Whether you've just begun your gardening experiences or you're an established gardener, building a window planter box is an easy way to start or expand your garden without a lot of work. Window planter boxes provide an ideal location for decorative plants and colorful flowers, as well as a variety of vegetables or herbs. If you live in the city or have an apartment, you will find window planter boxes particularly useful because they don't require a yard or other prepared bed.
Determine the length of your window planter box. In order to be aesthetically pleasing, the lengths of most window boxes match the widths of the windows in which you place them. Use your tape measure to measure the width of your chosen window, ensuring that you include the frame in your measurements. Write these measurements on your supplies list so you can cut wood in the correct length for your planter bottom, front and back.
Purchase the supplies you'll need for your window planter box at your local lumber or home supply store. Using cedar for your wood allows you to construct your window planter box out of a water-resistant material that doesn't use harsh chemicals, an important consideration if you're going to be putting edible plants in your planter box. Galvanized screws and nails are also essential to use since your window box will be exposed outside to the rain and moisture, which weakens traditional nails and screws.
Using a jig saw, cut three sections of 1-by-8-inch cedar boards that are the length you wrote on your supplies list in step one. These boards will serve as your window planter box bottom, front and back. Cut two 8-inch square pieces of 1-by-8-inch cedar boards for your planter box sides.
Assemble your window planter box. Fit the pieces together so they form a rectangular box shape (without a lid). Glue the pieces together with wood glue and clamp them into place. Drill holes at the four corners and every 6 to 9 inches along the bottom front edge of the front board; when drilling, make sure your drill bit also penetrates the side and bottom boards that are glued to the front board. Insert the 1 1/2-inch galvanized screws into the holes to fasten the front, bottom and side boards together. Repeat this process for the back of your window planter box.
Provide the finishing touches for your window planter box. Drill 1/8-inch holes in the bottom of your window box planter. According to Lowes.com, this allows water to drain out of your plant container. Using a paint brush, prime and paint the outside of your window box in colors that coordinate with your home or apartment.