Why a Window Box Is Worth the Effort

If you live in a terraced street or a flat above a shop, a window box may be the only patch of soil you own — and it is far more useful than it looks. Bees, hoverflies and butterflies move through towns along a loose network of gardens, allotments, parks and railway sidings. Where that network has gaps, the insects struggle. A planted box adds a refuelling stop, and if a few neighbours do the same, the whole street becomes a corridor.

Pollinators need two things above all: nectar for energy and pollen for protein. Most of our native wildflowers provide both, but many popular bedding plants — busy Lizzies, begonias, double petunias — offer almost nothing. They have been bred for show, not for wildlife. Choosing nectar-rich flowers instead costs you nothing in beauty. Lavender, salvia and thyme will fill a box with scent and colour and still be working for the bees in August.

Pick the Right Spot and Container

Most pollinator-friendly herbs and flowers want sun, so a south- or west-facing sill is ideal. Six hours of direct light is plenty; four will do at a push, though flowering will be less generous. If your only window faces north, you can still plant a box, but lean towards shade-tolerant options and accept a quieter display.

On the container itself:

• Size: aim for at least 60cm long and 20–25cm deep. Shallow troughs dry out in hours and leave roots cramped.
• Drainage: check for holes before you buy. If there are none, drill three or four, then cover them with broken crocks or a layer of grit so compost does not wash out.
• Weight: wet compost is heavy. A full 80cm box can weigh 20kg or more, so fix brackets into brick or masonry rather than relying on a plastic sill hook. Never hang a box over a pavement without checking it is secure.
• Material: terracotta looks lovely but dries fast and can crack in frost. Glazed ceramic or good-quality timber lasts well. Line timber boxes with a piece of old compost sack to slow rot.

The Best Nectar-Rich Plants

Three plants will carry a window box from early summer to autumn with almost no fuss.

• Lavender (Lavandula angustifolia 'Hidcote' is compact and reliable). Trim it lightly after flowering, never into old wood.
• Salvia — both the ornamental kinds, such as Salvia nemorosa, and culinary sage. They flower over a long period and bumblebees adore them.
• Thyme (Thymus vulgaris). Low-growing, drought-tolerant and smothered in tiny flowers that small solitary bees can reach.

Round these off with a few others: compact oregano, chives, marjoram, catmint, and annuals such as cosmos, verbena and nasturtium. Herbs earn their place twice over, since you can cut them for the kitchen. Avoid double-flowered varieties, which often have no accessible nectar at all.

Planting Up and Compost

Use a peat-free multipurpose compost mixed with about a quarter by volume of horticultural grit. The grit improves drainage and adds weight, which helps in a windy spot. Fill the box to within 2cm of the rim, firm gently and water before planting so the compost is moist rather than soaked.

Space plants closer than you would in open ground — around 15cm apart for lavender and salvia, 10cm for thyme. Close planting shades the compost, keeps roots cool and smothers weeds. Plant in late spring once frost has passed, water in well, and leave the box to settle for a week before moving it to its final position if you can.

Watering, Feeding and Deadheading

Window boxes dry out faster than any border. In high summer, check daily and water in the morning, when plants can take up moisture before the heat of the day. Water until it runs from the drainage holes. On very hot days, a second evening watering may be needed.

Feed little and often. A diluted liquid tomato feed once a fortnight from June to August keeps flowers coming without encouraging soft, sappy growth. Deadhead regularly — pinching off faded lavender spikes and salvia flowers redirects energy into new buds and stops the display fizzing out by July.

Never use insecticide on a pollinator box. If aphids appear, a strong jet of water or a squirt of diluted washing-up liquid is enough. A few aphids will also feed the hoverfly larvae, which are your allies.

Keeping It Going Through the Year

By late summer, the first flush fades. Tuck in a late-blooming plant such as sedum or a compact aster to carry the box into September, and leave the seedheads of thyme and lavender standing — they feed birds and provide winter shelter for insects.

In autumn, cut back the spent stems but leave the box planted. Move it against a sheltered wall if you can, and reduce watering. Come spring, top-dress with fresh compost, replace anything that has not survived, and start again. A window box is never quite finished, which is part of its charm — and every year it will bring a little more life to the street outside.