How to Make a Mini Wetland in a Container

A patch of marsh the size of a doorstep

A half-barrel of still water, mud and marsh plants is the smallest piece of habitat you can build that still behaves like a real wetland. An oak half-barrel about 70cm across and 40cm deep will hold roughly 130 litres once you stop 15cm below the rim — enough water to stay cool through a hot August, and enough depth to plant genuine British marsh species. Frogs find these things, even in the middle of a town, often in the first spring. Damselflies, hoverflies and bees arrive with the flowers. The whole job takes an afternoon.

Choosing the barrel and the right spot

• The barrel: a genuine oak half-barrel, not a plastic imitation. Oak insulates the water and the staves weather beautifully. Second-hand spirit barrels are ideal and cost far less than new.
• Position: four to six hours of sun a day keeps plants flowering and tadpoles growing, but the barrel will need topping up more often in full sun.
• Avoid: directly under deciduous trees, where leaves fall in and rot, and open, windy corners that blow litter across the surface.
• Ground: a firm, level base. Fill it where it stands — 130 litres of water weighs 130kg and you will not move it afterwards.

Lining with clay, the old way

Clay kept ponds watertight long before flexible liners, and it does two jobs at once: it seals, and it gives marsh plants the silty, nutrient-poor mud they actually grow in. Puddling clay or bentonite is what you want. Soak it in a bucket until soft, then press it into the barrel in a layer about 4cm thick across the base and up the sides, working out any air pockets with your fingers or the back of a spoon. Add water slowly, a few centimetres at a time, or the clay lifts and clouds.

One honest caveat: bare clay cracks if it dries out completely. Keep the level topped up weekly and the seal will hold; let it bake hard in a hot spell and you may be re-puddling next spring. If your barrel is old and leaking badly, run a butyl liner underneath and treat the clay as growing medium rather than a seal. Nobody looking into the water will ever know.

Planting: three plants do most of the work

Plant straight into the clay, without baskets, and let the roots knit it together.

• Marsh marigold (Caltha palustris): the star of the show, with butter-yellow flowers from March to May and glossy round leaves 30cm high. Sit it in water no deeper than 10cm.
• Soft rush (Juncus effusus): stiff upright green stems, 40–60cm tall, happy in 0–15cm of water. It gives the barrel its architecture and gives damselflies somewhere to perch and dry their wings.
• Water mint (Mentha aquatica): lilac flower heads from July to September and a magnet for bees, hoverflies and butterflies. It spreads, so plant it in a submerged basket if you want to keep it in check.

Add a bunch of hornwort, a native oxygenator that floats free and helps keep the water clear, and if you have room tuck in water forget-me-not and brooklime along one edge. Use bricks or a stack of stones to build a slope, so the depth runs from a shallow, sun-warmed 2cm at one end to 25cm at the other. That range of depths is what makes the difference between a tub of water and a working habitat.

Making it safe for the wildlife it attracts

Frogs and newts climb badly. A smooth-sided barrel is a trap unless you give them a way out, so lean a piece of weathered wood or a rough slate from the ground to the rim, and put a shallow gravel beach at one end so anything that falls in can wade out. Keep a log, a stone pile or a mound of leaf litter within a metre of the barrel; amphibians need damp cover to sit in, and they will not stay somewhere they cannot hide.

Fill it with rainwater from a butt, never tap water if you can avoid it — the nutrients and chlorine encourage algae and stress amphibians. And resist the temptation to add fish, which will eat every tadpole in the barrel within a fortnight.

A simple routine through the year

Top up with rainwater weekly from April to September, and twirl out blanketweed around a cane rather than reaching for chemicals. In autumn, net out fallen leaves but leave a few at the bottom for overwintering insects. Leave the stems and seed heads standing through winter; they are shelter for invertebrates, and frost on a rush stem is half the pleasure of having the thing. Cut the rushes back to about 5cm in late February, just before the frogs arrive. If the surface freezes, float a pan of hot water on it to melt a hole — never smash the ice, which sends shock waves through the water.

Leave the barrel alone between February and September, when there may be spawn, tadpoles or newt larvae in it. A new wetland always looks thin in its first summer. Let it be, and by the second the marigold will have swelled, the rush will have doubled, and the mint will be leaning out over the rim. When a frog calls back at you in the warm rain of early April, you will know it has worked.