Autumn cleanup tasks for garden ponds and filters

Why autumn matters more than any other season

For most of the year a garden pond looks after itself. Autumn is different. As daylight shortens and temperatures slide, the leaves come down, the marginal plants collapse, and the fish slow to a near-stop. Everything that falls into the water now stays there until spring, quietly rotting, feeding algae and consuming the oxygen your fish need most when the cold bites.

A tidy pond in November is a healthy pond in April. The work is neither heavy nor complicated, but it is time-sensitive, so it is worth setting aside a dry weekend in October to work through the list below.

Netting and leaf removal: the single most useful job

If you do nothing else, keep the leaves out. A single sycamore or willow overhanging a pond can deposit a wheelbarrow's worth of debris in a fortnight, and every leaf that sinks releases the nutrients that feed blanketweed and green water next year.

• Stretch a fine-mesh pond net over the surface in early October, before the main fall begins. Peg it taut at the edges so birds and hedgehogs cannot become tangled, and check it daily.
• Net the surface by hand each morning with a long-handled skimmer. Empty the basket well away from the pond, never onto the marginal planting, or you simply recycle the nutrients.
• Use a floating skimmer or a length of hose weighted at one end to draw surface debris towards a corner where you can scoop it easily.
• If the pond is small, a fine butterfly net fixed to a bamboo cane works perfectly and costs very little.

Where branches droop low, wrap a temporary barrier of fleece or mesh around the shrub rather than fighting the fall all winter.

Cutting back without stripping the pond bare

Waterside planting needs cutting back, but not shaving to the ground. Wildlife relies on the structure you leave behind.

• Cut back marginal plants such as irises, reeds and rushes to around 10–15cm above the waterline. Hollow stems left standing make winter refuges for invertebrates and newts.
• Trim yellowing lily leaves at the base of the stem, but leave any healthy floating pads in place as long as they last.
• Leave seed heads on sedges, teasels and grasses where you can — they feed finches and look handsome rimed with frost.
• Remove tender marinals such as water hyacinth and water lettuce before the first frost turns them to slime; compost them, never release them into the wild.
• Lift and divide overgrown lily baskets if they are pushing out of the water, then replant in aquatic compost with a gravel top.

Silt, sludge and the temptation to over-clean

Every autumn brings a layer of silt to the bottom of the pond, and every autumn someone decides to empty the whole thing and scrub it out. Please don't. A mature pond is a living system, and the sludge holds the bacteria, larvae and overwintering frogs that make it work.

Instead, use a pond vacuum or a net to lift out the worst of the debris from the deepest point, where rotting matter does the most harm. Remove no more than about a third of the silt in any one year. Leave a shallow shelf of mud undisturbed for amphibians, and never scrub the liner clean. If the water level has dropped in a dry spell, top it up with rainwater from a butt rather than tap water, which is high in nutrients and chlorine.

Filter servicing before the first frost

Your filter has worked hard all summer and is carrying the most bacteria it will hold all year. Service it gently, and service it before the cold makes outdoor plumbing miserable.

• Rinse filter sponges and media in a bucket of pond water, never under the tap. Chlorinated water kills the nitrifying bacteria you have spent months cultivating.
• Squeeze sponges lightly rather than washing them spotless; leave the brown staining — that is the good stuff.
• Check the impeller on pumps and remove any tangled debris or blanketweed. A pump that seizes in December is a pump you will be replacing in spring.
• Inspect seals and O-rings for cracks and replace any that look tired. Lag exposed pipework with foam lagging to prevent it splitting in a hard frost.
• Fit a new UV bulb if yours has run more than a year; output fades long before the light goes out.
• Keep the filter and pump running through winter on a stocked pond. Leaving them off lets the bacteria die back and the water stagnate.

Fish, wildlife and the last tasks of the year

As water temperatures fall below about 10°C, switch the fish to a wheatgerm-based food that is easier to digest, then stop feeding altogether once the pond sits below 5°C. Food that is not eaten simply rots. Leave a ball of barley straw in the pond if blanketweed has been a problem, and float a small bundle of oxygenating plants in a shallow basket for late-season cover.

Finally, resist the urge to tidy away every fallen leaf and seed head around the margins. A slightly rumpled, half-wild edge is exactly what frogs, toads and the insects they eat are looking for in the months ahead. Do the clearing, service the filter, then step back and let the pond go quietly to sleep.