Building a Simple Window Box for Year-Round Colour

A window box brings colour right up to the glass, close enough to brush against the curtains. It is also the most exposed piece of gardening you can do: whatever you plant there has to cope with wind, driving rain, reflected heat and, on a north-facing wall, weeks of gloom. Get the frame and the compost right and the planting becomes the easy part. Build for the worst week of the year rather than the best, and the box will look good in February as well as July.

Choose the box before you choose the plants

Aim for a trough roughly 60–90cm long, 20cm wide and at least 20cm deep. Depth matters more than length: shallow boxes dry out within hours on a sunny sill and leave no room for roots. Tapered sides look better against a wall than a plain rectangle, because the plants aren't crushed against the glass.

• Timber is the traditional choice. Use tanalised softwood or oak, treat it with a water-based preserver rather than creosote, and line the inside with a piece of permeable membrane to slow rotting.
• Terracotta looks lovely but cracks in a hard frost unless it is frost-proof. Line it or accept the risk.
• Plastic and glass-reinforced plastic are light, cheap and last for years. They warm up fast in sun, so keep them well watered.
• Metal conducts cold straight into the roots. Line it with a layer of bubble wrap or hessian before planting.

Do check the weight. A 90cm trough full of soaked compost and mature plants can reach 25–30kg, which is more than a plastic window sill was ever designed to carry. If in doubt, go shorter and deeper.

Fixing it so it survives a gale

A box that merely sits on the sill will lift in a gust and end up on the path. Fix it properly and check the fixings every autumn.

• Screw cast-iron sill brackets into the brickwork or the lintel, never into the mortar alone, and use stainless steel screws.
• Do not drill into the window frame itself, particularly uPVC. A loaded frame strains the opening and can void a guarantee.
• Leave a 2cm gap between the back of the box and the wall so air can move and damp doesn't sit against the render.
• On balcony railings, use over-the-rail hooks with a strap or cable tie as a backup.

Compost, drainage and a sensible mix

Drainage is the difference between a box that thrives and one that sours. Drill 8–10mm holes every 10–15cm along the base if they aren't already there. Then add a 2cm layer of coarse grit, a piece of permeable membrane to stop the compost washing through, and fill.

Garden soil is the one thing to avoid — in a box it sets like concrete. Instead, mix three parts John Innes No. 2 with one part multipurpose compost. The John Innes holds its structure and moisture far better over a full year. Stir through slow-release granules and a couple of handfuls of water-retaining gel, which genuinely earns its keep in a windy site.

Feeding starts about six weeks after planting, once the granules are spent. A weekly liquid tomato feed through the growing season keeps flowers coming. Water daily in summer and twice on a hot day; ease right back from November to February, when soggy roots do more harm than dry ones.

Planting for a sunny sill

South and west-facing boxes get hot and dry. Choose plants with small, tough leaves and plant them densely enough to shade their own compost.

• Upright: pelargoniums, nemesia, dwarf cosmos, osteospermum, lavender 'Hidcote' (add extra grit), and culinary herbs such as thyme, sage and chives.
• Trailing: ivy, verbena, calibrachoa, bacopa and trailing lobelia to soften the front edge.
• Spring: dwarf tulips, narcissi and wallflowers, planted as bulbs in October.

Planting for shade and a north-facing wall

North walls are cooler, damper and slower to wake in spring, but they hold colour longer in summer and are far kinder to foliage plants.

• Structure: a small euonymus, skimmia, hebe or clipped box to give the box a backbone through winter.
• Flower: hellebores, primroses, winter pansies, cyclamen coum and snowdrops.
• Foliage: hart's tongue fern, heuchera in bronze or lime, and trailing periwinkle.

In an exposed spot, avoid tall, soft-stemmed plants altogether. They will be flattened by the first equinoctial gale. Low, leathery and slightly dull is what survives.

Keeping colour going all year

The most reliable approach is a three-layer box: one small evergreen for structure, one seasonal flower for colour, and one trailer to spill over the front. Then simply swap the middle layer as the year turns.

• Autumn: plant bulbs, add heathers, cyclamen and ivy.
• Winter: hellebores, winter pansies and snowdrops carry the box through to March.
• Spring: bulbs flower through, then give way to primroses and wallflowers.
• Summer: pelargoniums, nemesia and trailing verbena in sun; ferns and begonias in shade.

Between changes, deadhead weekly, snip out anything leggy, and top-dress in late February with a couple of centimetres of fresh compost. Run a strip of copper tape around the rim if slugs find their way up. Do that much and a plain wooden trough will earn its place at the window for years.