Build Window Boxes

Cedar planters beneath windows have been catching geranium overflow and tomato vines since colonial doorways first got dressed up. The joinery is simple—butt joints and exterior screws—but the details matter. A properly built window box drains without rotting, holds soil without sagging, and mounts securely enough to handle forty pounds of saturated potting mix without pulling loose in a July thunderstorm. The goal is a planter that looks intentional, not slapped-on. That means matching the proportions to your window width, choosing lumber that can handle weather, and building in drainage from the start. Get the mounting right and these boxes will outlast the paint job. Get it wrong and you'll have a drooping, water-stained problem by August.

1. Cut the box frame to width. Measure your window width and subtract two inches—that's your box length. Cut front and back panels from 1x8 cedar to this length. Cut two end pieces at 8 inches. Your box will be roughly 7 inches deep and extend slightly past the window on each side when centered.
2. Assemble the main box. Stand the front panel on edge. Position end pieces flush at each corner and drive two 2-inch exterior screws through each joint. Attach the back panel the same way. The box should be rigid with square corners—check with a speed square before the screws set fully.
3. Attach the bottom with drainage gaps. Cut a bottom panel from 1x8 to fit inside the assembled frame. Instead of one solid piece, rip it into three strips on a table saw—this creates natural drainage gaps. Screw the strips across the bottom with 1/4-inch gaps between each. Drill three additional 1/2-inch drainage holes in each strip.
4. Add mounting cleats to the back. Cut two 1x2 cedar cleats to run horizontally across the back panel—one near the top, one near the bottom. Screw them flat against the back with 1-1/4 inch screws every six inches. These cleats create an air gap behind the box and provide solid mounting points that won't split out.
5. Seal and finish the wood. Apply two coats of exterior wood sealer or paint to all surfaces, inside and out. Let dry completely between coats. The inside doesn't need paint if you're using a plastic liner, but sealer prevents moisture wicking into end grain. Pay extra attention to drainage holes and cut edges where water penetrates fastest.
6. Locate studs and mark mounting points. Hold the finished box beneath your window at the desired height—typically centered below the sill with the rim 2-3 inches below it. Mark the cleat positions on the wall. Use a stud finder to locate studs behind the siding. Mark at least two stud locations that align with your cleats. If studs don't line up, plan to use wall anchors rated for outdoor use.
7. Mount the box with lag bolts. Drill pilot holes through the siding into studs at marked points. Use 3/8-inch by 4-inch galvanized lag bolts with washers. Drive bolts through the mounting cleats into studs, leaving a 1/4-inch gap between box and siding for air circulation. The box should feel absolutely solid—no flex when you push hard on the front edge.
8. Line and fill the box. Cut a plastic window box liner or heavy plastic sheeting to fit inside. Punch drainage holes that align with the wood gaps below. Fill with potting mix to within two inches of the rim. Plant as desired. Water thoroughly and check that drainage is flowing through the bottom gaps—you should see water dripping within thirty seconds.