Why chemical clarity comes at a cost

There is a moment, usually in late April, when a pond that looked pristine in March turns into something resembling pea soup with knitting in it. The instinct is to reach for a bottle. Please don't. Almost every algaecide, dye or "pond clear" treatment works by killing something, and in a wildlife pond that something is rarely just the algae. Copper-based products in particular linger in the sediment and are toxic to the mayfly nymphs, water fleas and snails that tadpoles, newts and dragonfly larvae feed on. You can end up with clear water and a pond that is, quietly, empty.

The reassuring news is that a well-planted wildlife pond largely clears itself. The measures that help are pleasant ones: a little planting, a little shade, and half an hour here and there with a bamboo cane and a fork.

Know your algae before you act

Not every green problem is the same, and treating them differently saves a great deal of frustration.

• Blanket weed (filamentous algae) forms long, slippery green strands in spring and early summer. It thrives on dissolved nutrients and bright, unshaded water.
• Green water is made of millions of single-celled algae. It is almost inevitable in a new pond in its first spring, and usually settles by midsummer as plants establish and graze it down.
• Duckweed is a tiny floating native plant, not an alga, but it spreads fast in nutrient-rich water. Because it floats, blanket-covering the surface, it starves submerged plants of light.

A new pond in its first year should be left alone to find its balance. Intervening too eagerly in that first season tends to make things worse, not better.

Plant for clear water

Plants are the cheapest and most effective filtration you will ever install. Aim for roughly half the pond surface covered by foliage by midsummer.

• Submerged oxygenators: hornwort, water milfoil and water crowfoot grow in the water and compete directly with algae for nutrients.
• Floating and deep-water plants: a native water lily, or frogbit in a smaller pond, casts the shade that keeps algae in check.
• Marginals: marsh marigold, water forget-me-not, ragged robin and yellow flag iris around the edges. Grow iris in a basket, as it can be thuggish in a small pond.
• What to avoid: non-native invaders such as parrot's feather, water fern and New Zealand pygmyweed. These spread alarmingly and are far harder to remove than any algae.

Shade, margins and a wildflower buffer

Sunlight is the algae's chief ally. Around 50 to 70 per cent surface shade by midsummer is a good target, achieved with plants in and around the pond rather than by planting a willow on the bank. Trees drop leaves, and leaves rot, and rotting leaves feed the algae you are trying to discourage.

What surrounds the pond matters too. If your wildflower meadow or long grass runs right up to the water's edge, that is ideal. Leave a metre or two of unmown, unfertilised margin as a buffer. Never use fertiliser, weedkiller or slug pellets anywhere near the pond; a single heavy rain can carry them straight in. A wildflower margin of knapweed, oxeye daisy and birds-foot trefoil gives frogs somewhere to hunt and gives you something lovely to look at while you are doing the weeding.

Removing blanket weed by hand — the right way

This is the heart of it, and it is genuinely satisfying work.

• Twirl the strands around a cane, a long fork or a garden rake, lifting them out like candyfloss.
• Work little and often from April to June, rather than in one heroic session.
• Never strip the pond bare. Take out no more than a third to a half at a time; removing everything at once causes an oxygen crash and releases nutrients back into the water, triggering a worse bloom within a fortnight.
• Pile what you pull out on the bank for a day. Small creatures, from water boatmen to newt tadpoles, will crawl back to the water. Return any that do not.
• Skim duckweed with a net in the same spirit, and compost the haul well away from the pond.

If you need to do a bigger clear-out, late September or October is the kindest time. Frogs are not spawning, and the creatures you disturb have time to settle before winter.

Gentle helpers and a sensible seasonal rhythm

Barley straw has a modest reputation for suppressing algae, though results are unpredictable. If you try it, use a small mesh bag of straw in early spring and remove it after six months, before it decays and deoxygenates the water. Bacterial "sludge busters" are unnecessary in a healthy pond and can be an expensive way of adding very little.

Top up with rainwater from a butt rather than tap water, which carries nutrients and phosphates that feed algae. Never introduce fish to a wildlife pond; they eat everything smaller than themselves and churn up the sediment. In autumn, net out fallen leaves but leave a few in the deepest part for frogs and newts to shelter under. In winter, do nothing at all — just watch, and let the pond rest.

Clear water in a wildlife pond is not a chemical result. It is the visible sign of a pond that is balanced, planted and gently tended, and it is worth the wait.