Why a tiny pond earns its space

Water is the single most useful thing you can add to a balcony. A bucket-sized pond will draw in bees looking for a drink, hoverflies and damselflies hunting over the surface, sparrows queuing for a bath, and — if you're lucky — a frog that has somehow tracked it down from three floors below. In built-up areas, clean water is genuinely scarce, so even a modest container becomes a small oasis. It also gives you something lovely to look at while you drink your morning tea.

You don't need much room. A container roughly the size of a washing-up bowl will work; something closer to a half-barrel is better. What matters far more than size is depth, planting and, crucially, an easy way in and out.

Choosing and siting the container

Look for something sturdy, frost-proof and free of drainage holes. Old glazed pots, half-barrels, sturdy trugs, ceramic planters and heavy-duty plastic storage boxes all do the job. Avoid thin terracotta, which splits in a hard frost, and anything that has previously held paint, weedkiller or fuel.

Aim for a water depth of at least 30cm and a volume of 45 to 60 litres. That's enough to buffer temperature swings and stop the water cooking on a hot afternoon.

Weight is worth thinking about. A 60-litre container filled with water, gravel and plants can weigh 70kg or more. Site it in a corner, close to a wall, or above a load-bearing beam rather than in the middle of a suspended timber deck. Once it's full, you won't want to move it.

Position it where it gets four to six hours of sun but is shaded during the fiercest midday heat, and out of the prevailing wind. Somewhere you can reach with a watering can is a bonus, as is a spot near a downpipe where you can park a small water butt.

Filling, planting and what to avoid

Fill with rainwater collected from a butt or an offcut of guttering. Tap water contains nutrients that feed algae, so use it only as a top-up if you have no alternative.

Native plants do the heavy lifting. They support British insects, and they cope with our weather.

• Oxygenators: hornwort, water starwort or curled pondweed. Sink two or three bunches, weighted with a pebble or a lead-free strip, to keep the water clear and give tadpoles cover.
• Marginals: brooklime, water forget-me-not, lesser spearwort and water mint, planted in small baskets of loam and topped with gravel. Water mint spreads, so keep it potted.
• Floating cover: a couple of small plants to shade the surface. Skip the dwarf water lily unless you have a very large container and good depth.

Steer clear of New Zealand pigmyweed, water fern and parrot's feather. They are invasive, spread at alarming speed, and are illegal to plant in the wild. Don't add fish, either — they eat everything else and turn a small pond into a maintenance job.

Making it safe for visitors

This is the part most people miss, and it decides whether your pond becomes a home or a trap. Any creature that goes in must be able to get out, and that goes double for frogs, which will find your balcony even if you're on the fourth floor.

• A gentle ramp: a piece of weathered timber, a rough slate, or stacked stones running from the water surface up to the rim. Keep the angle shallow — around 30 to 45 degrees — and the surface textured so wet feet can grip.
• A pebble beach: a shelf of smooth pebbles just breaking the surface. Bees, hoverflies and butterflies land here to drink without drowning.
• Half-submerged stones: scattered around the edge, they offer footholds at every level.
• A floating island: a wine cork or a small offcut of untreated timber gives insects and amphibians an emergency platform.

Leave a damp, shaded corner with a few fallen leaves and some moss or bark. Frogs need somewhere cool and moist to sit between dips, and they'll use it far more than the water itself.

Keeping it clear through the year

A new pond often goes green for the first fortnight or so. That's normal — it settles once the plants get going, so resist the urge to empty and start again, and never reach for algaecide.

Through the seasons, the routine is light:

• Spring: thin overcrowded oxygenators, scoop out dead leaves, top up with rainwater.
• Summer: top up weekly in hot spells, keeping at least half the surface open. Twirl blanketweed out with a stick and leave it on the rim overnight so any trapped creatures can crawl back.
• Autumn: net out falling leaves before they sink and rot.
• Winter: if it freezes, never smash the ice — the shock can kill frogs resting below. Float a ball or a sealed bottle to keep a small gap open.

Do those small things and your balcony pond will quietly become the busiest square metre on the whole building.