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One of the most commonly asked questions when constructing a garden pond is how deep should it be? The answer will depend on the purpose of the pond, and the overall size, shape, and location. A pond is an attractive visual feature, which can be the focal point of your garden. Here are the main points to bear in mind.
For a wildlife pond which supports a good range of plant life, it is recommended that the pond should be around 45cm (18 inches). This is ideal for growing oxygenating plants such as hornwort and willow moss, which help to keep the water healthy, and smaller lilies.
If you want to keep fish in the pond, then it should be around 60cm (2ft), though bigger fish like Koi need much deeper than this. This will help to avoid the water freezing solid in winter, and give the fish sufficient space to move around in. Any deeper than this, and the pond may have low oxygen levels, unless it has a correspondingly large surface area.
For larger ponds there’s really no benefit to going much deeper than 1m (3ft 3ins) as you will just be making more work for yourself, and this depth will accommodate large lilies nicely.
Shallow ponds of less than 45cm depth are more vulnerable to fluctuations in temperature. This means they heat up quickly in summer, which aids algae growth, and may completely overtake the pond. Conversely, in the winter months, a shallow pond can become too cold, and may even freeze solid, killing plants and wildlife.
When digging your pond, remember that the pond depth means the depth of the water, and not the actual excavation measurements. Allow for a few inches at the bottom of the pond to accommodate the lining, and at the top, because the water level will generally be a few inches below the upper ridge of the pond. About 6 inches is a good margin to add.
However, avoid digging the pond to the same depth over the entire surface area, which will result in vertical sides. This increases the possibility that the sides of the pond will collapse, and it also makes it more difficult for wildlife to enter and exit the pond.
Instead, create shelves around the sides of the pond. This provides a surface to place marginal pond plants on, which need their roots submerged, and the upper foliage above the surface. It is also safer for any small creatures that accidentally fall into the pond, such as hedgehogs, or even pets and children.
For a small pond the shelves should be at least 20cm wide to accommodate baskets up to 3 litres and in larger ponds a minimum of 30cm and up to 50cm or more: varying the width will make the pond look more natural and allow you to plant in groups. The depth should be a minimum of 15cm for very small ponds and 25cm for larger ponds.
It is not only the depth, but also the size and location of the pond that is important. Consider how well the size of the pond fits into the surrounding area. A small pond will look lost in a large garden, whereas a large pond will take up a disproportionate amount of space in a small garden.
Consider how much sun and shade the pond will get; ideally, it should have a good balance of both throughout the day. Sunlight is essential to help oxygenating plants function, although too much can mean the pond is more susceptible to nuisance algae. Avoid digging the pond directly under a tree or shrub, as the leaves will fall in and form a rotten sludge.