Flower window boxes enhance almost any type of house structure. However, they can be a real bonus for homes with limited growing space, such as apartments or condominiums. Fill your flower box with lovely annuals, such as pansies or petunias. Another options is to create an herb garden, a perennial garden with mini rose bushes or a bulb garden with tulips. Attach your flower window box directly under the outside of your window or set it on the indoor window ledge, which is perfect for an herb garden. Make extras and place them on the patio, deck, front porch or throughout the garden as unattached flower boxes.
Decide on the length of your flower window box. If you plan to attach it to the structure under the window, it looks best to make the box the same length as the window is wide.
Mark and saw two same-sized boards for the length of the window box. These will be the front and back. Mark and saw two same-sized boards for the width, 6 inches.
Paint all of your cut lumber on both sides with exterior paint. The boards are easier to paint flat, rather than when the box is complete. The exterior paint will seal the wood, which means the boxes will last longer.
Place the painted boards in a frame configuration, with the long front and back boards parallel to each other. Put the shorter, side boards, crosswise inside of the front and back boards, at the end.
Screw in 5.5-inch screws at each side-end of the front board and each side-end of the back board. Use your drill and attachment to drill the screws through to the sides of the shorter, end boards. You have now created the frame for your flower window box.
Measure the inside dimensions of the box. Mark and cut a board to those measurements. Paint the cut board on both sides.
Fit the cut and painted board to the inside, bottom of the flower box. Attach it with screws from the bottom outside of the box, into the sides of the cut board. Place a screw about every 4 inches.
Use a drill bit and drill to make holes through the bottom of the flower box, at random. This will create drainage holes for the plant water to escape, so it doesn't sit in a puddle at the bottom of the box.
Locate the wall studs to attach two to four 8-inch L-brackets to the underside of the window, depending on how wide you've made the window box. Be sure to screw them into the studs of the structure. The studs are not difficult to find, merely use a light tap until you no longer feel and hear the hollowness of the wall. You may also use a battery operated stud-finder, which is available at your local hardware store.
Mark the location of each screw, after measuring down from the window, so they are aligned evenly across. The distance down from the window is a personal choice. The flower window box can sit so the plants will be over the bottom or at the bottom of the window. Use 2-inch lag screws to attach the L-brackets, for strength.
Place the flower window box onto the top of the L-brackets. Slide the box forward so the end of the brackets do not show, about half an inch. Screw the underside of the box to the L-bracket with rust resistant screws. Your flower box is ready for planting.