How to Build and Install a Window Box Planter

Build a window box planter by cutting cedar boards to size, assembling with screws and waterproof glue, drilling drainage holes, and mounting securely beneath your window with heavy-duty brackets.

  1. Measure Once, Cut Smart. Measure your window width and decide on depth. A typical window box is 6-8 inches deep, 6-8 inches tall, and 2-4 inches shorter than your window width. This leaves room for mounting brackets and prevents the box from overwhelming the window. Sketch your design with exact measurements before cutting any wood.
  2. Shape Your Cedar Frame. Using 1x6 or 1x8 cedar boards, cut two side pieces to your chosen depth, one front piece and one back piece to your chosen width, and one bottom piece that fits inside the frame. Cedar naturally resists rot and insects, making it ideal for outdoor planters. Sand all pieces with 120-grit sandpaper to smooth rough edges.
  3. Open the Water Flow. Mark drainage holes every 4-6 inches along the bottom piece, staying 1 inch from the edges. Use a 1/2-inch spade bit to drill completely through. Proper drainage prevents root rot and extends the life of your plants. Test-fit all pieces together before proceeding to assembly.
  4. Lock the Frame Tight. Apply waterproof wood glue to the edges where pieces meet. Attach the front and back pieces to the sides using 2-inch exterior wood screws, drilling pilot holes to prevent splitting. The front and back pieces should overlap the side pieces. Check that corners are square using a framing square before the glue sets.
  5. Secure the Foundation. Place the bottom piece inside the assembled frame, resting on the lower edges of the side pieces. The bottom should sit about 1/2 inch up from the very bottom of the box to create a lip. Secure with 1.5-inch screws driven up through the sides into the bottom piece edges.
  6. Seal Against the Elements. Sand the entire box with 220-grit sandpaper for a smooth finish. Apply two coats of exterior wood stain or clear weatherproof sealant, allowing full drying time between coats. This step protects the wood from moisture and UV damage, significantly extending the planter's lifespan.
  7. Find and Anchor the Studs. Mark bracket locations on your house wall, ensuring they align with wall studs for maximum support. Use a stud finder to locate studs, then mark bracket holes. Install heavy-duty metal brackets rated for at least 50 pounds using 3-inch lag screws into the studs. The brackets should support the box from underneath.
  8. Level, Test, Plant. With help, lift the completed box onto the installed brackets. Ensure it sits level and secure before adding any weight. Test the installation by gently pushing and pulling on the box. Add a layer of landscape fabric to the bottom to prevent soil from washing out the drainage holes, then fill with quality potting mix.