How to Make a Concrete Planter

Cut your cement-impregnated fabric into roughly 1 foot by 1 foot squares. For a full-size planter you want about 40 squares; a small desk planter needs maybe 10 to 15. Small squares are far easier to wrap around a curved mold than wrestling with one big sheet.

The fabric is a polypropylene-and-cement composite (Sika is the common brand). Plain landscape fabric works in a pinch, but a true cement fabric gives you the strongest finished planter.

Pick a plastic container with the shape you want your planter to take. A round tub gives you a classic flowerpot profile; a square one gives you a planter box. Whatever you choose, soap up the entire inside surface generously with dish soap.

The soap is your release agent. Without it the cement will bond to the plastic and you'll never get the finished planter out cleanly. Coat every inch, including the bottom corners.

Mix plain Portland cement with water to a thick yogurt consistency. No additives, no modifiers - straight cement is enough. Half a bag will do the whole planter.

Take a soaked fabric square and use a putty knife to spread cement on both faces. Coating both sides means each layer laminates to the next, which is where the planter gets its strength. The fabric is so absorbent the cement pulls right through.

Before you start layering fabric into the mold, push a small air fitting (a tire valve or compressor coupling) into a pinched fold of cement-pasted fabric and place it at the bottom inside of the planter. This becomes your demold trick.

Once the planter cures you'll hook a compressor to this valve and blast air between the cured cement and the plastic mold. The pressure pops the planter out cleanly. Skipping this means hours of prying.

Press your first cement-pasted square against the inside of the soaped plastic container. The cement-rich side faces in, against the plastic - the smoother side - so when you demold, that's the surface that shows.

Smooth it flat with gloved fingers. Overlap each new square by a couple of inches so there are no gaps. Wrap the squares up over the rim of the mold so the planter has a finished edge when you flip it out.

Work your way around the inside of the mold, smoothing every seam with your hands. Press from the center of each square outward to drive trapped air toward the edge where it can escape.

You're looking for a fully covered surface with no white fabric showing through, no lifted corners, and no bubbles popping under the cement. Take five extra minutes here - it's the difference between a planter that lasts five winters and one that cracks in the first frost.

Repeat the same paste-and-press process for two more layers. Three layers is the sweet spot for a strong, lightweight planter. Stagger your seams so no two layers crack along the same line.

The layered fabric works exactly like a plaster cast on a broken arm - thin, light, but rigid once it cures. A round planter gets bonus strength because the pressure of the soil pushes equally against every part of the wall.

Take whatever cement you have left in your bucket and paste it across the inside surface of the planter in one final coat. This seals any white fabric showing through, hides the seam lines, and adds a strength layer that catches any spot you missed.

Use the putty knife to push the cement into every fold and corner. Look for tiny air pockets where the cement skin lifts and press them flat. Nothing has to go to waste - leftover cement is exactly what this coat is for.

While the cement is still workable, mix a small batch of white Portland cement and shape a smooth band around the top edge of the planter. White Portland sticks like crazy to the wet base layer and gives you a clean, decorative rim.

Bring the rim inward an inch or so. When you fill the planter with soil later, that lip keeps the dirt from spilling over the very top edge. Smooth the lines with wet fingers until it looks like a single curved band.

Cover the mold loosely with plastic and let the planter cure undisturbed for two full days. Cement hardens slowly, and rushing this step means cracked walls or a stuck planter.

When it's dry, hook your air compressor to the release valve and fire a steady blast of air between the cement and the plastic. The planter lifts straight out. Sand any rough spots, seal the outside with an exterior sealant, drop in a drainage layer and soil, and plant it up.