How to Make Large Concrete Planters | Mold It in a Weekend

Making a large concrete planter costs $100 to $150 in materials and takes a weekend, using two nested plywood molds, a 3:1 sand-to-cement mix, and a 24-hour cure before you remove the forms.

A single large concrete planter from a garden center runs $1,000 or more. Building one yourself with plywood molds and bagged concrete mix saves about 90% and lets you dial in the exact size you need. The process is straightforward: build an outer box, build a slightly smaller inner box, pour concrete between them, wait a day, and pull the forms off.

The key numbers to remember are a 2.5-inch wall thickness for strength, a 3:1 sand-to-cement ratio for the mix, and a full 24-hour cure before you touch the molds. Skip those three and you get cracks, thin walls, or a collapsed form. Hit them and you get a planter that outlives the house.

What You Need to Build the Molds and Mix

Two plywood boxes form the mold system. The outer box gives the planter its outside shape and size; the inner box creates the cavity for soil. Plan for a 2.5-inch gap between them on every side — that gap becomes your planter wall thickness. Houzewize’s large planter guide works through a 24-by-32-inch outer box with a 22-by-30-inch inner box to hit that wall thickness.

• Plywood (¾-inch) — Sturdy enough to hold wet concrete without bulging. Cut four side panels and one bottom panel for each box.
• Vegetable oil spray — Coat the inside of the outer box and the outside of the inner box so the concrete releases cleanly.
• Portland cement and sand — Mix at a 3:1 ratio by volume (three parts sand, one part cement). Add water slowly until the mix feels like peanut butter.
• Plasticizer and polypropylene fiber — Plasticizer (~1% by volume) makes the mix flow easier; fiber (~1–2% by volume) stops hairline cracks from forming as it dries.
• FrogTape® or any painters tape — Seals weep holes in the mold and holds the inner box down if it tries to float.
• 2-inch PVC pipe (~6 inches long) — Creates the drainage hole in the planter bottom. Cap one end with tape so concrete doesn’t get inside.

Step-by-Step: How to Make Large Concrete Planters

The two-source method below blends the best techniques from the Houzewize and Homemade by Carmona guides. Follow this order and you avoid the three biggest mistakes: floating inner forms, air pockets, and watery mix.

Step 1: Build the Outer Mold

Cut four pieces of ¾-inch plywood for the outer box sides — for a 24×32-inch planter, those are 24 inches tall by 32 inches wide. Cut a bottom panel 24×24 inches. Screw the side panels to each other and to the bottom panel, forming a sturdy open-top box. Drill a 2-inch hole in the center of the bottom panel for drainage.

Insert a 2-inch PVC pipe through that hole so it sticks up about 2 inches above the bottom. Cap the top of the pipe with tape or a push-fit cap so concrete can’t fill it during the pour. The gap under the pipe (between the pipe bottom and the ground) lets water drain out later.

Step 2: Build the Inner Mold

The inner box must be 5 inches narrower and 5 inches shorter overall than the outer box. For a 24×32 outer box, cut four side panels at 22 inches tall by 30 inches wide — then cut each one down the middle into two 11-by-32-inch halves. Halving the panels makes them much easier to remove after the concrete sets. Cut a bottom panel at 22×22 inches. Assemble the inner box the same way as the outer one.

Step 3: Lubricate Both Molds

Spray the inside of the outer mold and the outside of the inner mold with vegetable oil spray. A heavy, even coat is the difference between a clean release and a stuck mold that destroys the planter edges. Pay special attention to the corners.

Step 4: Mix the Concrete to Peanut Butter Consistency

Mix one part Portland cement with three parts sand in a wheelbarrow or plastic mixing tub. Add plasticizer and polypropylene fiber per the package instructions — these are optional but dramatically improve surface finish and crack resistance. Add water slowly while mixing until the concrete holds a ball shape in your hand without slumping. It should be thick but spreadable. Too watery is the single most common cause of cracked planters.

Wear an N95 mask while mixing and sifting — concrete dust contains silica that damages lungs.

Step 5: Pour the Base Layer

Pour a 2-inch layer of concrete into the bottom of the outer mold. Spread it evenly so the bottom of your finished planter will sit flat. If you want a thicker base for extra weight (recommended for tall planters), go up to 4 inches — just make sure the inner mold still has enough room to sit above the base when you set it in.

Step 6: Insert the Inner Mold and Space It Evenly

Lower the inner box into the outer box, centered. The gap between the two boxes should be 2.5 inches on every side. Screw 2.5-inch-wide spacer blocks (cut from scrap plywood) to the top edges of both boxes to keep the gap uniform. The inner box should sit about 1 inch above the base concrete layer — that 1-inch gap will form the planter’s floor thickness.

Step 7: Fill the Gap and Compact

Pour concrete into the gap between the two boxes. Work your way around, filling in layers. After every few shovelfuls, tap the sides of the outer box firmly with a rubber mallet — this settles the concrete and pushes air bubbles to the surface. Tapping is not optional; skip it and you get pockmarked walls. Homemade by Carmona’s planters tutorial recommends tapping until the surface looks smooth and no new bubbles rise.

As you fill, the inner box will try to float upward because concrete is heavier than air. Tape it down with FrogTape or painters tape stretched across the top of both boxes to keep it from rising.

Step 8: Smooth the Top Edge

Once the gap is full, run a straight piece of wood or a trowel across the top to level the concrete flush with the mold edges. This forms the rim of your planter. Smooth it like icing on a cake — any unevenness now will be permanent.

Step 9: Cure for 24 Hours (Minimum)

Leave the whole assembly undisturbed for at least 24 hours. Cover the top with plastic sheeting if you’re pouring outdoors in hot, dry weather — slow curing prevents surface cracks. The concrete will be hard enough to demold after one day but won’t reach full strength for about a week. Place the planter where you want it permanently before you fill it with soil; moving a large concrete planter after it’s full takes two strong people and a dolly.

Step 10: Demold and Finish

Unscrew and remove the outer box panels. If they stick, tap the plywood with a mallet — the vegetable oil should let it release cleanly. For the inner box, remove the spacer blocks and gently pry the half-panels loose with a flat screwdriver and mallet. Once all mold pieces are off, sand any sharp edges with 80-grit sandpaper or an orbital sander. Apply a concrete sealer (outdoor matte sealant if the planter lives uncovered; sealant is optional on covered porches).

Common Mistakes That Ruin Large Concrete Planters

Too much water in the mix. The mix should hold a ball shape, not drip. Excess water evaporates and leaves cavities, weak spots, and a chalky surface. Add water in small increments and stop when the texture looks like thick peanut butter.

Skipping the concrete sift. Bagged concrete often contains pebbles and clumps. Sift it through a ¼-inch hardware cloth before mixing (wear your mask). Unsifted concrete leaves rough patches on the planter surface that are hard to sand out.

Not securing the inner mold. The inner box floats upward as concrete fills the gap. Tape it to the outer box across the top. A floating inner mold can tilt, creating a wall that is paper-thin on one side and 4 inches thick on the other.

Prying the molds too early. Wait the full 24 hours. Concrete that feels hard at 12 hours is still brittle. Removing molds early snaps thin edges and corners off the planter.

Dirty or rocky sand. Sand for concrete should be clean masonry or concrete sand, not play sand or fill sand. Rocks and organic debris in the sand show up as blemishes on the finished surface and create weak points.

Cost Breakdown: DIY vs. Store-Bought Large Planters

Building your own makes financial sense for any planter over 18 inches tall. A 24×32-inch DIY planter costs about $120 in materials. A comparable concrete planter from a garden center or online retailer runs $800 to $1,500 plus shipping. The savings come entirely from labor and markup — the concrete itself costs about $30 for a 60-pound bag.

If you plan to buy your first large planter instead of mixing concrete, see our roundup of the best large concrete planters for tested options across sizes and styles.