Keeping a Garden Pond Clear Without Chemicals

Why Clear Water Isn't Just About Looks

Green, soupy water is the most common complaint from anyone who has dug a pond in a British garden, and it is worth saying at the outset that a little algae is not a fault. Algae sits at the bottom of the food chain, and it is part of the reason tadpoles, water fleas and dragonfly nymphs find your pond worth visiting at all. The trouble begins when nutrients outrun the plants that would otherwise mop them up, and the algae takes over. Chemical treatments will clear the water for a fortnight and leave you with a pond that is biologically poorer, so it is far better to build a balanced little ecosystem and let it settle. Here is how to do it.

Start With Native Oxygenators

Submerged oxygenators are the workhorses of a clear pond. They draw nitrogen and phosphate straight out of the water, compete with algae for light, and provide cover for the small invertebrates that graze on algae. Choose British natives and plant them in spring or early summer, when the water has warmed a little.

• Hornwort — often sold simply as a bunch, it has no true roots and can be dropped straight in, or weighted with a clean stone.
• Spiked water-milfoil — feathery and excellent at nutrient uptake, though it can spread, so thin it out each summer.
• Water starwort — a neat, bright green native that thrives in shallow margins and quiet corners.
• Curled pondweed — an undemanding native that seems to shrug off the murkier end of a garden pond.

Plant roughly one bunch per square metre of water surface, and expect them to die back in winter — that retreat is entirely normal and not a sign you have failed. Never strip them all out in autumn in a fit of tidiness; the remaining stems still shelter overwintering wildlife.

Marginals and Floating Leaves Do the Rest

Oxygenators work below the surface, but the plants that do the heaviest lifting against algae are the ones growing in baskets along the shelf, plus the floating leaves that shade the water. Together they form a sort of living filter, absorbing dissolved nutrients before algae ever gets a look in.

• Marsh marigold, water forget-me-not and brooklime for early flowers and frog cover.
• Water mint, purple loosestrife and flowering rush for scent and late-season interest.
• Yellow flag iris where you have room — handsome, but a thug in a small pond.
• A water lily or broad-leaved pondweed for floating shade. The native white water lily needs a generous pond, so choose a smaller lily for a barrel or half-barrel.

Aim for somewhere between half and two-thirds of the surface covered by leaves at midsummer, and always leave a patch of open water. Dragonflies and damselflies need clear water to lay their eggs, and birds need somewhere to bathe.

Use Shade Rather Than Chemicals

Sunlight is the fuel that drives algae, so the single most effective thing you can do is limit it. A pond in full glare from ten in the morning until six at night will always fight you. Site new ponds so they catch sun for part of the day and lie in dappled shade for the rest, ideally with shade arriving from the south or west.

If your pond is already built, you can create shade with a step of marginal planting on the sunny side, or by letting a shrub or small tree grow up nearby — an alder, a willow or a hazel all work well. Just keep leaves out of the water in autumn, because rotting leaves are pure nutrient for next spring's algae. A net stretched over the pond for a few weeks, or a brisk skim with a rake, does the job.

Top Up With Rainwater, Not Tap Water

Evaporation in a warm summer can drop a pond by several centimetres a week, and the obvious fix — a hose — is the one that quietly feeds the problem. Tap water carries chlorine and, more importantly, nitrate and phosphate that algae will gratefully devour. Letting a bucket stand overnight disperses the chlorine, but it does nothing about the nutrients.

• Fit a water butt to a shed or greenhouse and use that as your default top-up source.
• A downpipe diverter is cheap and keeps the butt full without overflowing in a downpour.
• If tap water is unavoidable, add it in small quantities during cool weather rather than pouring in a large volume in a heatwave.
• Avoid softened water, grey water and anything that has passed through a water softener, as the sodium does plants no favours.

Small Habits, and a Little Patience

Nutrients arrive from many directions, so it pays to keep an eye on the surroundings as well as the water. Do not fertilise borders within a couple of metres of the pond, keep lawn feed well away, and never use potting compost in planting baskets — a loam-based soil topped with gravel is far leaner. If you keep fish, feed them only what they clear in a few minutes.

Blanketweed, the stringy green nuisance, can be twirled out with a cane or a fork and left on the soil beside the pond for a day so that anything caught up can crawl back. A mesh bag of barley straw, floated in spring and removed after about six months, is a long-standing British remedy that helps in some ponds and not others. Silt should be cleared only every few years, not annually, and the dredgings left at the edge for twenty-four hours before you compost them.

Most of all, give it time. A new pond usually goes green in its first spring, and then clears as the plants establish and the invertebrates move in. That first murky season is not failure — it is a young pond learning to look after itself.