Window planter boxes aren't just for spring and summer. Before the winter chill sets in, get outdoors and collect some large pinecones. Tip them with white paint and create a display in your window planter box, along with some evergreen branches. You will be surprised by how easy window planter boxes are to build, with a few tools and some basic carpentry skills.
Measure the width of the window you wish to have a window planter box beneath. Your planter box should be this length or slightly less. Choose a 12-inch width or a narrower 8-inch width for the window planters.
Cut your boards, two for the length and two for the width. Redwood and cedar are good choices for lumber, as they are both climate-resistant. However, you can choose to use pine or any other wood that is available.
Drill galvanized screws into the long boards at the corners to attach the two side ends. Use three screws at each corner. This will create a frame-shape. Galvanized screws are rust-resistant and will hold your window planter box together longer.
Place the wood frame over the top of a piece of plywood or pressed wood. Draw around the frame. Cut the plywood at the markings. This will be the bottom of your window planter box.
Move the plywood cutout to the top of the frame and attach screws around all sides, catching both the plywood and frame boards. This needs to be extra sturdy, so place a screw approximately every 2 inches.
Drill holes through the plywood bottom, in several locations. This is to allow water to drain out easily. If you did not drill these holes, water would collect at the bottom of the planter, sopping roots. Many plants will die or become very unhealthy if they are allowed to sit in water.
Screw eye screws into the top edge of the window planter, on the side where it will be attached under the window. Place one approximately every 6 inches. This may seem like a lot, but remember that the planters will be very heavy after the soil and plants are inserted.
Paint or seal your window planter boxes with an exterior product, inside and outside. You may also paint a decorative motif, by hand or with stencils.