How to Make Large Cement Planters? | Build Your Own

Making large cement planters requires two nested molds with a 2-inch gap, a thick mix, and a 24-48 hour cure before demolding.

The method is straightforward: create an outer form for the outside shape and a smaller inner form for the planting cavity, then pour your mix into the space between them.

Whether you want a statement piece for the front porch or something practical for vegetables, the process stays the same. The real work is in the prep—getting the molds right and the mix consistency dialed in.

What You Need for the Molds

The mold setup decides everything about your finished planter. You’re building two boxes: one larger for the outside, one smaller for the cavity. The gap between them becomes your wall thickness.

For a large planter, aim for a 2-inch gap between the outer and inner molds.

• Outer mold: plywood works fine; it holds its shape and handles the weight of wet concrete.
• Inner mold: cut this one into removable halves where possible, since extracting it after curing is far easier.
• Drainage: a 2-inch PVC pipe placed near the bottom of the outer mold creates a clean drain hole.

Before pouring, coat every surface that will touch wet concrete with a release agent—plastic sheeting, dish soap, cooking oil, or vegetable oil all work. Skip this and you’ll fight to free your planter from the mold, and you risk cracking it.

Mixing the Concrete and Pouring

The mix consistency matters more than the exact ratio. You want something like thick peanut butter or pancake batter—workable but stiff enough to hold its shape when you scoop it.

Add water slowly and stop before the mix gets runny. Excess water is the main cause of cracking during curing.

Pour a base layer first, then set the inner mold in place. Fill the perimeter gap in stages, tapping the sides of the outer mold as you go to release air pockets that would leave voids in the finished wall.

Keep the inner mold weighted. Wet concrete is heavy, and an unsecured inner form will float upward as you fill around it, ruining your wall thickness. Sandbags or bricks set inside the inner mold handle this well.

Curing and Demolding

Once filled, cover the whole mold with plastic and let it sit. Longer is better if you can wait—additional curing time builds serious strength.

When you’re ready, carefully remove the outer mold first. If it sticks, a pry bar can help loosen it without damaging the concrete. Then work the inner form free, using the removable halves if you built them that way.

After the molds are off, sand any rough edges and fill small imperfections if you care about a clean look. For outdoor use, apply a concrete sealer once the planter is fully dry. This gives stain and water resistance and keeps the surface from degrading through freeze-thaw cycles.

Drainage and Care for Live Plants

If you’re planting directly in your cement planter, drainage holes are non-negotiable. Waterlogged soil kills roots fast.

A carbide-tipped masonry bit makes clean work of it.

For succulents or ornamentals that prefer drier conditions, add a layer of sand at the bottom of the finished planter before you add soil. And if you’ve sealed the planter, remember that sealer reduces porosity, which changes how fast the soil dries out—you’ll water less often than with unsealed concrete.

Your finished planter deserves a spot that shows it off. If you’re weighing sizes and styles before your next build, our roundup of the best rectangular cement planters covers the top commercial options and their dimensions.

Common Mistakes That Ruin a Planter

• Overwatering the mix. Weakens the concrete and invites cracking during curing.
• Skimping on release agent. The molds will stick, and prying them off can break the planter.
• Letting the inner mold float. Your walls end up uneven or the cavity collapses.
• Pouring too fast without tapping. Trapped air creates voids in the walls.
• Drilling drainage too early. Green concrete chips and cracks; wait the full two days.