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Almost every new pond turns pea-soup green in its first spring, and it is rarely a sign that you have done anything wrong. The culprit is suspended algae — single-celled plants so small they drift in the water column and tint it like weak tea. They feed on dissolved nutrients, chiefly nitrogen and phosphorus, which arrive from fish waste, rotting leaves, tap water top-ups and fertiliser washing off the surrounding lawn.
Add strong spring sunlight and warming water and you have perfect growing conditions. The reassuring news is that a new pond usually clears itself within four to six weeks as submerged plants establish, take up nutrients and shade the water. Panicking and reaching for a bottle is often what turns a temporary phase into a recurring problem.
It helps to know which algae you have. Green water is suspended algae. Blanketweed is the stringy stuff you can twirl around a stick. They need slightly different responses, but both are managed by the same principles: less light, fewer nutrients and more competition from higher plants.
Aim to have somewhere between half and two-thirds of the water surface shaded at midday. Water lilies are the classic answer — plant them in a basket of heavy loam, top with gravel, and they will cover a third to a half of a modest pond within a couple of seasons. Choose a variety suited to your depth, as some need 60cm or more while dwarf types are happy in 20–30cm.
Marginal plants do the same job around the edges. Yellow flag iris, marsh marigold, water mint and flowering rush all draw nutrients from the water and cast useful dappled shade. On the south side of the pond, a shrub or a clump of tall grasses will knock out the fiercest midday sun without turning the whole garden into a dark corner.
Floating plants are excellent shade-makers, but check before you buy — some, such as parrot's feather, are invasive and must never be released into the wild. If in doubt, ask at a reputable aquatic nursery for native or non-invasive options.
Barley straw remains the gentlest algaecide available to the gardener, and it is well worth using properly. As the straw breaks down in oxygen-rich water it releases humic substances and small amounts of hydrogen peroxide, which inhibit new algae growth without troubling fish, frogs or newts.
The details matter. Use barley straw, not hay — hay feeds the problem rather than solving it. Allow roughly 50g per square metre of pond surface, loosely packed in a mesh bag so water can flow through it, weighted down and sited near a pump or waterfall where circulation is best. Add it in spring once the water reaches about 10°C. It takes two to four weeks to become active and keeps working for four to six months.
Start before the bloom, not after it. Barley straw prevents algae rather than clearing an existing soup, so put it in as the frogs begin to spawn. Lift and replace the bag when the straw has turned dark and soft.
A healthy pond is a living filter. Nitrifying bacteria convert fish waste from ammonia into nitrite and then into far less harmful nitrate, which plants happily absorb. Other microbes digest sludge on the bottom. Both groups need oxygen and a surface to cling to, which is why a small pump, a trickling waterfall or even a solar fountain makes such a difference to clarity.
You can buy bacterial starters, and they are genuinely useful in a new pond or after a clean-out, but you can also encourage them for free. Never empty and scrub the whole pond — leave a few centimetres of sediment and keep filter sponges rinsed in pond water rather than tap water, which kills the very bacteria you are trying to keep.
Copper-based algaecides, blue dyes and blanketweed treatments will give you a briefly clearer pond and a long list of casualties. Copper is toxic to amphibians, fish and the small invertebrates that make up the pond food web. Killing algae also strips oxygen from the water as it decomposes, which is exactly when frogs and newts are most vulnerable.
Worse, these products often destroy the tiny grazing creatures that keep algae in check, so the bloom returns harder a few weeks later. A little algae is not a failure — tadpoles graze on it, and it is part of a working pond. If green water persists despite shade, plants and barley straw, look for the nutrient source rather than the symptom. Fix the cause and the clarity follows.
Making Beck shares practical, down-to-earth guidance on garden water features: designing, building and caring for streams, ponds and waterfalls. for readers across the UK.
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