This solution, a box-and-bracket design that is simple to build, install and maintain, came from Gardening Club of America member Joseph Plante of St. Paul, Minnesota. He has made window boxes like these for nearly every window on his house — and even one for the garage. Joe created a system that is convenient to access, allowing him to easily change the plants with each new season.
The box is sized to fi t around a plastic liner so that the rolled rim of the liner rests on the top edge of the box. (Be sure to measure the liner you purchase before you build the box in case you need to adjust the dimensions.) To allow for drainage, the window box has no bottom panel — only a couple of 2x2 cross braces. Joe added 2x2 decorative strips on the ends of the box to create visual interest and to increase the overall width of the box. You can eliminate that step or change the design to enhance the box and suit your home's style.
Corner braces are key to this window box system. Although the chunky brackets appear to do the heavy lifting, they are actually held up by the steel corner braces and lag screws, while two more lag screws secure each corner brace to the solid-wood window frame.
We made the box and brackets shown in the how-to steps out of cedar (which is a softwood) using hand tools and a cordless drill/driver. If you choose a hardwood such as poplar, you'll find that a power miter saw makes the work much easier.
After measuring and cutting the box front, back and sides, bore pilot holes and countersink holes at the ends of the front and back. Apply waterproof glue; then clamp and fasten with 1-1/2-in. deck screws.
To install the 2x2 bottom supports, first mark the screw positions on the front and back; then bore pilot and countersink holes, apply glue and drive the screws.
Using a combination square, mark the 45-degree beveled corners on the decorative strips. Clamp the workpiece to the workbench and cut the bevels with a fine-tooth handsaw.
Space the decorative strips evenly on the planter-box end and mark their positions. Bore two pilot holes for each strip and start the six screws from inside the box. Apply glue to the strips and hold them in place while driving the screws.
Make the 45-degree bevel cuts on the outer ends of the 4x4 bracket tops (E). To lay out the notch at the other end of each 4x4, first mark the depth of the notch (half of the 4x4's thickness) and then use the back piece (F) to mark the width.
Cut the notch in the bracket top with a handsaw. First make the perpendicular crosscut (thickness); then cut into the end of the 4x4 (with the grain) for the depth. Be sure to stay within the marked cut lines.
Cut the diagonal supports (G) to length before marking and cutting the 45-degree miter cuts at both ends. A handsaw will work, but a miter saw is more precise.
Join the bracket top and back (using glue and 2-1/4-in. screws); then clamp and fasten the diagonal support to the assembly. Check for square as you fasten the parts together. Use glue and two lag screws to attach a corner brace to the bracket.
The steel corner braces are secured to the window frame with two lag screws in each brace. This makes the box easy to remove when you want to replace plants as the seasons change.
How to Plant a Window Garden
Watch a video of Gardening How-To Editor Kathy Childers showing how to plant a window box. Go to HandyMagazine.Scout.com/Planter.