What's inside
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A cedar window box is a useful first spring project: the cuts are straight, the parts are small, and a few drainage slots prevent the most common failure. This plan makes a box about 30 inches long, 8 inches wide, and 7 inches deep. It fits many windows without becoming too heavy when filled with wet soil.
Contents
Materials and tools
Use 1-inch-thick cedar boards, usually sold as 1×8 and 1×6 stock. The finished dimensions of nominal lumber vary, so measure your boards before cutting. You will need:
- One 1×8 cedar board, at least 30 inches long, for the front
- One 1×8 cedar board, at least 30 inches long, for the back
- One 1×8 cedar board, at least 32 inches long, for the bottom
- Two end pieces, each about 6 1/2 inches wide and 7 inches tall
- 1 1/4-inch exterior-rated screws, preferably stainless steel or coated for cedar
- Exterior wood glue rated for outdoor use
For hand tools, a tape measure, pencil, square, drill, countersink bit, and saw are enough. A miter saw makes the repeated cuts easier, but a circular saw with a straightedge works just as well. A cedar project board is worth buying if you want cleaner, straighter stock than rough fencing boards provide.
| Board choice | Advantages | Drawbacks |
|---|---|---|
| Cedar project board | Straighter, smoother, fewer knots | Costs more; may still need trimming |
| Cedar fence board | Cheap and easy to find | Thinner, rougher, and often warped |
| Pressure-treated lumber | Durable and inexpensive | Heavier, harder to finish, and less pleasant to cut |
Fence boards are fine for a temporary or budget box if they are at least 5/8 inch thick and reasonably flat. Reject boards with deep splits, soft spots, or severe cupping. Avoid ordinary interior screws; cedar tannins and outdoor moisture can quickly stain the wood and corrode the fasteners.
Cut list and layout
These dimensions assume lumber that is close to 3/4 inch thick and a bottom board that fits between the front, back, and ends.
- Front: 30 inches long
- Back: 30 inches long
- Bottom: 28 1/2 inches long by about 6 1/2 inches wide
- Ends: two pieces, 6 1/2 inches wide by 7 inches tall
Cut the front, back, and bottom square. Mark the end pieces carefully and check that both are identical. Small differences will show at the corners and can leave gaps where water collects. Dry-fit all four sides around the bottom before drilling.
Cut the drainage slots first
Drainage is more reliable when water can escape across the bottom instead of sitting in a few drilled holes. Mark five slots on the bottom board, running across its width. Space them about 4 inches apart, with the outer slots roughly 2 inches from each end.
The simplest method is to drill two 3/8-inch holes at the ends of each slot, then cut between them with a jigsaw. Make each slot about 2 1/2 inches long and 3/8 inch wide. Sand the edges lightly so splinters do not catch on the landscape fabric or your hands.
If you do not have a jigsaw, drill eight to ten 3/8-inch holes in two rows instead. This works, but the holes can clog more readily with soil. Do not make the slots so large that potting mix falls through; a layer of fabric inside the box is better than oversized openings.
Assemble the box
Place the bottom on a flat surface and position the front and back boards against its long edges. Apply a thin bead of exterior wood glue, then clamp or hold the boards in position. Drill pilot holes before driving screws. Cedar splits easily near its ends, especially when using screws close to the corners.
Drive screws every 6 to 8 inches along the front and back, keeping them about 3/8 inch from the board edges. Countersink the heads slightly, but do not bury them deeply. A stainless steel exterior screw set costs more than standard coated screws but is the safer choice for cedar and wet soil.
Fit the end pieces between the front and back. Glue and screw through the front and back into each end, using two screws per joint. Check the box for square by measuring diagonally from corner to corner in both directions. If the measurements differ, push the longer diagonal inward before tightening the screws.
Sand and finish the cedar
Sand sharp corners with 100- or 120-grit paper. You do not need furniture-smooth surfaces; removing splinters and rounding the top edges slightly is enough. Leave the inside unfinished if you want the simplest maintenance. Cedar naturally resists decay, but it will turn gray outdoors.
For a colored box, use a penetrating exterior stain rather than paint on the inside. Paint can trap moisture if it begins peeling, while stain is easier to renew. Keep any finish off the drainage slots. A penetrating exterior wood stain is a practical choice when matching the box to trim or siding.
Mount and fill it safely
Do not rely on the window trim alone. A filled box can weigh 40 pounds or more, depending on its soil and plants. Use two or three heavy-duty brackets rated for the expected load, attach them to wall framing where possible, and use exterior structural screws suited to the wall material. Leave at least 1 inch of clearance between the box and siding so water and air can move behind it.
Line the inside with landscape fabric, allowing it to cover the slots without sealing them. Fill with lightweight potting mix, not garden soil. Garden soil compacts in a container and drains poorly. Leave about 1 inch below the rim so watering does not wash soil over the edge.
In early spring, choose plants that tolerate cool nights, such as pansies, violas, parsley, or leaf lettuce. Water until it runs from the slots, then water again only when the top inch of mix feels dry. If water remains in the box after several minutes, clear the slots before roots begin to rot.
Common mistakes to avoid
The most frequent problem is building a box with no slope or drainage. Keep the bottom flat for easy construction, but make sure every slot is open. Another failure is using boards that are too thin; 1/2-inch stock may bow under wet soil. Finally, do not screw into cedar without pilot holes. Splits around the fasteners can widen through the first freeze-and-thaw cycle.
Inspect the box each spring. Tighten loose brackets, clear the slots, and replace any board with deep rot. With sound cedar, correct drainage, and a secure mounting system, this small project should provide several seasons of useful growing space.