What's inside
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A wooden porch planter box looks simple, but the drainage is where most builds fail. A box that holds wet soil against its bottom will eventually rot, stain the porch, or become too heavy to move. This design uses a false bottom and a concealed drain outlet, so water leaves through the back or underside without a row of obvious holes in the front.
Contents
Project overview
The finished planter is approximately 36 inches long, 12 inches wide, and 14 inches tall. That size is large enough for seasonal flowers, herbs, or a small ornamental shrub, but still manageable when empty. Expect roughly four to six hours of work, including finish drying time. Cutting and assembly are suitable for a beginner who is comfortable using a drill and saw.
| Material | Best use | Trade-off |
|---|---|---|
| Cedar | Outdoor boxes with a natural finish | Costs more, but resists rot well |
| Pressure-treated lumber | Lowest-cost structural box | Can be wet, twisted, and difficult to finish cleanly |
| Exterior plywood | Hidden liners or internal panels | Edges must be sealed carefully |
| Redwood | Lightweight, rot-resistant construction | Usually pricier and softer than cedar |
For most porch projects, 1-inch-thick cedar boards are the sensible choice. Buy straight boards with tight knots. Avoid boards with end splits or soft patches. If you choose pressure-treated wood, let it dry before applying paint or stain; fresh boards may hold finish poorly.
Materials and tools
You will need enough 1×12 cedar or similar boards for the front, back, and ends; a 36-by-12-inch piece of exterior plywood for the false bottom; and four 1-by-1-inch cleats. Add exterior screws, waterproof wood glue, exterior caulk, and an outdoor finish. Use stainless steel exterior wood screws if the box will stay damp or you are using cedar. Coated deck screws are cheaper and acceptable for a painted pressure-treated box.
For drainage, gather a short length of 1-inch flexible tubing or PVC pipe, a 1-inch hole saw, and a small amount of pea gravel or lightweight drainage material. A landscape fabric roll keeps potting mix from washing into the drain cavity. You will also need a tape measure, square, drill, saw, clamps, sandpaper, and a caulking gun.
Cut the parts
Cut two long boards at 36 inches for the front and back. Cut two end boards at 10 1/2 inches if the long boards sit over the ends. This produces an outside width close to 12 inches when using 1-inch-thick stock. Cut the false bottom to about 34 by 10 inches, then test-fit it inside the box. Exact dimensions depend on the actual board thickness, which is often less than the stated size.
Cut four cleats about 10 inches long. Two support the false bottom near the front and back, and two shorter cleats can support the ends. Position the false bottom 2 1/2 to 3 inches above the real bottom. This creates a drainage chamber while leaving enough soil depth for most annuals and herbs.
Assemble the box
Sand the inside faces and break sharp outside edges before assembly. Dry-fit the four walls on a flat surface. Apply a thin bead of waterproof glue to each joint, drill pilot holes, and drive two screws into each corner. Pilot holes matter in cedar because the ends split easily, especially near the edge.
Check the box for square by measuring both diagonals. If the measurements differ, push the longer diagonal inward until they match, then clamp the box. Install the cleats with their top edges level and 2 1/2 to 3 inches above the bottom edge. Use screws from the outside when possible so the interior remains smooth.
Install the hidden drainage
Drill a 1-inch hole through the back wall, just above the real bottom and below the false bottom. Slope the hole slightly downward toward the outside. Insert a short tube through the hole, leaving its outside end flush or projecting no more than 1/2 inch. Seal around it with exterior caulk. The tube prevents water from sitting against the wood and gives the outlet a defined path.
Before installing the false bottom, drill several 3/4-inch holes through it. Space them about 6 inches apart. Cover the top of the false bottom with landscape fabric, leaving the drain openings exposed. Set the panel on the cleats and fasten it lightly with a few screws. Do not glue it in place; removable panels make future cleaning possible.
For extra protection, brush a wood preservative or exterior finish onto every interior surface, especially end grain and the underside of the false bottom. Keep finish away from surfaces that will directly contact edible plants unless the product is specifically labeled for that use. A clear exterior wood sealer is adequate for a natural-looking box, while exterior paint gives better protection if the wood is inexpensive or less rot-resistant.
Finish and fill the planter
Sand the exterior to 120 grit, then remove dust. Apply two thin coats of finish rather than one heavy coat. Pay particular attention to the bottom edge, corner joints, and the area around the drain tube. Leave the box off the porch until the finish is dry and no longer smells strongly.
Set the planter on four rubber feet, plastic furniture glides, or short cedar strips. Keep at least 1/2 inch of air space under the box. Without that gap, water can remain trapped between the planter and porch surface even if the hidden drain works correctly.
Fill with quality potting mix rather than garden soil, which is too dense for a container this size. Do not add a thick gravel layer above the false bottom; it consumes soil depth without fixing poor drainage. Water slowly and check that water exits the rear tube. If it does not, clear the tube before adding plants.
Seasonal maintenance and failure checks
In spring, inspect the drain opening for mulch, roots, and insects. During summer, lift or shift the box occasionally to check for trapped moisture underneath. In autumn, remove dead plant material so it does not clog the drainage holes. In climates with freezing winters, reduce watering and move the box under cover if possible. Saturated soil expands when frozen and can split the walls or false bottom.
The most common failure is a blocked outlet. The second is a false bottom installed too low, leaving too little soil for roots. A third is fastening directly into end grain without pilot holes, which leads to loose joints and cracks. If the box starts staining the porch, empty it, let the wood dry, and inspect the bottom before adding more sealant. Sealant cannot compensate for standing water or a missing air gap.