A pond is the best slug patrol you can build

Every allotment has a slug problem, and most of us spend the summer picking them off lettuces or scattering pellets around the brassicas. A small pond changes that equation. The moment it fills with water, you have created a breeding site for common frogs, and a single adult frog will eat its way through dozens of slugs and snails in a week. Smooth and palmate newts arrive to lay eggs in the folded leaves of water plants, and their larvae hunt slug eggs and small pests in the surrounding soil. Add dragonflies and damselflies and you have larvae that take aphids, midges and mosquito larvae as well.

You are not just adding a feature. You are installing a predator reservoir in the middle of your plot, one that works every night without you lifting a finger.

Choosing the right spot and size

Before digging, have a word with your site secretary or plot committee. Many allotment tenancy agreements require permission for anything that holds water, partly for safety and partly for insurance. Most sites are happy once they know the pond is small, planted and well maintained.

Size matters less than shape and depth. A pond roughly 2 metres by 1.5 metres is plenty for a half plot. Aim for a central depth of 60–75cm, which keeps the water cool in summer and stops it freezing solid in winter, and include a shallow shelf around 20–25cm deep for marginal plants.

• Site it in part sun, part shade — full sun encourages algae, deep shade discourages frogs.
• Keep it away from overhanging trees so you are not fishing out leaves all autumn.
• Leave at least a metre of rough grass, log pile or long planting along one side. Frogs need damp cover to travel between pond and veg beds.
• Avoid the lowest corner of the plot if it floods, and avoid anywhere that runoff from sheds, paths or neighbouring plots drains straight in.

Digging, lining and filling

Mark out the outline with string or sand, then dig in layers, keeping the spoil on a tarpaulin. Shape the sides with a gentle slope at one end — no more than 30 degrees — so hedgehogs, frogs and birds can walk in and out. Steep sides are a death trap.

Remove sharp stones and roots, then line with a butyl or EPDM rubber liner. Buy a piece at least 60cm wider and longer than your hole on every dimension, and old carpet or a layer of sand underneath protects it. Pre-formed rigid ponds work well too, but they are harder to settle level and offer no flexibility if you change your mind about the shape.

Fill with rainwater from a water butt if you can. Tap water contains chlorine and often chloramine, which will knock back the microscopic life you are trying to establish. If you must use tap water, let it stand in butts for a few days first. Leave the pond to settle for a fortnight before planting, and let the liner overhang sit for a week so it warms and softens before you trim it.

Plants that earn their place

Go native and go modest. A few well-chosen plants will oxygenate the water, provide egg-laying sites and shade the surface to keep algae in check. Plant marginals in baskets of heavy loam, not multipurpose compost, which is too rich and will fuel green water.

• Oxygenators: hornwort and water starwort — good for newts and tadpoles.
• Marginals: marsh marigold, brooklime, water forget-me-not and lesser spearwort.
• Deeper water: a small water lily in a basket, which shades the surface and gives frogs somewhere to sit.
• Edge planting: ragged robin, purple loosestrife and yellow flag iris, though keep the iris in a basket as it spreads.

Avoid floating pennywort, parrot's feather and New Zealand pigmyweed. They are invasive, illegal to plant in the wild, and will smother a small pond within a season.

Access, safety and shelter

Build in an escape route. A gently sloping gravel ramp at the shallow end, a few flat stones stacked like steps, or a piece of untreated timber leaning into the water all give wildlife a way out. Put a small log pile or a sheet of corrugated metal nearby to shelter frogs, newts and slow worms when they leave the pond.

On a site with children, keep the edges shallow and well planted rather than steep and open. A pond that is never more than knee-deep near the edge is far safer than one with a sheer drop, and the planting acts as a natural barrier without looking like a fence.

Year-round care without fuss

Little and often is the rule. Top up with rainwater in dry spells, and in a hot summer, top up in the evening to reduce evaporation loss. Clear fallen leaves in autumn with a net rather than a rake — you will scoop out tadpoles and newts otherwise. Once a year, in late September, thin the oxygenators so about a third of the water surface stays clear; leave the cuttings on the edge overnight so anything caught up can crawl back.

Never empty and scrub a pond. That silty layer at the bottom is the whole ecosystem. Scoop out excess silt by hand if it gets deep, but leave it be otherwise.

Come spring, listen. If you have frogs calling and dragonflies patrolling by June, the pond is doing its job — and your lettuces will be the better for it.