A problem almost every new pond shares

If you have dug a pond this spring and it has turned the colour of pea soup, take heart. Green, murky water is not a sign that you have done something wrong. It is the normal behaviour of a young pond finding its balance. In the first year or two, nutrients are plentiful, the microscopic algae that feed on them multiply fast, and the plants, insects and other creatures that would normally keep those nutrients in check have not yet established themselves.

That said, murky water is worth understanding rather than simply tolerating. The trick is to work with the pond's natural processes instead of reaching for a bottle of chemicals, which can strip the oxygen from the water and leave you with a clear but lifeless pond. There are gentler, more reliable routes to clarity, and most of them cost very little.

Work out what is actually clouding the water

Before you treat anything, decide what sort of murk you have. It makes a real difference to what you do next.

• Green water is suspended algae, so fine it passes through a net. It usually peaks in spring and early summer.
• Brown or grey cloudiness is normally silt or soil disturbed by fish, birds or heavy rain running in from the surrounding ground.
• Blanketweed is the stringy, hair-like algae that clings to plants and rocks. It thrives on the same nutrients as green water.

All three trace back to the same root cause: more nutrients in the water than the pond's plants can use. Reduce the surplus and the water clears itself.

Barley straw, used at the right time

Barley straw is the best-known traditional remedy, and it does work, but only if you use it properly. As the straw breaks down it releases compounds that inhibit the growth of new algae. It does not kill algae already present, so it is a preventative rather than a cure.

• Use roughly 50 grams per square metre of pond surface, loosely packed into a mesh bag or old netting so water can flow through it.
• Place it where water moves, such as near a pump outlet or a shallow edge, rather than in a still corner.
• Put it in from April to June, before the algae gets going, and replace the bag every four to six months.
• Never add it in winter. Cold, still water holds little oxygen, and rotting straw can use up what is left.

Give it four to six weeks to take effect. If the pond is small, half a bag of straw in a water butt of rainwater, left to steep, can be poured in as a mild liquid dose.

Plants do the heavy lifting

The single most effective long-term fix is a well-planted pond. Plants compete with algae for the same dissolved nutrients, and shading cuts the light that algae need to grow.

• Oxygenators grow below the surface and are the workhorses. Native hornwort and spiked water-milfoil are excellent choices. Aim to cover around a third of the pond floor.
• Floating plants such as water lilies and frogbit shade the water. Keep them to about half the surface at most, leaving open water for birds and insects.
• Marginal plants like iris, reeds and marsh marigold draw nutrients up through their roots and give wildlife somewhere to shelter.
• Avoid invasive non-natives such as floating pennywort and parrot's feather, which will take over and are illegal to allow to spread into the wild.

If you must use tap water to top up, remember it can carry nitrates and phosphates that feed algae. Rainwater from a butt is always the better choice.

Feed the fish sparingly, and never in cold weather

Overfeeding is one of the most common causes of green water in garden ponds, and the easiest to fix. Every uneaten flake of food sinks and decomposes, releasing exactly the nutrients algae love.

• Feed only what the fish will clear in two to three minutes, once a day at most in warm weather.
• Stop feeding entirely when the water is below about 10°C. Fish barely digest food in the cold and the rest just rots.
• Net out any uneaten food rather than leaving it to sink.
• Keep fish numbers modest. A heavily stocked pond will always struggle to stay clear, however careful you are.

Small habits that keep nutrients out

Clarity is as much about what you keep out as what you add. A few simple routines make a genuine difference.

• Never use lawn feed, weedkiller or fertiliser within a few metres of the pond, and check that run-off does not flow towards it.
• Net the surface in autumn to stop fallen leaves rotting on the bottom.
• Remove dead and decaying plant material in spring rather than leaving it in the water.
• Top up with rainwater, and avoid changing the water wholesale, which unsettles the pond's chemistry.
• Resist the urge to scrub everything clean. A little silt and algae on the sides is food and habitat for the creatures that keep the pond healthy.

Most new ponds settle within two or three summers once plants, insects, amphibians and the microscopic life that grazes on algae have all arrived. Patience, planting and a light hand with the fish food will do far more for your water than any chemical ever could, and the frogs, damselflies and newts will thank you for it.