Why Chemicals and Ponds Don't Mix

When the water turns the colour of pea soup in June, it is tempting to reach for a bottle of algaecide. Please don't. A garden pond is not a swimming pool but a small, self-contained ecosystem, and the chemicals sold to clear green water are rarely selective. Copper-based treatments and dyes knock back the algae, but they also harm dragonfly and damselfly larvae, water boatmen and the amphibians that live in the pond. Frogs, toads and newts absorb water and oxygen through their skin, so anything dissolved in the water ends up inside them. The best pond management works with the wildlife rather than against it.

A pond that finds its own balance needs very little intervention — some planting, a bundle of barley straw, and a willingness to tolerate a bit of messiness.

Plants Do the Filtering for You

Submerged oxygenators are the quiet workhorses of a clear pond. They grow below the surface, taking up the dissolved nutrients that would otherwise feed algae, and releasing oxygen during the day. Plant them in spring, either in baskets or weighted and dropped in, allowing roughly one good bunch per square metre of surface.

• Hornwort — dense, feathery and happy in still water
• Spiked water-milfoil — a native that suits deeper water
• Water starwort — neat, creeping and ideal for shallow margins

Thin them in late summer so they never cover more than about half the water.

Around the edges, marginals such as marsh marigold, yellow flag iris, purple loosestrife and water mint draw nutrients up through their roots. Floating leaves — water lilies above all — shade the surface and starve algae of light. Aim for a third to a half of the surface covered by midsummer.

Do check what you are planting. Floating pennywort, water fern and parrot's feather are invasive non-natives that will smother a small pond, and surplus plants should never be tipped into a stream or ditch.

Barley Straw: The Old Gardener's Standby

Barley straw is the traditional, chemical-free way to keep water clear. As it breaks down in oxygenated water it releases compounds that inhibit algae without harming anything else. Use roughly 50g of loose straw per square metre of pond surface, loosely packed into a mesh bag with a float attached so the bundle sits just below the surface rather than on the bottom, where there is little oxygen.

Put it in during spring, once the water reaches about 10°C, and replace it every six months. Spent straw is exhausted and beginning to rot, which simply adds nutrients, so remove the old bundle when the new one goes in. Be realistic about what it can do: barley straw prevents problems and keeps a balanced pond clear. It will not clear a dense mat of blanketweed overnight.

Netting, Leaves and Gentle Housekeeping

Falling leaves are the single biggest source of the nutrients that turn ponds green the following spring. From October until the leaves are down, stretch fine netting over part of the pond on a raised frame of canes or timber. Never let netting lie flat on the water — frogs, newts and hedgehogs will become tangled and drown. Leave a section open, and keep a gently sloping edge or a piece of roughened board so that anything that does fall in can climb out.

Blanketweed in spring is best twirled out with a stick or lifted by hand. Leave what you remove on the bank for a few hours so that shrimps, snails and tadpoles can crawl back into the water.

Resist the urge to empty and scrub the pond. That destroys the whole community in one afternoon. If silt builds up, take out a little at a time in early spring, and leave some leaf litter in the deepest part for frogs and dragonfly larvae to overwinter in.

Water Levels and What You Top Up With

Top up with rainwater from a butt whenever you can. Tap water is treated with chlorine and carries nitrates and phosphates that feed algae. If you have no alternative in a heatwave, fill a bucket and let it stand overnight before adding it slowly.

Keep the surrounding garden in mind, too. Fertiliser, weedkiller and slug pellets all find their way into a pond, and slug pellets are particularly bad news for frogs and hedgehogs. A rough strip of unmown grass and wildflowers around the edge acts as a buffer, filtering run-off and giving amphibians somewhere to shelter.

Let the Wildlife Finish the Job

If you want frogs, newts and dragonflies, do not stock fish. Even a few goldfish will make short work of a season's tadpoles and dragonfly larvae. Tolerate a little blanketweed in May — it is a nursery for tadpoles and shelter for invertebrates, and newts graze on it. A pond that is slightly green in spring and clear by midsummer is a healthy one, not a failing one.

Keep the water topped up, leave the edges wild, and sit quietly at dusk with a cup of tea. The dragonflies hawking over the surface and the frog sitting on the lily pad are the sign that your pond is working exactly as it should.