A window box bursting with blossoms will brighten any window, from inside or out. If you buy a stock one, you'll have your choice of designs, but you'll also be limited to standard lengths—typically 2 to 6 feet, in 6-inch increments.
Building the paneled box shown here offers you the chance to tailor your planter to the width of your window and select molding to match your home's style. PVC boards come in white but can be painted any color. Either way, they mimic the look of painted wood without being a target for rot and mold. They're available at home centers in the same dimensions as traditional lumber. If you prefer to work in wood, choose a water-resistant species, like teak or cedar.
Download and print the cut list for building a PVC windowbox.
PVC windowbox cut list
We built a 30-inch-wide window box designed to sit beneath a 32-inch-wide window. For boxes longer than 48 inches, add a third bracket at the center of the box.
1x10 PVC front and back pieces: two @ 30 inches
1x10 PVC end pieces: two @ 7¾ inches
1x10 PVC bottom piece: one @ 30 inches
1⅛-by-1 ⅛-inch PVC outside-corner molding. Cut to fit.
PVC cap molding. Cut to fit.
Make the frame.
Use a sliding compound miter saw or a circular saw to size the pieces according to the cut list. Sandwich the box's end pieces between the front and back pieces, applying PVC cement to the captured edges. Clamp the assembly together. Tack the corners with 1½-inch brads. Use a ½-inch spade bit to drill drainage holes every 6 inches along the centerline of the bottom piece, and adhere and tack the piece to the box.
Attach the molding.
Measure the built box, and cut the molding to fit, making 45-degree miter cuts at the corners. Apply cement and tack the molding in place with ¾-inch brads, covering the seam at the bottom.
Install the brackets.
Position a support bracket on the wall just below the window, to catch the sill framing, 3 to 6 inches in from the window's sides. Drill ¼-inch pilot holes through the bracket and into the house, squeeze caulk into the pilot holes, and fasten the bracket with ⅜-by-3-inch lag screws. For brick or stone, use a masonry bit and masonry anchors. Use a 4-foot level to position the second bracket before installing it.
Hang the box.
Center the box on the brackets and fasten it with 1¼-inch wood screws, installed up through each bracket and into the box.