Why clear water is a wildlife issue, not just a looks issue

A pond that turns soup-green in June is a common complaint, but the cause is usually a simple imbalance rather than bad luck. Suspended algae and blanketweed thrive on nutrients — nitrates and phosphates — combined with warm water and plenty of sunlight. In a young or overstocked pond, there are simply not enough plants drawing those nutrients out of the water, so the algae get first refusal. The result is murky water that blocks light from submerged plants, starves the pond of oxygen overnight and makes life harder for frogspawn, tadpoles and dragonfly nymphs. Clear water, by contrast, is simply a sign that a working ecosystem is doing its job.

Hornwort: the quiet workhorse of the British pond

Hornwort (Ceratophyllum demersum) is the most useful native oxygenator for most garden ponds. It has no true roots, so it drifts through the water column and takes up dissolved nutrients directly through its feathery leaves. That puts it in direct competition with algae for the same food, and because it grows quickly it usually wins. Its dense stems are excellent cover for tadpoles, newt larvae and small invertebrates, and dragonfly nymphs use it as a hunting platform.

Buy it as loose bunches or weighted bundles and simply drop it in. A handful per square metre of surface area is plenty to start with — it will find its own level. In hard winters it may collapse to the bottom, but generous tips normally survive and regrow in spring. Thin it in midsummer if it fills more than about a third of the water column, and never tip spare pieces into a wild pond or stream, where it can overwhelm small watercourses. If your local garden centre sells non-native oxygenators such as parrot's feather or floating pennywort, leave them on the shelf: both are invasive and illegal to plant in the wild.

Water forget-me-not: pretty, useful and native

Water forget-me-not (Myosotis scorpioides) is the charming counterpart to hornwort's hard graft. A native marginal, it sits with its roots in shallow water and its flowers above the surface from May to September, offering small sky-blue blooms that bees and hoverflies adore. Because it roots in the silt at the pond edge, it draws out nutrients that would otherwise feed algae, and its low mat of foliage provides damp, shaded cover where froglets can hide from predators.

Plant it in an aquatic basket at the margins, with the crown roughly 0 to 10 cm below the waterline, and divide clumps every two or three years so it does not crowd out other marginals. A group of three plants along one sunny edge is usually enough to make a visible difference.

Sun, shade and the balance between them

Most British garden ponds do best with three to five hours of direct sunshine a day, ideally in the morning, followed by dappled shade in the afternoon. Full sun all day warms the water quickly, which encourages algae and can stress fish and amphibians. Deep shade, on the other hand, stunts flowering plants and leaves the water cold and sluggish.

A position that catches morning sun and is screened from the strongest afternoon rays by a hedge, fence or light-canopied tree is close to ideal. Avoid siting a pond under willows, poplars or large conifers, whose roots and heavy leaf fall will make maintenance a misery. Aim for roughly half the water surface to end up covered by foliage — hornwort below, a native water lily suited to your pond's size above, and forget-me-nots fringing the edge.

Planting and seasonal care that keeps water clear

Get the planting right in spring and the pond largely looks after itself.

• April to May: add hornwort once the water has warmed slightly, then plant marginals such as water forget-me-not in baskets of aquatic compost topped with grit.
• Early summer: thin hornwort and divide congested marginals so light and oxygen can circulate.
• Autumn: net out fallen leaves before they sink and rot, which releases nutrients back into the water.
• Winter: leave hollow stems and seed heads standing for overwintering invertebrates rather than cutting everything back.

Top up the pond with rainwater from a butt rather than tap water, which carries its own nutrients and minerals. Never use fertiliser, weedkiller or pesticide anywhere near the pond, and keep fish numbers low — goldfish are nutrient factories, and a wildlife pond often needs no fish at all.

If the water still turns green

New ponds almost always go green in their first spring, and that is simply the ecosystem finding its feet. Given time, and a good density of oxygenating plants, it usually clears by midsummer on its own. If the water stays murky, work through the likely causes: run-off from a lawn or vegetable bed, decaying leaves on the bottom, too many fish, or too little shade across the surface.

A small barley straw bundle can help early in the season, and adding a few more bunches of hornwort costs very little and competes harder for nutrients. Resist the urge to drain and scrub the pond clean — that removes the very life you are trying to encourage, and the algae will simply return. Patience, the right native plants and a little afternoon shade remain the best clear-water tools a British gardener has.