Managing algae in garden water features

Why algae takes hold in the first place

Algae needs two things above all else: light and nutrients. Every pond, stream and waterfall has both, which is why a certain amount of algae is perfectly normal. The trouble starts when the balance tips — when more nutrients arrive than your plants can absorb, or more sunlight reaches the water than the surface can cope with.

In a garden pond you will usually meet one of two characters. Suspended single-celled algae turns the water a murky pea green, often within days in spring. Filamentous algae, better known as blanketweed, arrives as long green strands that tangle round pumps and float up in ropes on warm afternoons. Both respond to the same treatment: cut the food supply and cut the light. Chemicals rarely solve it for long, and they often make things worse.

Cut the nutrient supply at the source

Nutrients come from you, your garden and the weather. Work through this list honestly and you will usually find the culprit.

• Fish food is the single biggest contributor in a stocked pond. Feed only what the fish clear in a couple of minutes, once or twice a day in summer, and far less as the water cools. Scoop out anything uneaten.
• Decaying matter — fallen leaves, grass clippings, dead lily pads, sunken plant debris — releases phosphate and nitrate as it rots. Skim the surface weekly and net the bottom in autumn before it breaks down.
• Run-off from lawns, borders and compost heaps carries fertiliser straight into the water. Leave a buffer of gravel or dense planting around the edge so it soaks away first, and keep lawn feed well away from the pond.
• Tap water top-ups can add nutrients and phosphates. Rainwater from a butt is better, though even that picks up whatever is on the roof. Top up little and often rather than in one big gush.
• Overstocking simply means more waste than the system can process. Fewer, well-fed fish make for clearer water.

Plant for shade and competition

Plants are your best long-term ally. Aim to cover roughly half to two-thirds of the surface by midsummer with floating foliage, which shades the water below and starves algae of light. Hardy water lilies are the classic choice, but water hawthorn, floating heart and frogbit all do the job in smaller ponds. Check before buying anything vigorous — some floating plants spread far too readily and are best avoided in the UK.

Submerged oxygenators such as hornwort and water crowfoot take nutrients directly out of the water, competing with algae for the same food. Around the margins, irises, marsh marigold, sedges and rushes shade the edges and use up nutrients through their roots. If the pond sits in full sun all day, plant shrubs or tall perennials on the southern side to cast dappled shade — just avoid anything that drops heavy leaf litter in autumn.

Movement, aeration and grazing

Stagnant water favours algae, so keep the water moving. A stream or small waterfall introduces oxygen and discourages the still, warm conditions blanketweed loves. Check for dead spots where the flow does not reach, and consider a simple bog filter or planted gravel filter if your pond is heavily stocked — it is one of the most effective nutrient-stripping systems you can build yourself.

A UV clarifier will clear suspended green water by clumping algae so a filter can catch it, but it does nothing for blanketweed. Barley straw, added as a bale or an extract in early spring before algae gets going, releases compounds that gently inhibit growth. It is a slow preventive, not a rescue remedy.

Encourage the grazers too. Tadpoles, water snails and daphnia all nibble algae, so leave a little natural cover and resist the urge to keep the pond pristine.

Removing what is already there

Twirl blanketweed out with a rough stick, a garden rake or your hands, turning it like candy floss, and compost it well away from the water. Do this little and often. Pulling out huge mats in one go disturbs everything and releases the very nutrients you are trying to remove.

Be wary of algicides. When a large mass of algae dies at once, the decomposition strips oxygen from the water and can kill fish, newts and other wildlife overnight. If you feel you must treat, do it in sections, only in well-aerated water, and never during hot, still weather.

A simple seasonal routine

• Spring: net out debris, divide and repot congested plants, top up with rainwater, add barley straw, and begin feeding sparingly.
• Summer: skim weekly, feed lightly, remove blanketweed often, keep the water level steady and check the pump is running clean.
• Autumn: cover the pond with netting if trees are nearby, cut back marginals, and lift out rotting foliage before it sinks.
• Winter: stop feeding once the water drops below about 10°C, keep a small area of ice open by resting a pan of hot water on the surface, and leave plant cover in place for wildlife.

New ponds take a season or two to settle, and every pond has a green week or two in spring. Keep the nutrients out, shade the surface, keep the water moving, and clarity will come — usually without a single bottle of chemicals.