Start with the Cause, Not the Cure

Every British pond keeper knows the two faces of the same problem. One spring morning the water is clear and full of tadpoles; a fortnight later it is either strung with green knitting or turned the colour of pea soup. Neither is an invasion. Both are symptoms.

String algae — the blanket weed most of us wrestle with — is a filamentous green alga, mostly Cladophora, that anchors itself to stones, liner and plant stems. Green water is something different: billions of single-celled algae suspended in the water column, blooming because nothing is eating them quickly enough. Both are responding to the same two things — light and dissolved nutrients — and both can be managed by taking those away and putting something else in their place.

Pouring in algicide is the one approach that reliably makes things worse. It kills the algae, which then rots, releasing the nutrients it absorbed straight back into the water and feeding the next bloom. It also takes out the grazers and the oxygenators that were quietly doing the work for you.

Shade Is Your Cheapest Algae Treatment

Light is the throttle. In an open, sunlit pond you are effectively running an algae farm, and no amount of netting will keep up. Aim for between half and two-thirds of the surface shaded by midsummer, keeping a patch of open water for birds, frogs and dragonflies to use.

• Plant a native water lily such as Nymphaea alba or the smaller 'Alba Minor' in the deepest section. Its floating leaves shade the water beneath while offering cover for fish and emerging insects.
• Add floating natives like frogbit and water soldier to smaller ponds. Both spread readily, so thin them with a rake when they cover more than you want.
• Shade the south and west edges with marginals — yellow flag iris, purple loosestrife, marsh marigold — or with a hedge planted a couple of metres back from the liner.
• Avoid deciduous trees directly over the pond. Leaf fall is a nutrient delivery service, and it arrives just as the pond is slowing down for winter.

Depth matters too. A pond that is at least 60cm deep in the middle stays cooler and more stable than a shallow saucer, and cooler water holds less algae-friendly conditions.

Crowd the Water with Competition

Algae thrive in the gaps. Fill them. Oxygenating plants grow fast, take up nitrogen and phosphate from the water, and release oxygen at the same time — the opposite of what a dying algal bloom does.

Good British natives include hornwort (Ceratophyllum demersum), water starwort and curled pondweed. Hornwort has the advantage of having no true roots, so it can simply be bundled and weighted to sink into the pond, and it does much of its nutrient uptake straight from the water. Plant generously — a carrier bag's worth in a modest pond is not excessive — and expect to thin it in high summer by the same token.

Buy from a reputable aquatic nursery and never lift plants from the wild or from a canal. Invasive non-natives such as floating pennywort, Australian swamp stonecrop and water fern are doing enormous damage to British waterways, and a pond is the perfect launchpad for them.

Put Grazers to Work

This is where a wildlife pond outshines a fish pond. Daphnia, the water fleas that filter green water out of the column, breed explosively in a pond without fish — and are eaten to extinction in one with them. If pea-soup water is your problem, keeping fish is the problem. A fish-free pond often clears itself within a fortnight once daphnia establish.

Alongside them, encourage water lice, ramshorn and bladder snails, freshwater shrimp, tadpoles and the larvae of mayflies and caddis. They graze algae from every surface, and they are all native pond wildlife in their own right. A handful of sticklebacks will help with string algae in a larger pond, though they will also take their share of tadpoles.

Starve the Bloom: Nutrients and Barley Straw

Nutrients arrive from rainfall runoff, garden fertiliser, fish food, rotting leaves and tap water. Tap water in much of Britain carries both nitrate and phosphate, which is why filling a pond from the hose is a slow-motion algae feed. Top up with rainwater from a butt instead, and never let lawn feed or compost heap runoff reach the pond.

• Net the surface in autumn and lift leaves out before they sink.
• Feed fish sparingly, if at all, and never more than they clear in a few minutes.
• Cut back and remove — rather than leave — excessive oxygenator growth, so the nutrients leave the pond with the plants.
• Twist string algae out with a stick or rake in spring. Pull it out, don't let it die in the water.
• Add barley straw in early spring, roughly 25–50g per cubic metre, loosely bundled and floating where water flows past it. It works best as a preventative in well-oxygenated water, not as a rescue.

A Season of Small Habits

Natural algae control is not a product you add; it is a balance you nudge. Aim for a pond where plants, grazers and shade hold the algae at a dull background level, and accept that a flush of blanket weed in April and a greenish tinge in June are normal parts of a living pond. Clear the worst of it by hand, keep the nutrients out, and let the wildlife do the rest. By late summer a well-planted British pond should be clear enough to see the newts.