What's inside
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This intermediate planter box has four corner posts made from mitered boards, giving each corner a clean, furniture-like look instead of exposing the end grain of a single square post. The finished box is about 30 inches long, 18 inches wide, and 18 inches tall. It is large enough for herbs, flowers, or a compact vegetable planting, but still manageable to move before it is filled.
Contents
Materials and design choices
Use exterior-rated wood throughout. Cedar is the best balance of durability, weight, and workability. Ground-contact pressure-treated lumber lasts longer, but it is heavier and more likely to move as it dries. Do not use ordinary indoor pine unless you are treating the box as a short-term project.
| Material | Advantages | Drawbacks | Best use |
|---|---|---|---|
| Cedar | Light, naturally rot resistant, easy to cut | Soft; dents and costs more than pine | Most residential planters |
| Pressure-treated lumber | Durable and widely available | Heavy, wet when new, can twist while drying | Large planters or damp locations |
| Exterior pine | Cheapest option | Shorter service life and more checking | Protected areas or temporary planters |
For this plan, buy four 1×8 boards at least 6 feet long for the side panels, two 1×4 boards for the bottom cleats and legs, and one 2×2 or 2×3 board for internal supports. Actual 1×8 boards measure about 3/4 inch by 7-1/4 inches. You will also need exterior screws, waterproof wood glue, landscape fabric, and a non-toxic exterior finish if you want to preserve the color. A cedar fence board selection can be economical, but check every board for splits and inconsistent thickness before buying.
Cut list and dimensions
Cut the long side panels to 30 inches and the end panels to 16-1/2 inches. That end length fits between the long panels, producing an outside width of approximately 18 inches. Cut eight corner-post strips from 1×4 stock, each 18 inches long. Rip or use the existing edges so every strip is the same width, ideally about 3-1/2 inches.
Each corner post is made by joining two strips along a 45-degree miter. The visible outside corner becomes a crisp 90-degree point. Also cut four bottom cleats at 26 inches and two or three cross supports at about 16-1/2 inches. These supports carry the soil load and keep the bottom from sagging.
Measure your actual boards before cutting. A nominal 1×8 can vary enough to change the end-panel length. If your side boards are not exactly 7-1/4 inches wide, assemble the corner posts first, then set the final box height from the real parts.
Build the mitered corner posts
Set the saw to 45 degrees and make a test cut on scrap. The two beveled faces must meet without a visible gap. A small gap at the outside point is usually caused by an inaccurate saw setting, a bowed board, or pushing the stock sideways during the cut.
Apply a thin coat of exterior wood glue to one miter and clamp the two strips around a straight scrap block or a square piece of plywood. The glue provides alignment and stiffness; screws installed later do most of the structural work. If you want faster assembly, use right-angle woodworking clamps, but verify that the joint remains square before tightening them.
Drive two 1-1/4-inch exterior screws from the inside face of each post into the adjoining strip. Predrill first, especially in cedar near the mitered tip. Keep the screws at least 3/4 inch from the ends. Wipe away glue squeeze-out; hardened glue prevents finish from soaking into the wood.
Assemble the box
Lay two corner posts on a flat surface and position the long panels between them. The top edges of the panels should align with the top of the posts. Drill clearance holes through the panels, then drive two 1-5/8-inch exterior screws into each post at each joint. Repeat with the opposite side and the two end panels.
Check the box for square by measuring both diagonals. If one diagonal is longer, push the longer corners inward until both measurements match. A difference of more than 1/8 inch will make the bottom supports fit poorly and can leave the liner under tension.
Do not depend on glue alone for these joints. Seasonal movement, wet soil, and the leverage created by the corner posts can break a glued edge. Screws should clamp the panels to the posts, while glue mainly seals and reduces movement.
Install the bottom and legs
Turn the box upside down and attach the four bottom cleats inside the lower edge, using 1-1/4-inch screws. Set each cleat about 3/4 inch above the bottom so the bottom boards sit slightly recessed. Install the cross supports between the long cleats. Space them no more than 8 inches apart; a wider span allows wet soil to bow the bottom.
Cut bottom boards to about 16-1/2 inches and lay them across the cleats with 1/8-inch gaps for drainage. Drill several 3/8-inch drainage holes through each board. Add four short feet or blocks underneath the corners to lift the planter at least 1/2 inch from a deck or patio. Without clearance, water stays trapped beneath the box and accelerates rot.
For a box this size, exterior wood screws with a corrosion-resistant coating are sufficient. Stainless steel screws are better near salt air or constantly wet locations, but they cost more and are not necessary for a sheltered backyard planter.
Line, finish, and fill
Sand sharp edges with 120-grit paper, paying special attention to the pointed miter corners. Do not aggressively round them; a small eased edge is enough to prevent splinters while preserving the design.
Staple landscape fabric or a breathable planter liner to the inside walls, leaving the bottom open over the drainage holes. Avoid plastic sheeting unless you add many drainage openings; a sealed liner turns the box into a tub and keeps the soil saturated. A raised-bed landscape fabric liner is the cheaper, more forgiving choice.
Apply two thin coats of an exterior penetrating oil or water-based outdoor finish to the outside surfaces, following the label’s drying times. Leave the inside unfinished if you are concerned about coating contact with soil. Put the planter in its final location before filling it; wet soil can make it weigh more than 200 pounds. Use a quality potting or raised-bed mix rather than garden soil, which compacts and can hold too much water in a container.
Seasonal maintenance
Inspect the miters and bottom edges each spring. Tighten loose screws, clear blocked drainage holes, and touch up the finish before the wood becomes weathered. Expect cedar to turn gray if left unfinished; that is a cosmetic change, not immediate structural failure. If a corner opens, remove nearby soil, add a replacement screw after predrilling, and reseal the joint. The most common cause of early failure is not the miter—it is standing water trapped under the planter.