Why a Spring Window Box Earns Its Keep

A window box is the smallest garden most of us will ever own, and somehow it works harder than any of them. Even a modest box — say sixty centimetres long and twenty deep — gives you room for three distinct jobs: something to trail over the edge, something upright to give the whole thing height and shape, and something generous and scented to fill the gaps in between.

Plant in early spring, or in autumn the year before if you are organised, and you will have colour from March right through to the end of May. The combination that never fails is trailing ivy, upright tulips and wallflowers. It is a classic for good reason: the ivy softens the box, the tulips lift the eye upwards, and the wallflowers bring that honey-and-clove scent that drifts in every time you open the window.

The Three Plants That Do the Work

Trailing ivy gives you a green framework that looks good before the flowers arrive and after they have gone. Choose a small-leaved variety such as Hedera helix 'Glacier' or 'Ivalace' rather than the rampant large-leaved forms. A single plant at each end is usually plenty, and you can trim it with scissors whenever it starts to look straggly.

Upright tulips provide the vertical interest. In a shallow box, height is your friend, but so is a bit of restraint: look for mid-season varieties with sturdy stems, or the shorter species tulips if your box sits in an exposed, windy spot. Plant them in a block rather than a line, five to seven bulbs in a sixty-centimetre box, so they read as a clump rather than a row of lonely soldiers.

Wallflowers (Erysimum cheiri) are the workhorses. They flower for weeks, come in russet, apricot, gold, deep red and cream, and smell wonderful. They are biennials, so you either buy them as plants in spring or sow them in late summer for the following year. Two or three plants will fill out the middle of the box within a fortnight.

Drainage: The Bit Most People Skip

Nearly every window box failure I have seen has been a drainage failure. Tulip bulbs in particular will rot in waterlogged compost, and wallflowers sulk badly if their roots sit wet through winter. Sort this out before a single plant goes in.

• Drill or enlarge drainage holes — aim for one every fifteen centimetres along the base, plus a couple at the ends.
• Cover the holes with broken crocks or a layer of coarse gravel so compost does not wash through and block them.
• Add a two-centimetre layer of grit or gravel at the bottom as a reservoir-free drainage zone.
• Use a loam-based compost such as John Innes No. 2, mixed roughly three parts to one with multipurpose and a good handful of horticultural grit.
• Raise the box on small feet or battens so water can escape freely and does not stain the sill.
• If you have a tray beneath, empty it after heavy rain. Standing water is the enemy.

Planting Up, Layer by Layer

Start with the bulbs if you are planting in autumn. Set them pointed end up, about ten centimetres deep and roughly ten centimetres apart. If you are planting potted tulips in spring, treat them gently — they resent root disturbance — and set them at the same depth they were growing in their pots.

Next, position the wallflowers in the middle ground, firming the compost around their roots with your fingers. Finally, tuck the ivy at the two front corners, angling the trailing growth outwards over the rim so it cascades rather than tangles. Leave about two and a half centimetres of clear space below the rim: this is your watering channel, and without it every drink will simply run off onto the pavement.

Water everything in thoroughly, then top up with a thin layer of grit or gravel. This keeps the compost off the leaves, suppresses weeds and stops squirrels from treating your box as a larder.

Watering and Care Through the Season

Window boxes dry out far faster than borders. They are up in the air, exposed on several sides, and often sheltered from rain by an overhang. In a dry spell, plan on watering once a day, and twice a day during warm, breezy weather. Water slowly until it runs out of the drainage holes — a quick splash does nothing but dampen the surface.

Once the wallflowers and tulips start forming buds, switch to a high-potash liquid feed, diluted as the bottle directs, every ten to fourteen days. Deadhead the wallflowers regularly; they will keep producing new flowers for weeks if you do not let them set seed. Tulips do not need deadheading for flowering, but removing the faded petals keeps the display tidy and prevents botrytis in damp weather.

Keeping the Display Going

Late frosts are the one thing that can catch you out. Keep a piece of fleece or horticultural fabric to hand and drape it over the box on cold nights in April — it makes a real difference to tender new growth.

When the tulip flowers finally go over, resist the urge to cut the foliage straight away. Leave it for six weeks so the bulb can build up reserves, then either lift the bulbs and dry them for next autumn, or tip the whole lot out and replant with trailing verbena, bacopa and a few pelargoniums for summer. Either way, you will have had four good months of colour from a box no bigger than a windowsill — which is a very decent return on a morning's work.