Our cities are getting denser, and the private garden is shrinking with them. Where a Victorian terrace once commanded a long strip of lawn, new-build flats often offer a courtyard the size of a parking bay, or a balcony barely wide enough for a folding chair. It is tempting to assume that wildlife gardening belongs to those with mature oaks and a wildflower meadow. In practice, the opposite is often true. A well-designed small space can punch well above its weight, because the insects it supports do not need acreage — they need the right conditions, arranged intelligently and maintained with a light touch.

Start with structure, not colour

Most small gardens fail at the first hurdle because they are planted for a single season. The instinct is to fill a courtyard with flowering pots in May, watch it blaze through August, and then leave it bare until the following spring. Pollinators, birds and invertebrates need continuity, and that means thinking in layers.

Think of a compact garden as having four storeys:

• The canopy: even a 3-metre space can take a small tree in a pot — a crab apple, a rowan or a hazel on a dwarfing rootstock. These give blossom for early bees, berries for thrushes and a perch for sparrows.
• The shrub layer: climbers and medium shrubs. A north-facing wall is ideal for pyracantha or ivy, both of which flower late and fruit reliably. Ivy is arguably the single best plant for urban wildlife, offering nectar in October when little else is open.
• The herbaceous layer: perennials with open, daisy-like flowers — hardy geraniums, echinacea, rudbeckia, lavender. Single-flowered varieties are far more useful than doubles, whose petals block access to nectar.
• The ground and litter layer: ground cover such as bugle, wild strawberry or ajuga, plus a deliberate patch of undisturbed leaf litter where beetles and overwintering larvae can shelter.

If you achieve all four layers in a courtyard, you have effectively recreated the structural complexity of a hedgerow in miniature.

Water, however small

Nothing transforms a small garden's wildlife value faster than a pond. Research into garden ponds in the UK has repeatedly found that even a modest one — a sunken washing-up bowl will do — supports more invertebrate diversity per square metre than most other features. The crucial requirement is a shallow edge, created with stones or a gently sloping ramp, so that hedgehogs, frogs and thirsty birds can get in and out without drowning.

If space or safety rules out open water, a bird bath is a worthy substitute, provided it is refreshed regularly and positioned near cover so that birds can dash to safety. Moving water — a simple solar-powered bubbler — deters mosquitoes, which breed only in still water, and the sound draws birds from some distance.

Group water with your planting rather than isolating it. A bird bath surrounded by lavender and a dense shrub is a habitat; the same bath on a bare patio is merely ornament.

The importance of gaps and neglect

One of the more counter-intuitive lessons of urban ecology is that tidiness is the enemy. Seal every fence panel, mortar every brick joint and sweep every corner, and you eliminate precisely the microhabitats that small creatures depend on.

In practice, this means allowing a few deliberate imperfections:

• A gap at the base of the fence. A 10 centimetre opening — often hidden behind a shrub — allows hedgehogs to move between gardens. In many towns these gaps are now the only remaining connectivity between properties, and a hedgehog's nightly range can cover a dozen gardens.
• Bare soil patches. Solitary bees, which make up the vast majority of our bee species, need exposed ground to nest. Leave a sunny, undisturbed square of soil, or a south-facing bank of sand, and they will find it.
• Dead wood. A small log pile tucked behind a pot provides shelter for beetles, centipedes and overwintering ladybirds. There is no need to make it pretty.
• Seed heads left standing. Teasels, fennel and sedum left over winter feed goldfinches and provide hollow stems for stem-nesting bees.

None of this requires abandonment. A single hidden corner of managed untidiness is enough, and it can be screened by a pot of grasses from the seating area.

Choosing plants that earn their place

In a small plot, every plant must work harder. Go for long-flowering, nectar-rich species with recognised wildlife value rather than whatever is on offer at the till. Salvia, catmint, oregano, thyme, foxgloves, hellebores and sedum all flourish in containers and earn their keep across the seasons. Avoid bedding plants such as begonias and petunias, which offer almost nothing to insects despite their brightness.

Berries matter as much as flowers. If you have room for two or three shrubs, choose between them on the basis of what they provide in autumn and winter — callicarpa, cotoneaster and honeysuckle all bridge the hungry gap. For very tight spaces, fruit trees grown as fans or espaliers against a wall deliver blossom, fruit and structure in a single footprint.

Finally, resist the temptation to clear everything in autumn. Tidy the paths, remove diseased foliage, and then step back. A garden that looks slightly unkempt in November is a garden that will hum in June.