Building a planter box from scratch is one of the most accessible woodworking projects a beginning gardener can take on. The design is simple — four sides, a bottom, and drainage holes — and the result is a custom-sized, high-quality planter that outperforms most store-bought options at lower cost per square inch of growing space. This guide walks through the complete process using cedar lumber and basic tools, producing a 24x12x12-inch planter box suitable for herbs, peppers, compact tomatoes, or mixed annuals.
Why Build vs. Buy
Commercial planter boxes frequently use thin boards, mixed-grade lumber with significant sapwood, and fasteners that rust within a season or two. A DIY cedar planter uses materials you select yourself, allows you to match exact dimensions for a specific space, and typically costs less than a comparable retail product of the same quality. Building also makes it easy to scale up — the same techniques that build one 24-inch planter build a 48-inch planter or a series of matching boxes for a garden border.
For context on how this size of planter fits into a broader container garden strategy, the container gardening for small spaces guide discusses how different container sizes suit different planting goals.
Materials and Tools
Lumber
This build uses #1 or clear western red cedar 1x6 boards (actual dimension: 3/4 inch x 5.5 inches). The lumber list for one 24x12x12-inch planter:
- Two boards at 24 inches (long sides, top course)
- Two boards at 24 inches (long sides, bottom course)
- Four boards at 10.5 inches (short sides — two courses of two boards each)
- Three boards at 22.5 inches (bottom slats)
Total lumber needed: approximately 14 linear feet of 1x6 cedar. Buy 16 feet to account for end cuts and any defects.
Hardware
- 1.5-inch stainless steel or hot-dipped galvanized screws (box of 100)
- 2.5-inch stainless steel or hot-dipped galvanized screws (box of 50) for corner assembly
- Exterior-grade construction adhesive (optional but recommended for corner joints)
Tools
- Miter saw or circular saw
- Drill/driver
- Drill bit set (including countersink bit)
- Speed square
- Tape measure and pencil
- Clamps (4 minimum)
- Sandpaper (120-grit for light smoothing of cut ends)
Step-by-Step Assembly
Step 1: Cut the Lumber
Cut all boards to length according to the cutting list above. Use a speed square to confirm cuts are square — out-of-square cuts make joints difficult to close and gaps visible from outside. Sand cut ends lightly with 120-grit sandpaper to remove splinters and fuzz, which speeds up weathering if left rough.
Step 2: Assemble the Long Side Panels
Each long side is two 24-inch boards stacked vertically. Lay both boards face-down on a flat surface, butted together along the long edges. Drill pilot holes and drive 1.5-inch screws every 8 inches along the seam (from the back face) to join them into a single panel. This creates two long-side panels, each 24 inches wide by approximately 11 inches tall.
Step 3: Assemble the Short End Panels
Each short end is two 10.5-inch boards stacked the same way. Join them as above to create two end panels, each approximately 10.5 inches wide and 11 inches tall.
Step 4: Join the Corners
Stand one long-side panel upright and clamp it to your workbench or a helper's hands. Apply a thin bead of exterior construction adhesive to the inside edge of the end panel and clamp it to the end of the long-side panel, flush at the top. The end panel sits inside the long panel — this makes the exterior corners clean. Pre-drill through the long panel face into the edge of the end panel with a countersink bit, then drive 2.5-inch screws. Place two screws per board course (four total per corner). Repeat for all four corners.
Check squareness at each corner before the adhesive sets. A diagonal measurement from corner to corner, checked on both diagonals, should be equal if the box is square. Adjust the clamps if needed.
Step 5: Install the Bottom Slats
Turn the assembled box upside-down. Space three 22.5-inch bottom slats evenly across the opening, leaving a 1/4- to 3/8-inch gap between each slat for drainage. Drill pilot holes and attach each slat to the bottom edge of each long panel with 1.5-inch screws. Do not glue the bottom slats — leaving them screw-mounted only makes them replaceable if they rot first.
Step 6: Drill Drainage Holes
Flip the box upright. With a 1/2-inch drill bit, drill drainage holes through the bottom slats — aim for one hole per 6 to 8 square inches of base area. For a 24x10.5-inch base, drill 8 to 10 holes distributed evenly. Drilling at a slight angle (not perfectly vertical) helps water clear the hole quickly. The planter box drainage guide covers drainage configurations and why hole placement and size matter for different soil depths.
Step 7: Sand and Finish
Go over all external surfaces with 120-grit sandpaper to smooth any rough grain and remove any ink stamps or grading marks from the lumber. Wipe down with a damp cloth and allow to dry. At this point the planter is ready to use — cedar does not need finishing for durability. If desired, apply a single coat of penetrating oil (teak oil or linseed oil) to the exterior surfaces to slow weathering and preserve the color. Do not oil the interior surfaces.
Scaling the Design
The same assembly process scales directly to other sizes. A 48x12x12-inch planter uses the same techniques with longer boards and three equally-spaced corner supports inside the long walls to prevent bowing. For planters over 12 inches tall, add a third course of boards to each side panel and use corner posts (2x2 cedar cut to height) at each corner for additional rigidity.
This approach to modular planter construction pairs well with the principles in the complete raised beds guide — the structural logic is similar, with the main differences being the use of a solid bottom rather than open soil contact and the taller wall relative to the footprint.
Frequently Asked Questions
What type of wood screws should be used for a cedar planter box?
Use stainless steel or hot-dipped galvanized screws only. Standard zinc-plated screws will rust within one to two seasons when exposed to outdoor moisture and cedar's natural acids — the rust stains the wood and weakens the joint. Stainless steel is more expensive but will outlast the wood itself. Hot-dipped galvanized is a cost-effective alternative that performs well in most climates.
Is construction adhesive necessary for a planter box?
No, but it significantly improves joint strength and longevity. Exterior-rated PL Premium or Titebond III are suitable choices. Apply adhesive to one mating surface only, clamp until the adhesive sets (usually 30 to 60 minutes), then drive screws. The screws provide clamping force while the adhesive cures; the adhesive handles shear stress that screws alone resist less effectively.
How do I prevent the bottom boards from rotting before the sides?
The bottom is the most moisture-prone part of any planter box. Two strategies extend bottom life: first, use thick stock (3/4 inch minimum, 1 inch preferred) for the bottom slats — thicker wood rots slower. Second, elevate the planter slightly on feet, blocks, or casters so the bottom is not sitting in standing water. A cheap rubber foot screwed into each corner costs under $5 and can add years to the life of the bottom boards.
Can this planter box be made from pressure-treated lumber?
Yes, modern ACQ pressure-treated lumber (rated for ground contact) can be used for the bottom slats, where rot risk is highest, while using cedar for the visible sides. This hybrid approach maximizes durability in the highest-moisture zone while keeping the exterior appearance natural. Avoid using CCA-treated wood and do not use pressure-treated lumber for any surfaces that will contact edible crops directly.
How long does this build take from start to finish?
With all lumber pre-cut and hardware on hand, the assembly takes 1 to 2 hours for a beginner working alone. Adding the cutting step, a full build from raw lumber to finished planter takes 2 to 3 hours. Building multiple identical planters in a batch is more efficient — cut all pieces for all boxes first, then assemble one after another. A set of four 24-inch planters can be built in a single afternoon.