How to dig a large pond
Building a large pond is a rewarding challenge for you and for wildlife. In summer, you should see pond skaters and water beetles within a few weeks. You might get dragonflies, damselflies, frogs and newts within a year. You can build a pond at any time of year. Autumn is ideal, as the ground isn't too wet or dry and is perfect for digging.
Choose your location. Have you got room in your garden for a large pond? By large pond, we mean one which is 2m x 2m in area and sometimes much more. If you do, it’s one of the most rewarding things you can do for wildlife.
Choose a spot with plenty of sunshine and that doesn't present any flood risk - think where excess water will go if it overflows.
Ponds often look best in the places they would form naturally – the lowest point in the garden – but it isn't essential. You are likely to create quite a lot of spoil as you dig, so decide where that is going to go. You could create raised beds and earth mounds as another wildlife-friendly feature elsewhere in the garden.
Think safety. If you have young children, or you have young visitors, the pond will need to be somewhere they can only get to if supervised.
Design the shape of your pond. The golden rules for large ponds are:
- Shallow shelving margins - make sure that anything such as hedgehogs that fall in can easily get out
- Deeper areas, but you don't need to go too deep (600mm in the middle is fine)
- Plenty of submerged aquatic pondweed
- Plenty of emergent vegetation.
Get creative with your shape, using wiggly margins or a perfectly geometric shape - wildlife won't mind either way. Try doing a sketch on paper, and use string to set out your edges before digging.
Choose and buy your pond liner. Although it’s possible to line a pond with concrete, we recommend using either a pre-moulded liner - usually fiberglass, they are expensive but durable, or a flexible liner - materials include plastic (PVC), butyl rubber and EPDM rubber (such as Firestone).They’re the easiest way to create a pond to the dimensions you want.
Flexible liners don't bend round right angles so use them for more naturally curving ponds. A flexible liner will come as a rectangle. The liner dimensions you will need will be (the maximum length of the pond plus twice the maximum depth) by (the maximum width plus twice the maximum depth). As a rough guide, a 3m x 3m liner will cost £50 - £100. Buy the best quality you can afford so that it won’t leak and will last longer.
Dig your hole. Mark out where your pond is with a hosepipe laid on the ground or lines of builders' sand.
For really big ponds, you may want to hire a mini-digger to take some of the strain. Create horizontal shelves where you will be able to put aquatic plants in pots. Carefully check it is level across all sides with a spirit level.
Line your hole. If you are using a ready-made moulded liner, lower it into the hole to test that you have dug the correct shape. Dig a little bit more earth out than the size of the liner so that it has a bit of wriggle room. Then pack the base with a protective layer of sand, lower the liner into position and carefully pack sand around the sides.
If using a flexible liner, dig a slightly larger hole than you need (about 50mm extra all the way round and down). Then make sure you remove all rocks, roots and sharp objects from the hole. Don’t cut the liner yet!
Protect the liner. To ensure the liner doesn't get punctured, place a 50mm layer of sand all around the hole. Then over this put sheets of man-made pond underlay, so that rocks and roots can't penetrate it.
Now unfold your liner across the hole. Remember that EDPM and rubber liners are very heavy, so you may need some help. Push it loosely into the contours. Smooth out the liner and tuck in folds as required.
You can also place some washed gravel into the base to provide cover, and protect the liner. But if you want to use soil, only ever use the special aquatic compost, devoid of almost all nutrients – garden soil will give you algae problems.
Fill with water. It’s best to use rainwater, as tap water contains nitrates and phosphates which may give you algae problems. Leave the pond for a week or so to settle.
Edging the pond. Once the pond is filled, trim the liner that is sticking out around the outside. Tuck rocks or logs around the edge to give it a natural look and hide any remaining liner.
Buy some aquatic plants - but choose carefully. You want a range of plants which look attractive and perform different functions to keep your pond healthy. Choose ones which won't grow too large for your pond, and above all avoid any invasive non-native species. Only buy from a reputable company.
Get plenty of oxygenating weed and let it establish and spread. Leave to settle in early spring before any other new planting. Submerged pondweeds are also useful for pond creatures to hide and lay their eggs. Choose rigid hornwort, whorled water-milfoil and starwort.
Floating aquatics like pondweeds (look for those called Potamogeton), fringed water-lily Nymphoides peltata (which is smaller and less likely to overrun the pond than most other waterlilies, but NOT in Ireland, where it's non-native) and water soldier provide a useful surface cover and shade for the wildlife under the water. They can be placed in deeper water too.
Emergent and marginal plants like marsh marigold, water plantain and flowering rush can be planted around the edge to provide cover for resting and nesting wildlife.
Planting up. In early spring plant lots of marginals onto your pond shelves using aquatic baskets and soil.
Aftercare. For the first few months, it’s normal to get algae or blanket weed. Remove with a rubber rake, or get children to remove it by winding it around a stick. Before composting, allow it to sit at the side of the pond for critters to escape back into the water.
You may need to top the pond up in hot weather – try to use rainwater from a water butt.
Watch wildlife arrive! Most of the creatures that use ponds are expert at finding new ones. So there's no need to bring in water or wildlife from other ponds, as this can bring in pathogens.