Why a tub of water earns its space

Water is the single most effective thing you can add to a small garden for wildlife, and it does not need to be a pond. A glazed pot, a half barrel or a sturdy trough on a patio or balcony will start pulling in insects within days of being filled. Bees and hoverflies land on wet stones to drink. Damselflies patrol the rim. Birds drop in to bathe, and on a warm evening the surface reflects the sky and the whole corner feels calmer than it has any right to.

The catch is that a container is a closed system. There is no soil to buffer it and no stream to flush it, so the plants you choose matter far more than they would in a garden pond. Get the planting right and looking after it takes minutes a week.

Choosing the container and where it sits

Go bigger than you think you need. A container 45cm across and at least 30cm deep holds a steadier temperature, gives roots room and is far less likely to turn green overnight. It must be watertight: no drainage holes, unless you seal them or drop in a rigid liner. Glazed ceramic and plastic-lined pots are the safest choices, as unglazed terracotta can crack in a hard frost.

Site it where it gets five or six hours of sun, with a little afternoon shade if you can manage it, since strong sun feeds algae. Then check the practicalities:

• Stand it on level paving or a bed of gravel, not directly on soil where it will silt up.
• Keep it clear of overhanging trees. Falling leaves rot and feed algae.
• Choose a spot away from the main route where children and dogs charge about.
• Add a ramp of stones or a piece of untreated timber so frogs, hedgehogs and anything else that tumbles in can climb back out.

Native marginals: the plants that do the visible work

Marginals sit in shallow water with their roots submerged and their crowns just above the surface. They flower, they feed pollinators, and they give the feature its shape. Plant them in aquatic baskets so they can be lifted and divided later.

• Marsh marigold (Caltha palustris) opens golden cups in March and April, when almost nothing else is offering nectar.
• Water forget-me-not (Myosotis scorpioides) flowers all summer in soft blue and is a magnet for small bees.
• Brooklime (Veronica beccabunga) stays low and glossy with tiny blue spikes, good for the front edge.
• Water mint (Mentha aquatica) is aromatic, flowers late, and behaves itself in a basket.
• Ragged robin (Lychnis flos-cuculi) brings pink raggedy heads that hoverflies adore.

Two or three species is plenty in a tub. Leave out yellow flag iris and bulrushes, which will burst the space within a single season.

Oxygenators: the invisible engine

Oxygenators live entirely underwater. They release oxygen, soak up the nutrients that algae would otherwise use, and give damselfly nymphs and tadpoles somewhere to hide. You will barely notice them, but without them the water will turn to pea soup.

• Hornwort (Ceratophyllum demersum) has no roots at all, so you simply drop it in.
• Water starwort (Callitriche species) forms neat green mats near the surface.
• Spiked water milfoil (Myriophyllum spicatum) is feathery and establishes quickly.

Roughly one generous bunch for every 20 to 30 litres of water is about right. Thin them in midsummer if they start pressing against the sides.

Floating plants, and what to avoid

Frogbit (Hydrocharis morsus-ranae) is the best native floater for a tub, with small lily-like leaves and white flowers, and roots that dangle where small creatures shelter. Broad-leaved pondweed is another good native option. Keep coverage to about a third of the surface so light and oxygen can still reach the water beneath. If you long for a water lily, choose a dwarf cultivar in a large tub, but accept that most are not native.

Never introduce floating pennywort, water fern, parrot's feather or New Zealand pigmyweed. These are invasive non-natives, illegal to plant in the wild, and they will smother a container in weeks.

Planting up, topping up and keeping it healthy

Use a loam-based aquatic compost, never multipurpose. Multipurpose is too rich and too light: it floats, clouds the water and feeds algae. Line your baskets with hessian if the mesh is wide, then finish the surface with a 2cm layer of washed gravel to stop compost escaping. Lower the planted baskets in gently, add the oxygenators last, and fill with rainwater from a butt wherever you can. Tap water brings chlorine and nutrients with it.

After that, the routine is short:

• Top up in dry spells, little and often, to keep the level stable.
• Twirl a stick through any blanketweed and lift it out rather than reaching for chemicals.
• Remove yellowing leaves and spent flowers to stop them rotting.
• Leave seed heads standing through autumn for insects and seed-eating birds.
• In winter, do not smash the ice, as the shock can kill overwintering frogs and newts. Float a small ball on the surface instead, which keeps a patch clear and lets gases escape.

Do all of that and by next summer you will have bees drinking at the edge, hoverflies working the mint, and quite possibly a frog sitting on a stone, wondering why it took you so long.