How To Build A Cedar Planter Box For Your Patio Or Deck

Building your own cedar planter box is a rewarding woodworking project that adds natural beauty to any outdoor space. Western red cedar is the ideal timber for this task because it is lightweight, naturally rot-resistant, and carries a pleasant aromatic scent that deters insects. Whether you place it on a deck, patio, or front porch, a well-constructed planter becomes a focal point that showcases your favorite flowers and trailing vines. This project, based on the approach demonstrated by This Old House general contractor Tom Silva, produces a freestanding planter that looks like a piece of furniture rather than a simple garden box. The design emphasizes durability and drainage, two factors that determine how many seasons your planter will survive outdoors. For a related project that mounts to your windowsill, see our guide on how to Build A Cedar Window Planter Box For Your Home Diy Sizing Assembly And Mounting.

Understanding The Design And Materials

Tom Silva designed this planter with four tapered legs that elevate the box off the ground, allowing air to circulate underneath so the deck or patio stays dry and cleanable. The interior uses two milk crates lined with fabric grow bags to hold the potting mix, creating an air gap between the damp soil and the cedar boards. This clever separation dramatically extends the lifespan of the wood because direct soil contact is the main cause of rot in outdoor planters. Water drains freely through the grow bags and crates, landing on the ground rather than pooling against the cedar.

The project calls for approximately 32 feet of 5/4×6 western red cedar for the rails, top, and stringers; 10 feet of 4×4 western red cedar for the legs and crosspieces; and 16 feet of 1×4 western red cedar for the side panels. When selecting lumber at the home center, choose boards that are straight, free of large knots, and have tight grain patterns. Cedar with visible checking or warping will only worsen after exposure to sun and rain. All fasteners must be stainless steel to prevent the black staining that occurs when regular steel reacts with tannins in the cedar. You will need 2.5-inch stainless steel deck screws, 1.25-inch stainless steel wood screws, and stainless steel pocket screws. For inspiration on choosing the right plants and arranging them, read about Designing A Stunning Container Garden Essential Tips For Pots Planters And Outdoor Displays.

Cutting And Preparing The Cedar Pieces

Start by cutting all pieces to length using a miter saw. The legs require special attention because they are ripped from 4×4 stock down to 2.25 inches square, and then tapered on the inward-facing sides. Silva uses a tapering jig on the table saw to cut a 2.75-degree taper starting 8 inches from the bottom of each leg. This subtle taper gives the planter its furniture-like appearance and prevents the legs from looking blocky. Once the taper cuts are complete, smooth the edges with sandpaper and round over the outer edges using a router with a 1-inch-radius round-over bit. Take your time with the router because a clean, consistent profile makes the finished planter look professional. A block plane can help make a slight chamfer on the long outside edges of the panel pieces for a refined detail. If you are looking for a different planter configuration for railings, check out this guide on How To Make A Deck Rail Planter from Bob Vila.

For the side panels, mount a 3/8-inch stacked dado blade on the table saw and set the blade height to 3/8 inch. Cut a full-length dado groove along one edge of each rail, offset about 1/8 inch from the centerline. Then cut 3/8 by 3/8 inch rabbets on both ends of every panel piece. When assembled correctly, this offset creates a clean 1/4-inch reveal between the panel pieces and the rails on the outside faces of the planter. Test the fit on a scrap piece before cutting your actual lumber. A dado that is too tight will force the panels out of square, while one that is too loose will leave unsightly gaps.

Assembling The Legs And Rails

Make the top rails from 5/4×6 cedar ripped to 2.75 inches wide, and the bottom rails ripped to 5 inches wide. Use the dado blade to cut grooves along one edge of each rail where the panel pieces will seat. Drill evenly spaced pocket holes through the inside faces at both ends of each rail. The pocket holes should be spaced about 4 inches apart for maximum holding strength.

To attach a top rail, clamp a leg to the workbench with the rounded edge facing down, set the rail on 1-inch spacer blocks with pocket holes facing up, align the undadoed side flush with the top of the leg, and secure with pocket screws. For the bottom rail, rest it on 1-inch spacers, push the rail and panel pieces against the leg, then slide the bottom rail 1/8 inch away from the top rail before fastening. This 1/8-inch gap is critical because it allows the wood to expand when it gets wet without cracking or buckling. Silva explains this principle clearly: if the bottom rail fits too tightly against the panel pieces, seasonal moisture changes will cause the cedar to push against itself and eventually split. That small floating gap prevents this problem entirely.

If you enjoy repurposing techniques for outdoor structures, you might be interested in how to Build A Custom Outdoor Grilling Station From Repurposed Planters for your backyard.

Building The Side Panels And Joining The Box

Slide the panel pieces into the rail dadoes one by one, ensuring the 1/4-inch reveal is visible on the outside face. Work from one end toward the other, pressing each panel firmly into the groove. Clamp a second leg to the workbench and rest the rails on 1-inch spacer blocks so the top rail sits flush with the top of the leg. Verify there is a 1/8-inch expansion gap between the rails and panel pieces before driving pocket screws through the rails into the leg. Check for square by measuring diagonally from corner to corner. If the two diagonal measurements match, your assembly is square.

Repeat this process to create three identical sides. For the fourth and final side, screw the top rail to both legs first, then fit the panel pieces into the top rail dado, and finally slip the bottom rail over the bottom ends of the panels before fastening it with the 1/8-inch gap. The table below summarizes the key dimensions for each component in this project.

For more information on working with cedar in curved applications, see the techniques covered in Bending Cedar Shingles Curved Surfaces.

Building The Top Frame And Base Support

Create the top by mitering 5/4×6 cedar boards to the lengths shown in the cut list. The mitered corners give the planter a refined look that matches its furniture-grade design. Drill pocket holes in the ends of the long pieces, apply wood glue to the mitered joints, and clamp them together until the faces are flush. Make sure the glue does not squeeze out onto visible surfaces because dried glue will resist staining and show up as white patches. Once the glue cures for about one hour, sand the top with an orbital sander using 220-grit sandpaper, followed by 320-grit for a smooth finish. Pay extra attention to the corners where the miter joints meet. Attach the completed top to the planter box with pocket screws, leaving a 1-inch overhang on all sides. This overhang creates a subtle shadow line that makes the top appear to float above the box.

For the base, cut four crosspieces from the 4×4 rip offcuts and two stringers from the 5/4×6 stock. These offcuts come from the material removed when you tapered the legs, so do not discard those strips. Arrange the crosspieces on the stringers in a ladder-like frame and fasten with 2.5-inch deck screws. The spacing between crosspieces should match the width of your milk crates so they sit securely. Slide the base between the bottom rails with the dadoed edge facing down. Position the lower edge of the stringers 3 inches above the lower edge of the bottom rails, then fasten with 1.25-inch wood screws.

Finishing Touches And Planting

Once the box is fully assembled, apply a penetrating oil finish designed for outdoor use to protect the cedar from UV rays and moisture. Unlike film-forming finishes that can peel and crack, penetrating oils soak into the wood fibers and provide protection from within. Reapply the oil once a year to maintain the color and water resistance. This treatment helps the wood weather gracefully to a silvery gray rather than cracking or warping. Allow the finish to dry completely before moving the planter to its final location. A full 24-hour drying period is recommended before exposing the finish to rain or heavy dew.

Line two milk crates with fabric grow bags or landscape fabric, fill them with high-quality potting mix, and set them inside the planter. The crates rest on the crosspieces, keeping the grow bags elevated above the bottom of the box. Water drains freely through the fabric and crates, landing on the ground below rather than pooling inside the cedar structure. This drainage design is what keeps the wood dry and extends the planter life for many seasons. When selecting plants, consider a mix of the following:

• Upright focal plants such as dwarf evergreens or ornamental grasses for height
• Trailing plants like ivy, sweet potato vine, or calibrachoa to spill over the sides
• Filler flowers such as petunias, marigolds, or geraniums for color throughout the season
• Foliage plants like coleus or heuchera for texture contrast

If you are using cedar for roofing as well, understand the safety requirements for Fire Retardant Treatment For Cedar Roof Shakes And Shingles.

Place the planter where it catches morning sun and is visible from your main seating area. With proper care, your cedar planter will remain a beautiful, functional part of your outdoor space for years. Should your cedar ever darken from exposure, you can learn How To Restore And Lighten Darkened Cedar Paneling Bleaching Techniques For Interior Wood to restore its original appearance.