How to make a window box planter

Inspired by David Domoney's window box guide. This guide draws on "How to Make a Window Box" from David Domoney — Chartered Horticulturalist, TV presenter on ITV's This Morning and Love Your Garden, and one of the UK's most recognised gardening experts. David's approach is practical and encouraging, and he covers both the construction and the planting, which is exactly what you need if you are coming to this fresh.

1. Choose your timber and measure up

Softwood is fine — pine or spruce decking board, 22–25mm thick, is easy to work with and takes paint and stain well. Hardwood such as oak or iroko lasts much longer untreated but costs more. Avoid MDF or chipboard outdoors: they absorb water and fall apart fairly quickly.

Measure the window sill width and allow a small overhang each side if you want the box to look generous. A box 150–200mm deep is enough for most plants, and 180–200mm wide is a practical working depth. Cut a front panel, back panel, two end panels, and a base.

2. Drill drainage holes in the base

Before assembling anything, drill five or six 15–20mm drainage holes along the base panel. Space them evenly so no part of the base holds water. Waterlogged compost kills roots quickly — drainage matters more than almost anything else in a container planting.

If you are going to paint or treat the inside of the box, do it now while the base is still loose. Brush two coats of a water-based wood preservative onto all internal faces, paying particular attention to the base and the bottom edges of the side panels.

3. Assemble the box

Clamp the front and back panels to the base, then fix with 50mm exterior screws, pre-drilled to avoid splitting. Apply exterior wood glue to each joint before screwing for extra strength. Fit the two end panels in the same way. Check the box is square as you go — a diagonal measurement between opposite corners will confirm it.

Fill any gaps at the joints with an exterior wood filler once the glue has dried. Sand the whole thing lightly with medium-grade abrasive paper, paying attention to sharp corners and edges.

4. Treat or paint the outside

A window box takes weather from above and from below, so decent treatment is worth the time. Two coats of exterior wood paint or a quality fence and shed stain gives good protection and can match the colour of your front door or window frames.

Apply with a brush rather than a roller to get the paint into any gaps around fixings and joints. Allow each coat to dry fully before applying the next. There is no rush — the box is not going anywhere while the paint dries.

5. Fix the brackets to the wall

A planted window box is considerably heavier than an empty one, so the fixings need to be robust. Use purpose-made window box brackets or L-brackets at minimum 300mm long, screwed into solid masonry with 60mm plugs and screws. On a timber-framed house, fix into the studs.

Hold the box in position and mark the bracket positions. Remove the box, fix the brackets, then refit the box and screw it to the brackets from below. Check it is level before tightening fully. Give it a good shake to confirm it is not going anywhere.

6. Add crocks and compost

Put a layer of crocks (broken terracotta pot fragments) or large gravel over the drainage holes before adding compost. This stops compost washing through while keeping drainage free. Use a good multi-purpose compost with some slow-release fertiliser mixed in — it saves a lot of feeding through the season.

Fill the box to within 30mm of the top rim. That rim gives you watering headroom and stops compost splashing out in heavy rain.

7. Plant up and water in

Plan the planting before you remove anything from its pot. As a general rule: a tall or structural plant at the back, medium-height fillers in the middle, and trailing plants at the front. Water each plant well before removing from its pot, then plant, firm in gently, and water the whole box thoroughly once planted.

Window boxes in full sun dry out quickly, especially in summer. Check daily during warm dry spells and water when the top 25mm of compost feels dry. A weekly liquid feed keeps displays going through the growing season.

When to call a handyman

Call Richard if you want a set of matching window boxes made and fitted across the front of the house, or if the wall surface is tricky — old lime render, tile hanging, or render on a timber frame all need different fixings. Getting the brackets right is not something to cut corners on at height.