A window box is one of the hardest-working pieces of garden you can own. It is also one of the easiest to get wrong — a glorious burst of colour in July, then eight months of bare compost. The secret is to treat it like a miniature border: build it around a permanent framework of evergreens, then let bulbs and bedding come and go around that skeleton. Get the planting right and the same box will carry you from January to December.

Choose the box and the compost first

Before you buy a single plant, work out which way the window faces. A south or west-facing sill is hot, bright and dries out fast; a north or east-facing one is cooler, damper and suits foliage over flowers. This single detail will decide half your plant choices.

Whatever the aspect, go bigger than you think you need. A box at least 20cm deep and 20cm wide holds enough compost to buffer against drought and frost; shallower ones need watering twice a day in summer and still sulk. Length is a matter of taste, but a 90cm box is a good practical size.

• Weight matters. Wet compost is heavy — a metre-long box can top 35kg. Fix brackets into brick or masonry with proper anchors rather than mortar joints, or stand the box on a windowsill saddle.
• Drainage is non-negotiable. Check for holes, then add a 2cm layer of crocks or broken polystyrene. Standing water kills more window box plants than cold ever does.
• Use a soil-based mix. John Innes No. 2 blended with a little multipurpose compost and a handful of grit gives roots something to hold. Avoid pure garden soil, which sets like concrete.
• Add slow-release feed and a little water-retaining gel at planting time, mixed through the lower half of the compost.

The backbone: evergreens and structure

This is the part most people skip, and it is why their boxes look empty for two-thirds of the year. Aim for three to five structural plants in a 90cm box, spaced so they will just touch when mature.

Small box (Buxus) is the classic choice, though box blight and box moth mean many growers now prefer alternatives such as Ilex crenata, Euonymus japonicus 'Microphyllus' or Lonicera nitida, all of which clip neatly and hold their shape. For something with more character, try Hebe 'Green Globe', a dwarf skimmia for red winter buds, or a variegated sedge such as Carex oshimensis 'Evergold' to light up a shady sill.

Use trailing ivy or a prostrate euonymus at the front corners to soften the box edge, and repeat the same plant at each end for a sense of rhythm.

Bulbs: the quiet workers

Bulbs give you flowers at exactly the moments when nothing else is willing. Plant them in layers, like lasagne, in autumn.

• Lowest layer (15–20cm deep): dwarf narcissi such as 'Tête-à-Tête', and short tulips like 'Red Riding Hood' or species tulips, which cope with wind far better than tall varieties.
• Middle layer (8–10cm): crocus, dwarf iris and muscari.
• Top layer (5cm): snowdrops and cyclamen coum, planted close to the surface at the front.

After flowering, deadhead but leave the foliage for six weeks so the bulb can fatten up for next year. Top up with a fresh handful of bulbs each autumn rather than trying to keep the same ones going forever.

Bedding to bridge the gaps

Bedding plants fill the space between bulb displays. Buy them small and healthy, plant in odd numbers, and resist cramming — roots need room to work.

• Spring: pansies, violas, primulas and wallflowers, which bring scent right to the window.
• Summer: pelargoniums, calibrachoa, nemesia and bacopa, with trailing lobelia or trailing verbena spilling over the front edge.
• Autumn: winter heathers, small chrysanthemums and ornamental cabbages, which hold colour well into November.

Feed weekly with a liquid tomato feed from April to September, and the display will keep going far longer than the plants have any right to.

Looking after it through the year

Watering is the main job, and rain rarely does it for you — an overhanging roof or eaves means a window box can stay dry through a downpour. In summer, water daily and twice on hot days, until it runs from the drainage holes. In winter, ease off but never let the compost dry out completely, and water on mild mornings rather than during a frost.

Each spring, scrape off the top 5cm of compost and replace it with fresh, then add slow-release granules. Trim evergreens lightly after flowering, cut back tired trailing growth, and deadhead regularly. Watch for vine weevil in the compost and aphids on soft new tips.

Every two to three years, tip the whole lot out, refresh the compost and replant. Done this way, a single window box becomes a genuine year-round feature — and the best-value square metre of growing space in the house.