What's inside
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A wooden window box with hidden drainage looks cleaner than one with visible holes in its floor, but water still needs a dependable route out. The beginner-friendly solution is a waterproof planting liner set inside a wood shell, with drainage holes in the liner and a concealed outlet through the back or underside. The wood stays drier, and you can remove the liner to clean it.
Contents
Choose dimensions and layout
Measure the window sill and the space below it before buying lumber. A practical starter size is 30 inches long, 8 inches deep, and 8 inches tall. Keep the box narrower than the sill’s usable depth, and don’t make it so long that you can’t lift it safely once planted. Wet potting mix is heavy: a box this size can weigh 35 pounds or more, depending on the soil and plants.
This plan uses ¾-inch cedar boards for the visible shell and a plastic planter liner that fits inside it. Cedar is naturally more rot-resistant than untreated pine, but it still benefits from drying between wet spells. Pressure-treated lumber is a cheaper option for a hidden frame, but it may be harder to cut cleanly and is unnecessary for a small decorative box. Avoid old painted or treated scrap if you don’t know what it contains.
Materials and tools
For a 30-by-8-by-8-inch box, buy enough ¾-inch cedar for a front, back, bottom, and two ends, plus a liner roughly 28 inches long and 6 inches wide. Exact liner dimensions vary, so buy the liner first and size the box around it. You’ll also need exterior-rated screws, waterproof construction adhesive, sandpaper, and a finish rated for outdoor use if you want to change the cedar’s color.
Useful tools include a tape measure, square, drill-driver, saw, and clamps. A cedar board search can help compare board availability and prices. For a beginner, a pre-cut liner is usually simpler than building a watertight tray from sheet goods; a plastic window-box liner search is a useful starting point.
Compare hidden-drainage options
| Method | What it hides | Trade-off |
|---|---|---|
| Removable plastic liner with rear outlet | Most holes and the water path | Easy to clean; liner must fit the shell |
| Wood floor with screened holes | Holes from a normal viewing angle | Wood stays wetter and needs more inspection |
| Sealed wooden tray | All drainage hardware | Coating failures can trap water against wood |
The removable liner is the best choice for a first build. A screened wooden floor costs less if you already have suitable lumber, but it doesn’t keep water away from the wood. A sealed tray can work, but seams and coatings are common failure points; it is not automatically more durable just because it looks seamless.
Build the wood shell
Cut the front and back boards to 30 inches, the ends to 6½ inches, and the bottom to 30 by 6½ inches. These measurements assume ¾-inch side boards and a 6½-inch inside width; adjust them to your liner and actual board thickness. Dry-fit the pieces and check that the corners are square before fastening.
Apply a thin bead of exterior construction adhesive at the end joints. Drill pilot holes to reduce splitting, then drive two exterior screws through each end into the front and back. Fasten the bottom to the sides with screws about every 6 inches. Leave small gaps only if the liner covers them; the shell is decorative, not the watertight container. Sand sharp edges and remove sawdust before finishing.
Make the hidden drainage path
Set the liner in the shell and mark a drainage route near the back, where it will be less visible. Drill two or three ⅜-inch holes through the liner’s back wall, about ½ inch above its floor. Space them several inches apart. This leaves a shallow water reserve below the outlets while allowing excess water to escape. Check the liner’s shape first: if it has molded drainage points, use those instead of drilling new holes.
Place the box with a slight outward slope—roughly ⅛ inch from back to front—so water does not run toward the house. A small plastic shim beneath the back edge is enough. Don’t block the outlet against siding or caulk it closed. If your liner drains through the bottom instead, set it on plastic feet or spacers so the holes remain open, and let the water discharge clear of the wall and sill.
Mount and plant safely
Do not rely on the box’s decorative screws to hold it to a wall. Attach it to framing, masonry anchors, or a purpose-made bracket rated for the filled weight. If you can’t locate solid framing or verify the wall material, use a freestanding support or ask for help before mounting. Keep the box below the sill, leave access for cleaning, and avoid placing it where runoff can stain siding or drip onto people.
Fill the liner with a lightweight potting mix, not garden soil, which compacts and stays wet in a shallow container. Leave about 1 inch below the rim for watering. For a sunnier window, use drought-tolerant plants; shade plants need more frequent checks if the box is sheltered from rain. A lightweight container potting-mix search can help you compare bag sizes and ingredients.
Check for failures
Water the box slowly after planting and watch the outlets. If water backs up, the holes may be too high, blocked by roots, or pressed against the wall. If the shell stays wet for days, remove the liner and check for trapped water between plastic and wood. In spring and fall, lift the liner, clear debris, inspect screws, and touch up damaged finish. In freezing climates, empty or remove the liner before hard freezes if it may hold water; frozen water can split plastic and force joints apart.