Installing a pond skimmer to clear fallen leaves

Why leaves cause more trouble than they look

A handful of leaves drifting about in October looks charming. A thousand of them sitting on the bottom by December is another matter. As they break down they strip oxygen from the water, release the phosphates and nitrates that feed blanketweed and green water, and build a layer of black sludge that is miserable to clear. In a pond with fish, that autumn spike in decay is often what tips water quality over the edge.

Netting by hand still has its place, especially on a small wildlife pond. But on anything bigger than a couple of square metres, or wherever trees overhang, a skimmer does the job far better — it lifts leaves off the surface before they ever get the chance to sink.

How a skimmer works — and which type suits your pond

A skimmer draws water from the top few centimetres of the pond, which is exactly where floating leaves collect. There are three common arrangements.

• Floating skimmers sit on the surface, attached to a pump or to a hose running to a pump on the bank. They rise and fall with the water level, so they keep working without fiddling. This is the easiest option for most garden ponds and the one I would suggest for a retrofit.
• In-wall skimmers are set into the pond wall with a hinged weir door. Neat and effective, but they need a stable water level: drop more than about 40 mm below the weir and they stop drawing leaves in. Best kept for formal ponds with an automatic top-up.
• Surface intakes and skimmer baskets plumbed into a pump chamber or gravity-fed filter line. Handy if you already run a bottom drain and filtration, as the surface draw joins the same system.

If yours is a wildlife pond with newts and frogs, choose a floating unit with a coarse guard and leave an escape ramp — a strip of rough mesh or a piece of cork bark inside the basket — so small amphibians can climb out rather than tiring themselves against the current.

Choosing the right size and siting it properly

Aim to turn the top layer of water over roughly every hour or two. As a rule of thumb, a skimmer flowing 3,000 to 6,000 litres per hour suits a pond of 2,000 to 8,000 litres. Oversizing wastes electricity and can draw in small fish and tadpoles; undersizing leaves half the surface untouched.

Siting matters as much as flow. Our prevailing south-westerlies push leaves into the north-east corner of a pond, so put the skimmer there, or wherever you notice debris gathering, with a clear run of open water in front of it.

Set the weir opening 50 to 75 mm below the normal water line — deep enough that a modest drop in level will not stop it working. Check levels in summer, when evaporation is at its worst, and top up with rainwater where you can.

Outdoors, run everything from an RCD-protected socket, with cable in conduit or armoured cable buried below the lawn. Low-voltage pumps are the safest choice and are perfectly adequate for skimmer duty.

Keeping it running through autumn and winter

• Check daily in peak leaf fall — late October through November, and again after every gale.
• Empty the basket at about a third full. A blocked basket starves the pump, and a pump left running dry will ruin its impeller within hours.
• Rinse rather than scrub. Leave the biofilm behind, and never tip the contents down a drain.
• Listen to the pump. A change of note usually means grit or a leaf fragment in the impeller. Isolate the power before clearing it.
• Switch off for winter. Once the leaves are down, drain, dry and store the skimmer and pump indoors — frost cracks plastic weirs and splits housings that hold water.
• Keep a spare basket so you can swap a full one for a clean one in seconds.

What to do with the leaves you collect

What you pull out is wet, cold and rich in nutrients — a nuisance in the pond, an asset on the compost heap. Never tip it back into the water and never wash it into a drain, where those nutrients simply feed algae further along.

• Compost it. Leaves are carbon-rich, so layer them with grass clippings, vegetable peelings and a little soil. Turn the heap now and then; a mixed heap will deal with them in six to nine months.
• Make leafmould. If it is mostly leaves, pack them into a bin bag punched with a few holes, or a wire cage, keep them damp and leave for a year. The crumbly result is a superb mulch for ferns, hostas and astilbes at the pond edge.
• Keep aquatic weeds separate. Fragments of floating pennywort, parrot's feather and other invasive aquatics should never reach the heap — bag them and dispose of them as your council advises.

A skimmer will not solve every autumn problem, but it turns a fortnight of daily netting into a two-minute job, and it keeps the muck out of the water in the first place — which is where it does the most harm.