Why a pond earns its keep on the plot

Every allotment has a slug problem, and most plot-holders eventually work out that the cheapest, kindest answer is a pond. Frogs, toads, newts and hedgehogs all feed on slugs, beetles and other pests, and a pond gives them somewhere to breed, drink and shelter. Even a puddle-sized pond of a square metre or two will be found and colonised within a season — you will not need to introduce anything.

Build it well, with gently sloping edges and a good mix of native plants, and it will do more for your brassicas than any number of blue pellets. It is also one of the few bits of allotment kit that improves with age and looks after itself.

Choosing a spot and a sensible size

Pick a place that gets sun for at least half the day. Warm water means more insect life, more tadpoles and more frog activity. Keep it away from overhanging trees and shrubs: leaves drop in and rot, roots can puncture a liner, and heavy shade suppresses the plants you want.

Avoid the lowest corner of the plot if that is where water drains to. Run-off carries nutrients and phosphate, which feed blanket algae rather than the wildlife you are after. Stay a metre or two back from the boundary, too, so you are not causing a dispute or digging into a neighbour's roots.

For size, roughly 1.5 metres by a metre is ample. Depth matters more than width: aim for 60–75cm at the deepest point, which lets frogs and newts overwinter in the silt below the ice. Anything shallower than 45cm can freeze solid in a hard winter.

Do check your tenancy agreement or site rules first. Some allotment sites restrict open water, particularly where children visit. A sunken half-barrel or a raised trough makes a perfectly good compromise and needs no digging at all.

Digging and shaping: slopes, shelves and depth

Mark out an irregular shape with string, sand or a hose. Curved edges look natural and give you far more edge habitat than a rectangle.

Lift the turf first and stack it upside down nearby — it will rot down into good planting soil for the margins. Then dig in stages:

• The beach. Leave one end sloping gently from ground level into the water, roughly one in four. This is the escape route for hedgehogs, birds, bees and frogs, and without it small creatures drown.
• The shelf. Cut a flat ledge 20–30cm below the waterline around at least a third of the edge, about 25cm wide. Marginal plants sit here.
• The centre. Dig the middle out to your 60–75cm depth.

Check the rim is level all the way round using a straight edge and a spirit level. If it is not, the liner will show above the water on one side. Pick out sharp stones and any thick roots before you go any further.

Lining, filling and hiding the edge

Butyl or EPDM liner at 0.75–1mm will outlast most of us. Put down an underlay first — old carpet, a layer of sand or proper geotextile. Rigid preformed liners are simpler but usually have sides too steep for wildlife to climb.

To work out the size you need: length plus twice the depth plus 60cm overlap, and the same for the width. Dig a shallow trench about 20cm out from the rim all the way round.

Lay the liner loosely, then add about 5cm of water to let it settle into the shape before you start folding corners. Tuck the surplus into the trench, backfill with soil, and hide the edge with turf, stones and plants so it looks like it has always been there.

Fill from a water butt if you possibly can. Tap water is not harmful as such, but it carries nutrients and phosphate that will give you a green soup of algae in the first summer. If you must use the hose, let the pond stand a fortnight before planting.

Planting it up with natives

Use native plants, bought from a reputable aquatic nursery, and never take them from the wild. A modest scheme is better than a jungle:

• Marginals for the shelf: marsh marigold, brooklime, water forget-me-not, water mint, lesser spearwort or purple loosestrife.
• Oxygenators for deeper water: hornwort or water starwort, which keep the water clear and give tadpoles cover.

Plant in aquatic compost in baskets, or straight into the silt on the shelf. Avoid invasive non-natives such as New Zealand pygmyweed, floating pennywort, parrot's feather and water hyacinth — they will take over and are illegal to let loose in the wild.

Never add fish; they eat tadpoles and spawn. And do not import frog spawn from elsewhere. Amphibians find new water astonishingly quickly on their own.

Looking after it, and keeping it safe

Maintenance is deliberately light:

• Top up in dry spells with rainwater, and never let the pond dry out in spring while tadpoles are developing.
• Rake out fallen leaves in autumn, but leave the silt — it is the frogs' winter bedroom.
• Thin duckweed with a net if it covers more than half the surface.
• Keep slug pellets, weedkiller and fertiliser well away from the water.

If children visit the site, fence the pond or cover a raised one with sturdy mesh, and always leave a sloping beach or a rough plank in the water so anything that falls in can climb out.

Finally, give your amphibians somewhere to live on dry land: a log pile, a heap of stones and a patch of long grass beside the pond. A frog with no cover will not stay, however nice the water is. A well-built pond costs about the same as a couple of seasons' slug pellets and then keeps on working, year after year.