix small batches if you’re working alone; the concrete needs to stay consistent across the whole pour.

Before pouring, spray the inside surfaces of the outer mold and the outside surfaces of the inner mold with vegetable oil spray, or line them with plastic wrap. This keeps the concrete from bonding to the wood or cardboard and makes demolding a 5-minute job instead of a fight.

Pouring and Tapping: Getting a Solid, Bubble-Free Box

Pour a 1- to 2-inch layer of concrete into the bottom of the outer mold first. Then place the inner box into that base layer and immediately fill it with rocks, sand, or any heavy material — the weight presses the inner form down and stops it from floating up when you add concrete around the sides. Tape across the top to the outer rim if the inner box still tries to rise.

Slowly pour concrete around the inner box, working your way around until the mix reaches the top edge of the outer form. While pouring, continuously tap the sides of the outer mold with a rubber mallet or hammer — this vibration releases trapped air bubbles and compacts the concrete, giving you a smooth, dense wall with no weak spots.

Once filled, cover the entire assembly with plastic sheeting to trap moisture. Concrete needs humidity to cure properly; dry air causes surface cracking.

Curing, Demolding, and Finishing

Let the planter cure for at least 4 days, and do not attempt to demold before 2 days. Removing the molds too early leaves you with brittle, easily broken concrete. A temperate environment (around 60-75°F) is ideal.

To demold, tear the cardboard corners or unscrew the plywood forms. If the inner box is stuck, use a flat screwdriver and mallet to pry it loose from the inside. Once the molds are off, let the planter sit for another day before handling. Smooth rough edges with a 220-grit sanding block or a diamond polishing pad. For outdoor placement, always seal the concrete with a suitable outdoor sealer to protect against moisture, frost damage, and mildew growth on porous surfaces.

If you’re ready to buy rather than build, check out our roundup of tested best concrete planter boxes available now — these pick up where DIY leaves off.

Common Mistakes to Avoid

• Skipping the weights inside the inner box. Without enough weight, the inner mold floats upward during the pour, giving you uneven walls and a reduced soil cavity.
• Using non-silicone caulk on mold seams. Regular caulk won’t seal against wet concrete, and leaking slurry ruins the mold shape.
• Demolding before 2 days. Green concrete is fragile; pulling the forms too early causes corner breaks and hairline cracks that become failure points later.
• Not tapping the mold sides. Air pockets left in the walls create weak spots that crack during freeze-thaw cycles.