Why a heatwave tests every pond

Warm water holds less dissolved oxygen than cool water, so the first thing to go wrong in a long hot spell is the invisible chemistry. At the same time, evaporation is quietly lowering the water level, and every centimetre lost means less volume to buffer any problem. Algae, which has been sitting quietly all spring, suddenly finds the conditions it has been waiting for and grows explosively. Fish and amphibians, meanwhile, are more active and therefore using more oxygen just when there is less of it about.

None of this is a reason to panic. A few simple habits, kept up through July and August, will carry most ponds through the worst of it without fish gasping at the surface or frogs abandoning ship.

Top up with rainwater, not tap water

Rainwater is the single best thing you can put in a pond. It is soft, roughly the right pH, and free of the phosphorus and nitrogen that feed algae. Tap water, by contrast, brings nutrients with it, along with chlorine or chloramine that will stress fish and amphibians. If you can, keep a water butt topped up through the winter and spring so you have a reserve ready when you need it.

Practical points worth knowing:

• Top up in the cool of the evening rather than the middle of the afternoon, so less is lost to evaporation straight away.
• Add no more than a couple of inches at a time. Big, sudden changes in temperature and pH upset fish more than a slightly low water level does.
• Never let the level fall more than about 10cm, especially in a small pond. That is when a pond starts to feel crowded and oxygen-poor.
• If you have no rainwater and must use tap water in an emergency, treat it with a dechlorinator first. Standing tap water for 24 hours only helps with plain chlorine, not chloramine.
• Check the level in the morning. Losing a couple of centimetres a day in a genuine heatwave is normal; losing much more may mean a leak.

Shade the surface — aim for about half

If roughly a third to a half of your pond surface is covered by floating leaves, you will noticeably cool the water beneath and slow algae down. Water lilies are the classic choice, but water hawthorn, floating heart and even a few well-placed marginal plants in pots will do the job. If your pond is too deep or too small for lilies, a shade net stretched over part of the water during the hottest hours works perfectly well as a temporary measure.

Do leave some open water. Dragonflies and damselflies need it for egg-laying, bees and hoverflies come down to drink at the edges, and a fully covered pond becomes stale and airless.

Tackle blanket weed before it takes over

Blanket weed — those long, slippery green strands — loves warm, nutrient-rich water. Twirl it out with a cane or a rough stick, then leave what you have pulled on the side of the pond for a day so that any trapped tadpoles, newts and water snails can crawl back in before you compost it.

Please resist chemical algicides. They kill oxygenating plants along with the algae, and the mass of dead matter that follows strips yet more oxygen from the water at exactly the wrong moment. Better approaches:

• Scoop out fallen leaves, dead plant material and uneaten fish food. Decay is what feeds the algae.
• Avoid feeding fish heavily in hot weather. Their appetite drops, and uneaten pellets rot.
• Never use garden fertiliser, or let lawn feed or weedkiller run off, anywhere near the pond.
• Keep topping up with rainwater to avoid adding nutrients.
• Plant more oxygenators such as hornwort, cabomba and elodea, and more floating plants to out-compete the algae for light and nutrients.
• Skim duckweed regularly with a fine net before it carpets the surface; a little is fine, a solid mat is not.

Barley straw is a useful preventative in spring, but it needs oxygen to work, so it is not much help in the middle of a heatwave.

Keep oxygen levels healthy

Oxygen is lowest in the early morning, after a night in which plants have been consuming it rather than producing it. If you see fish gulping at the surface, or amphibians huddled at the edge looking listless, you have a problem. The remedy is movement: a small solar fountain, a pump with a waterfall, or simply an air pump with an air stone run through the night will make a real difference. Keep any pump and filter clean, since a clogged unit is doing very little.

Also remove dying plant leaves before they rot, and avoid stirring up the bottom of the pond in hot weather — the sediment there is deoxygenated and best left alone.

Look after the wider wildlife

A pond in summer is a lifeline for far more than its own inhabitants. Hedgehogs, thirsty birds, bees and wasps all come looking for water, and many will drown in a steep-sided pond. Keep at least one gently sloping edge or a ramp of stones, and put out a shallow dish of water with a few pebbles in it at ground level for hedgehogs and a bird bath for the birds.

Long grass and dense planting around the margins keep the water cooler and give frogs, newts and insects somewhere to shelter. If your pond does dry out completely, don't despair — many British pond species are adapted to it, and the pond will recover with the autumn rain. Keeping the deepest section topped up with rainwater through the worst weeks gives everything the best chance of coming through the summer in good shape.