How to Build a Cedar Planter Box That Lasts 15 Years

Why Cedar Outlasts Every Other Material

Western red cedar (Thuja plicata) contains natural oils called thujaplicins that act as fungicides and insect repellents. These oils are concentrated in the heartwood, the dense inner portion of the tree. When you buy cedar lumber, look for boards marked "heartwood" or "H" grade. Sapwood, the lighter-colored outer rings, lacks these oils and rots at the same rate as pine.

Cedar also has a low shrinkage factor of 2.4 percent across the grain, compared to 5.1 percent for pine and 7.8 percent for oak. Low shrinkage means the planter box joints stay tight through seasonal moisture cycles. A pine box that is snug in April develops 1/8-inch gaps at the corners by September as the wood dries and shrinks. Cedar moves minimally, so the box remains structurally sound without re-tightening hardware.

The two grades of cedar available at most lumber yards are clear cedar (knot-free, $6 to $9 per board foot) and common cedar (contains tight knots, $3 to $5 per board foot). For a planter box, common cedar is sufficient. Tight knots add character and do not compromise structural integrity in this application. Reserve clear cedar for furniture projects where appearance is the primary concern.

Complete Cut List and Material Breakdown

This planter box measures 48 inches long by 18 inches wide by 18 inches tall (exterior dimensions). The interior soil depth is 16 inches, which provides adequate root space for most vegetables, herbs, and annual flowers. Adjust the length in 6-inch increments by adding or subtracting side boards; the width and height remain fixed for structural stability.

Lumber

You need seven boards of 1x6 western red cedar at 6 feet long (actual dimensions: 3/4 inch thick by 5.5 inches wide). Two boards form the long sides (cut to 48 inches each), two boards form the short ends (cut to 16.5 inches each), and three boards form the bottom (cut to 16.5 inches each). At $3.50 to $5 per board, the lumber total is $25 to $35.

For the four corner posts that reinforce the joints, buy one 2x2 cedar board at 4 feet long (actual dimensions: 1.5 x 1.5 inches). Cut it into four 16.5-inch pieces. A 4-foot 2x2 cedar costs $4 to $6.

For the bottom support cleats that hold the bottom boards, buy one 1x2 cedar board at 4 feet long. Cut it into four 14-inch pieces. A 1x2 cedar costs $2 to $3.

Hardware and Supplies

Use exterior-grade deck screws, not drywall screws. Deck screws have a corrosion-resistant coating that withstands constant moisture exposure. Grip-Rite or DeckMate 2-inch exterior deck screws cost $8 to $12 per box of 100. You will need approximately 40 screws for this project. For the bottom boards, use 1-5/8-inch deck screws to avoid poking through the 3/4-inch side boards.

A roll of 6-mil polyethylene sheeting ($10 for a 10x25-foot roll) lines the interior to slow soil-to-wood moisture transfer. A box of 1/2-inch galvanized roofing nails ($5) attaches the liner. A 1/2-inch drill bit ($3) bores drainage holes through the bottom boards. A tube of Titebond III wood glue ($7) reinforces the corner joints.

Total Cost

Lumber: $31 to $44. Hardware and supplies: $33 to $37. Total: $64 to $81. If you already own a drill, saw, and drill bits, subtract $15 to $20 for those items. The realistic cost for someone with basic tools is $45 to $65.

Tools Required

A cordless drill/driver is the only power tool required. A DeWalt 20V MAX drill/driver ($99 with one battery) or a Milwaukee M18 ($129) handles every screw in this project. A circular saw speeds up the cutting, but a hand saw ($15) works if you do not own one. A jigsaw ($40) is useful for cutting the polyethylene liner but is not strictly necessary; a utility knife cuts 6-mil plastic cleanly.

Additional hand tools: a tape measure, a speed square ($8) for marking 90-degree cuts, a pencil, clamps (two 12-inch Irwin Quick-Grip clamps at $12 each hold boards during assembly), and a 1/2-inch spade bit or hole saw for drainage holes. Sandpaper (120-grit) smooths the cut edges. Safety glasses and work gloves complete the list.

Cutting the Lumber: Exact Dimensions

Measure twice and cut once. Write each measurement on the board with a pencil before cutting. Cedar is soft enough that even a dull blade cuts it, but a sharp blade produces cleaner edges with less tear-out.

Side Boards

Cut two 1x6 boards to exactly 48 inches. These form the long sides of the planter. Cut two more 1x6 boards to exactly 16.5 inches. These form the short ends. The short end measurement accounts for the 3/4-inch thickness of the two long side boards: 18 inches minus 1.5 inches equals 16.5 inches. This calculation ensures the short ends fit flush inside the long sides.

Bottom Boards

Cut three 1x6 boards to 16.5 inches. These span the width of the planter and sit on the support cleats. Three boards at 5.5 inches wide each cover 16.5 inches, which matches the interior width exactly. The bottom boards rest on the cleats and are screwed through the side boards into the cleat ends.

Corner Posts

Cut the 2x2 into four pieces at 16.5 inches each. The corner posts sit inside the planter at each corner, flush with the bottom edge and extending 16.5 inches upward. The top edge of the corner posts sits 1.5 inches below the top edge of the side boards (the 1x6 boards are 5.5 inches tall, and two stacked boards equal 11 inches; the corner posts extend from the bottom to 16.5 inches, which is 1.5 inches below the 18-inch total height). This positioning keeps the posts hidden inside the planter.

Support Cleats

Cut the 1x2 into four pieces at 14 inches each. These cleats screw to the inside face of the long side boards, 1 inch up from the bottom edge, and support the bottom boards. Position two cleats per long side, spaced evenly: one cleat 4 inches from each end.

Assembly: Building the Box

Assembly takes 45 minutes to 1 hour. Work on a flat surface, preferably a concrete garage floor or a workbench. Uneven ground causes the box to rack (twist out of square), which makes the bottom boards fit poorly.

Step 1: Attach the Corner Posts

Lay one long side board (48 inches) flat on your work surface. Place two corner posts at each end, flush with the bottom edge of the board. The posts should be positioned 3/4 inch in from the end of the board (the thickness of the short end board). Apply a bead of Titebond III wood glue along the face of each post where it contacts the side board. Clamp the posts in place with your 12-inch clamps. Drive two 2-inch deck screws through the outside face of the side board into each corner post, positioned 1 inch from the top and bottom edges. Repeat for the second long side board.

Step 2: Attach the Short Ends

Stand the two long side assemblies upright, with the corner posts facing inward. Position a short end board (16.5 inches) between the corner posts at one end. The short end board should be flush with the top and bottom edges of the long side boards. Apply wood glue to the edges of the corner posts where they contact the short end board. Drive two 2-inch deck screws through each long side board into the ends of the short end board, positioned 1 inch from the top and bottom edges. Repeat for the second short end. Check the box for squareness by measuring diagonally from corner to corner. Both diagonal measurements should be within 1/16 inch of each other. If they are not, push or pull the box until the measurements match, then drive the remaining screws.

Step 3: Install the Support Cleats

Mark the inside face of each long side board 1 inch up from the bottom edge. Screw the 14-inch cleats to these marks using 1-5/8-inch deck screws, two screws per cleat end. Space the cleats evenly: one pair 4 inches from the left end, one pair 4 inches from the right end. The cleats should be flush with the bottom face of the long side boards when viewed from inside the box.

Step 4: Install the Bottom Boards

Place the three bottom boards (16.5 inches each) across the cleats. They should fit snugly between the two long side boards. Leave a 1/4-inch gap between each bottom board for drainage. This gap is critical; without it, water pools on the bottom and accelerates rot. Drive 1-5/8-inch deck screws through the long side boards into the ends of each bottom board, two screws per end. Do not screw through the bottom boards into the cleats; the boards need to expand and contract with moisture, and fixed screws would cause splitting.

Step 5: Drill Drainage Holes

Drill five 1/2-inch drainage holes through each bottom board, spaced evenly along its length. A total of 15 holes provides adequate drainage for a 48x18-inch planter. Without drainage holes, soil becomes waterlogged within hours of rain or watering, and roots suffocate and rot. Position the holes at least 1 inch from the board edges to maintain strength.

Lining the Box and Finishing Touches

The polyethylene liner serves a specific purpose: it separates the soil from the wood, slowing moisture transfer that causes rot. The liner is not waterproof; water must still drain through the bottom. The liner covers the four interior sides and the bottom, with holes punched through it to align with the drainage holes.

Installing the Liner

Cut a piece of 6-mil polyethylene sheeting 50 inches long by 40 inches wide. This gives you 1 inch of overlap at the top edges on all sides. Press the liner into the box, folding the corners like wrapping paper. Use a utility knife to trim the excess at the top edge so the liner sits 1 inch below the rim. Secure the liner to the inside face of the side boards with galvanized roofing nails, spaced every 6 inches along the top edge. Punch holes through the liner with a pencil to align with each of the 15 drainage holes in the bottom boards.

To Finish or Not to Finish

Untreated cedar weathers to a silver-gray patina within 12 to 18 months of outdoor exposure. This is a cosmetic change only and does not affect the wood's structural integrity or rot resistance. If you prefer the original amber color, apply a penetrating exterior oil finish such as Penofin Brazilian Rosewood Oil ($32 per quart) or Messmer's UV Plus ($28 per quart). These oils penetrate the wood fibers and contain UV blockers that slow the graying process. Apply one coat with a brush or rag, let it soak in for 20 minutes, and wipe off excess. Reapply annually. Do not use film-forming finishes such as polyurethane or varnish on outdoor cedar; they peel and flake within two years as the wood expands and contracts with moisture changes.

Soil Volume and Planting Guidelines

The interior dimensions of the finished planter are approximately 46.5 inches long by 16.5 inches wide by 16 inches deep, yielding a soil volume of roughly 6.5 cubic feet. A 2-cubic-foot bag of Miracle-Gro Potting Mix costs $12 to $15, so filling this planter requires three to four bags, or $36 to $60 in soil.

Do not fill the planter with soil dug from your yard. Yard soil compacts into a dense mass in a container, blocking drainage and suffocating roots. Use a bagged potting mix that contains perlite or vermiculite for drainage, peat moss or coco coir for moisture retention, and a slow-release fertilizer. For vegetable gardening, mix in 2 cups of Espoma Garden-tone ($8 for a 4-pound bag) per cubic foot of soil before planting.

Fill the planter to within 1 inch of the rim. This 1-inch gap prevents water from overflowing when you water heavily. For tomatoes, space plants 18 inches apart in a single row down the center of the planter; four plants fit comfortably in 48 inches. For herbs, plant in a grid pattern with 6 to 8 inches between plants. For flowers, follow the spacing on the plant tags, which varies by species from 4 inches for alyssum to 18 inches for zinnias.

Maintenance Schedule

Cedar planter boxes require minimal maintenance. Inspect the joints and bottom boards once per year in early spring before planting. Tighten any screws that have backed out due to wood movement. Replace the polyethylene liner every 3 to 4 years; it becomes brittle and cracks from UV exposure. A new liner costs $5 in materials and takes 15 minutes to install.

If you applied an oil finish, reapply once per year in late spring after the wood has dried from winter moisture. Sand the surface lightly with 120-grit sandpaper before reapplying to remove any loose fibers. If you left the cedar unfinished, no maintenance is required. The silver-gray patina is the natural weathered state of the wood and provides the same rot resistance as fresh cedar.

At the 10-year mark, inspect the bottom boards for soft spots by pressing firmly with a screwdriver. Cedar that has softened indicates the beginning of rot. Replace individual bottom boards rather than the entire box. The side boards typically outlast the bottom boards by 5 to 8 years because the bottom boards sit in constant contact with wet soil.

Key Takeaways

Western red cedar is the only material that combines rot resistance, chemical-free safety for edible plants, and a 15-year lifespan without maintenance. The critical construction details that determine longevity are: 1/4-inch gaps between bottom boards for drainage, a polyethylene liner that separates soil from wood, corner posts that reinforce the joints against racking, and exterior deck screws rather than drywall screws that rust and fail. Total material cost is $45 to $70, and assembly takes under 2 hours with basic tools. The resulting planter holds 6.5 cubic feet of soil, enough for a productive vegetable or herb garden in a compact footprint.