A window box is one of the highest-impact projects in the whole garden: a few feet of trailing petunias, cascading ivy, or a tidy row of herbs transforms a plain facade into something people slow down to look at. Building your own beats buying a flimsy plastic trough — you get the exact length to fit your window, the wood and finish you want, and a box strong enough to carry wet soil for years. The two things that separate a window box that lasts from one that fails are drainage and mounting: a box that can’t drain rots out in a season, and a box that isn’t anchored properly can pull loose and fall — soil, plants, and all — which is genuinely dangerous below a second-story window. This guide walks through a clean, durable build with an exact cut list, the right dimensions, real drainage, and three safe mounting methods.
Key Takeaways
- Size the box to the window: make it about 2–4 in. shorter than the window width, and 8 in. deep and 8 in. tall for healthy roots.
- Use cedar, redwood, or exterior plywood — naturally rot-resistant woods last far longer on a wet, sun-baked box.
- Drainage is non-negotiable: drill 1/2 in. holes every 6 in. along the floor or the box will waterlog and rot.
- Mount into solid framing or masonry with heavy brackets — a soil-filled box is heavy and a poorly fixed one can fall.
- Line the box or use a drop-in liner to protect the wood and make seasonal replanting easy.
Measure Your Window First
Before you cut anything, measure the width of your window opening or trim. The classic look is a box that runs nearly the full width of the window — make it 2 to 4 inches shorter than the trim so it reads as part of the window without looking cramped. For proportions that hold enough soil to keep plants alive through summer, aim for an inside depth (front to back) of about 8 inches and an inside height of about 8 inches. Anything shallower dries out hourly in the sun and starves roots. If your window is wider than about 4 feet, plan to add a center support bracket, because a long box full of wet soil sags and racks in the middle. Note whether you’ll mount on wood siding, brick, stucco, or directly under a sill — that decides your hardware.
Materials and Cut List
This cut list builds a window box with an inside cavity about 36 in. long, 8 in. wide, and 8 in. tall — a good fit for a roughly 40-inch window. Scale the length to your own measurement. Cedar is the best all-round choice: rot-resistant, light, and it takes paint or stain beautifully. Exterior-grade plywood is cheaper and works well if you seal and paint every face. Avoid bare pine, which rots fast in this wet, exposed application.
- Front and back: two 1×8 cedar boards, 36 in. long.
- Two ends: two 1×8 cedar boards cut to the inside width plus board thickness — about 9.5 in. (so the assembled inside is ~8 in. deep).
- Floor: one 1×8 cedar board, cut to fit inside the assembled frame (about 34.5 in. × 8 in.).
- Optional trim: 1×2 or decorative molding for the front face and a top cap.
- Fasteners: 1.5 in. and 2 in. exterior/stainless deck screws plus waterproof exterior wood glue.
- Mounting hardware: two or three heavy steel L-brackets or dedicated window-box brackets, plus appropriate screws or masonry anchors.
- Finish: exterior primer and paint, or penetrating exterior wood oil/stain.
- Tools: circular or miter saw, drill/driver, 1/2 in. spade bit, tape measure, square, clamps, level, and a stud finder.
Step-by-Step Build
Step 1: Cut and dry-fit the boards
Cut the front, back, two ends, and floor to your dimensions. Dry-fit them into a box: the front and back overlap the ends, and the floor sits inside on the bottom edges. Check that everything sits square and tight before you commit any glue or screws. Sand the cut edges smooth — splinters in cedar are common and a quick pass with sandpaper makes the whole project look more finished.
Step 2: Assemble the frame
Run a bead of exterior wood glue along the joints, then clamp the front and back to the two end boards to form the rectangular frame. Pre-drill to prevent splitting and drive 2-inch screws through the front and back into the end grain of the end boards — two or three screws per corner. Check the box for square by measuring both diagonals; they should be equal. Glue plus screws gives a joint that survives years of wet-dry cycling far better than screws alone.
Step 3: Fit the floor and drill drainage
Drop the floor board inside the frame so it rests on the bottom edges of the front, back, and ends. Glue and screw it down with 1.5-inch screws driven from the outside faces into the edge of the floor board. Now the most important step of the whole build: drainage. Drill 1/2-inch holes through the floor every 6 inches along its length, in a row or two. A window box with too few holes turns into a stagnant trough that drowns roots and rots the wood from the inside out. You truly cannot over-drain a window box.
Step 4: Add trim and finish
Add any decorative trim or a top cap with glue and small finish screws or brads. Then protect the wood: fill screw heads, sand, prime, and paint with a quality exterior paint, or brush on a penetrating exterior oil if you want to show the cedar grain. Coat every face, including the bottom and the inside, and let it cure fully. The finish is what stands between your box and the weather — don’t skip the inside, since that’s where water sits.
Step 5: Line the interior
Even a sealed wood box lasts much longer with a liner between the soil and the wood. The easiest approach is a rigid plastic or metal drop-in liner sized to slip inside; it makes replanting each season trivial — you just lift the whole liner out. Alternatively, line the inside with heavy landscape fabric or perforated pond liner, leaving the drainage holes open. Whatever you use, never block the drainage holes.
Mounting It Safely
A window box full of wet soil can weigh 40 to 80 pounds, so mounting is a safety issue, not just an aesthetic one. Always fasten into something solid — never into siding or stucco alone, which won’t hold the load.
- On a wood-sided house: use a stud finder to locate the framing, then screw heavy steel L-brackets or window-box brackets into the studs or the window header, not just the cladding. Use at least two brackets, three for boxes over 3 feet.
- On brick or masonry: drill into the mortar or brick and use proper sleeve or expansion anchors rated for the load. Don’t rely on plastic wall plugs.
- On a deck or porch rail: use clamp-on rail brackets sized to your rail dimension, and check that they grip tightly with no wobble.
Set the box on the brackets, check it’s level, and screw the box down to the brackets so it can’t be lifted off by wind. Give it a firm tug to confirm it’s solid before you fill it.
Soil, Plants, and Care
Fill with a light potting mix — not garden soil, which compacts and weighs the box down. Mix in slow-release fertilizer and water-retaining crystals to ease the relentless drying that comes with a shallow, sun-exposed container. For a full look, follow the classic “thriller, filler, spiller” recipe: an upright centerpiece (geranium, dwarf grass, salvia), mounding fillers (petunia, calibrachoa, marigold), and trailers to spill over the front (ivy, lobelia, sweet potato vine). A box of herbs — basil, thyme, parsley, chives — is a productive twist that’s perfect right outside a kitchen window. Because the soil volume is small, water daily in summer and feed every couple of weeks. This is an ideal project for a small-space or balcony garden where ground beds aren’t an option, and it complements other curb-appeal projects in our outdoor living guides. Building more this season? Our raised bed with legs plans use the same tools and skills.
Common Mistakes to Avoid
The classic failures are too few drainage holes, an unsealed interior, undersized mounting hardware, and using rot-prone bare pine. Any one of them shortens the box’s life or makes it unsafe. Drill plenty of holes, seal every face, anchor into real framing or masonry with heavy brackets, and choose cedar or sealed plywood. Get those right and your window box will look good and stay put for many seasons.
Frequently Asked Questions
How big should a window box be?
Make it 2–4 inches shorter than your window’s width so it frames the window without crowding it, with an inside depth of about 8 inches front-to-back and 8 inches tall. Those proportions hold enough soil to keep plants alive through summer heat without drying out hourly.
What wood is best for a window box?
Cedar and redwood are the best — naturally rot-resistant, light, and they take paint or stain well. Exterior-grade plywood works if you seal and paint every face. Avoid bare pine, which rots quickly in such a wet, sun-exposed location.
How many drainage holes does a window box need?
Drill 1/2-inch holes through the floor every 6 inches along its length. Drainage is the single most important detail — too few holes turn the box into a stagnant trough that drowns roots and rots the wood from the inside. You cannot over-drain a window box.
How do I mount a window box securely?
Fasten heavy steel brackets into solid framing (studs or the window header) on a wood house, or into brick/mortar with proper masonry anchors. Use at least two brackets, three for boxes over 3 feet, and screw the box to the brackets so wind can’t lift it. Never fix into siding or stucco alone.
What should I plant in a window box?
Use the “thriller, filler, spiller” recipe: an upright centerpiece like a geranium or salvia, mounding fillers like petunias or marigolds, and trailers like ivy or lobelia to spill over the front. A box of herbs outside a kitchen window is a productive alternative. Water daily in summer and feed every couple of weeks.