Autumn Leaves and Keeping Your Pond Clean

Autumn Arrives and Your Pond Changes Overnight

There is a fortnight each autumn when a British pond seems to change character completely. The water goes still and dark, the marginals collapse back, and the first proper gale strips the trees with a single gust. If you have a pond in a small garden — a butyl-lined pool of two or three square metres, perhaps — it can look as though someone has emptied a wheelbarrow of leaves into it by the second week of October. Left alone, those leaves will quietly work against everything you have spent the summer encouraging.

None of this is a reason to panic. A little planning in late September and a few minutes with a net through October and November will keep your water clear and your wildlife healthy, and every leaf you lift out is a free gift for the garden.

What Falling Leaves Actually Do to the Water

Leaves are not inert. Once they sink and soften, they become food for bacteria and fungi, and that process has consequences.

• They consume oxygen. The microbes breaking down leaf tissue use dissolved oxygen that your fish, newts and invertebrates need. In a small pond with little surface movement, oxygen can drop surprisingly fast.
• They release tannins. Oak and beech are the worst offenders. The water turns a weak-tea brown, the pH can drift downwards, and the pond looks tired rather than natural.
• They build sludge. Leaf litter at the bottom compacts into a black, smelly layer where decay happens without oxygen. If you have ever poked the bottom of a neglected pond in spring and released a sulphur smell, that is last autumn's leaves talking.
• They feed algae. Every leaf is a package of nutrients. What does not rot away over winter will be released as the water warms in spring, feeding blanketweed and green water just when you want the pond at its best.

A handful of leaves is harmless and genuinely useful habitat. A blanket of them is a slow-motion problem.

Netting: The Easiest Insurance You Can Buy

If only one job gets done this autumn, make it this one. Netting stretched over the pond catches the majority of leaves before they ever reach the water, and it takes an hour to put up.

• Put it up early. Late September or early October, before the first real leaf fall, not after.
• Choose the right mesh. A fine mesh of around 10 to 15 millimetres stops leaves and also keeps herons and cats off, but it will also block light and trap frogs if it sags into the water.
• Keep it taut and slightly above the surface. Duck tape, bamboo canes or short timber posts at the corners will hold it clear of the water. A net resting on the surface is a trap for amphibians, hedgehogs and birds.
• Leave a gap or a gentle ramp. Wildlife still needs to get in and out. Pond margins with a shallow, planted edge give frogs and hedgehogs an escape route.
• Take it off once the trees are bare. Usually mid to late November, once the last leaves have dropped. Do not leave it on all winter — it blocks light, collects debris and can freeze into the surface.

Skimming Little and Often

Netting will not catch everything, and a pond is always going to collect some debris from windblown leaves and nearby trees. The trick is to skim regularly rather than in one heroic session.

• Use a long-handled pond net with a fine mesh, and work from the edge rather than leaning over the water.
• Skim from the surface only. Do not drag the net through the silt at the bottom — that disturbs overwintering wildlife and releases nutrients back into the water.
• Work the windward side. On a breezy day, leaves gather against one edge. Ten minutes there is worth an hour elsewhere.
• Empty the net well away from the pond. Tip the contents onto the compost heap or a sheet of tarpaulin on the lawn, not on the bank where the next downpour will wash them back in.
• Check the pump and filter. If you run a filter, inspect the intake weekly in autumn. A clogged intake is the most common cause of a pond pump failing quietly in November.

Don't Empty the Pond — Tidy Gently and Leave the Wildlife

Autumn is not the season for a thorough clean-out. Frogs, toads, newts, dragonfly larvae and countless invertebrates are already settling into the bottom silt for winter, and some will spend the coldest months buried there. Draining the pond or scrubbing the bottom in October or November destroys that overwintering population, and you will notice the absence next spring.

Confine yourself to surface skimming, trimming back dead foliage from marginals, and cutting back any plants that have flopped into the water. Leave the silt alone until late spring, and even then only remove a portion of it. A pond with a little leaf litter at the bottom is a healthier, more natural pond than one scrubbed clean.

Turn the Leaves Into Leafmould for the Garden

Every leaf you lift out is worth keeping. Rather than binning them, make leafmould — one of the best soil improvers you can produce for nothing.

• Collect in a wire frame or bin. A simple ring of chicken wire in a corner of the garden is ideal, or fill old compost bags punched with a few holes.
• Keep them damp. Dry leaves rot slowly. Water the heap if autumn turns dry.
• Be patient. Sycamore, lime and horse chestnut break down in about a year. Oak, beech and hornbeam can take two.
• Use it everywhere. Once crumbly and dark, leafmould makes an excellent mulch, a soil conditioner for borders, and a component of seed-sowing mixes.
• Avoid diseased material. Leaves from trees with obvious fungal problems are better kept out of the vegetable patch, though a hot compost heap will usually deal with them.

An hour with a net on a bright October afternoon, a net over the pond through the worst of the fall, and a wire bin quietly working in the corner: that is the whole job. Come spring, you will have clearer water, livelier wildlife and a garden that has quietly benefited from every leaf you rescued.