A window box is a simple carcass — four sides and a base — but two things separate one that lasts from one that doesn't: drainage that actually clears water, and brackets fixed into something solid enough to carry it full and wet. This one is sized for a typical window, cut from a single length of cedar board, and hung on brackets rated for its filled weight rather than screwed straight into render.

A Cedar Window Box Planter on Mounting Brackets

What you need

1 length 3.6m x 19 x 184mm cedar board (nominal 1x8) for the front, back, ends and base
1 box 32mm stainless or coated exterior deck screws for assembly
1 small bottle waterproof exterior wood glue optional, for the corner joints
3 galvanised steel window box brackets, rated at least 20kg each
8 75mm exterior wood screws, or masonry screws and wall plugs for brick to fix the brackets to the wall
1 strip landscape fabric or weed membrane, cut to the base optional liner over the drainage holes
1 tin exterior wood oil or stain optional — cedar can also be left to weather silver-grey

Step by step

Cut the front and back panels to 900 x 184mm from the cedar board.
15 min
Cut the two end panels to 146 x 184mm, bracing each short piece against a stop block or clamp on the mitre saw rather than holding it freehand near the blade.
15 min
Cut the base panel to 900 x 184mm, the same size as the front and back.
10 min
Mark and drill eight to ten 10mm drainage holes through the base panel, spaced evenly, clamping it to the bench so the spade bit can't catch and spin it.
15 min
Pilot-drill the end panels near their edges to stop the thin cedar splitting, then glue and screw the front panel to both end panels, keeping the outside faces flush.
20 min
Glue and screw the back panel onto the same two end panels, forming the four-sided carcass, and check it's square by measuring the diagonals.
20 min
Pilot-drill around the perimeter of the base panel, then glue and screw it up into the bottom edges of the front, back and end panels, flush all round.
20 min
Sand all edges and the inside corners smooth, easing anything sharp enough to catch a hand.
15 min
Apply exterior oil or stain if you're using one, following the tin for coats and recoat times, or leave the cedar bare to weather.
15 min
Mark bracket positions on the wall below the window, spaced evenly, locating solid framing or masonry rather than render or thin cladding alone.
15 min
Fix the brackets with the 75mm screws, or masonry screws and wall plugs into brick.
15 min
Lift the box onto the brackets and secure it through the brackets' fixing holes into the underside.
10 min
Line the base with the fabric strip if you're using one, then fill with compost once any finish has fully cured.
10 min

Afterwards

Vacuum sawdust from inside the box before planting — trapped dust holds moisture against the wood. If you've oiled or stained the cedar, leave it to cure fully per the tin, typically 24 to 48 hours, before filling with wet compost. Check the bracket screws again after the first couple of waterings; planted compost is heavier than it looks, and new fixings can settle slightly as the timber beds in.

Questions

Can I use pine instead of cedar?

Yes, but pine rots outdoors without a finish, where cedar resists rot untreated. If you use pine, prime and paint or oil it fully, including the end grain, and expect to recoat it more often.

Will the brackets really hold it?

A 900 x 184 x 184mm box holds around 25 litres of compost, which at roughly 1.2kg a litre fully wet comes to 25-30kg once planted. Choose brackets rated well above that between them, and fix into solid framing or masonry rather than render or thin cladding alone.

Do I need to line the box?

A strip of landscape fabric over the drainage holes stops compost washing out without blocking the water. It's worth doing, not strictly required.

When's the best time to build and plant it?

Build and fix it any time of year — it's the planting that's seasonal. Built now, it's ready for spring bulbs for an early show, or for bedding plants once the frosts have passed.