How green is your pond?

A pond will attract a variety of wildlife into the garden such as frogs, damselflies, dragonflies, water boatman and pond skaters. Many different types of birds will visit for a bath and a dip; I even had a kingfisher dive in for a fish.

But what should you do if your pond looks like a bowl of green soup? The good news is that this can be fixed. Here’s how.

A natural water balance

The cause of this greening is algae. Usually a pond is fairly clear over winter until spring arrives and the ecology starts to change. As temperature and sunlight levels increase, the water warms up, blooms of algae appear, and your pond turns green.

For algae to thrive in your pond it needs sun, minerals and nutrients to feed on. The key to maintaining clear water is to create an ecological balance which reduces these elements, in turn, inhibiting the algae.

Reduce sunlight to the pond

Cutting down the amount of sun on your pond will make the conditions less suitable for algae. This may seem counterintuitive since most pond plants like a good amount of sun. So to keep algae in check you’ll need to come up with a clever way to reduce the amount of sunlight to the water without shading the plants.

The answer is to cover a good part of the pond’s surface with plants that will act as a shield to the water underneath. Floating plants, submerged plants and water lilies are ideal. You should aim to cover about half of the pond’s surface.

Reduce nutrients in the pond

Algae feed on the nutrients in your pond, so reducing nutrients in the water will inhibit algae growth. Avoid constructing a pond near deciduous trees and shrubs. When leaves fall into a pond they sink to the bottom, rot down and make the water more nutrient rich. In addition they also release toxins which pollute the water and endanger pond life. If leaves do fall into the pond, it’s best to skim them off with a net and remove.

It may be tempting to use ordinary compost when planting into a pond, but it’s full of nutrients. It’s better to use sterile aquatic compost which is free from peat and nutrients wherever possible.

There is also the thorny issue of whether to introduce fish. Fish are attractive, but they excrete, which adds nutrients to the pond and feeds the algae.

Oxygenate the water

Algae grows fast and can rapidly deplete the water of oxygen. It’s important to oxygenate the water to support the plants and wildlife which in turn keep the water clear. Submerged oxygenating plants are invaluable to the natural balance of the pond. Try things like ranunculus aquatilis (water crowfoot), hottonia palustris (water violet), potamogeton crispus (curly pondweed), and myriophyllum verticillatum (milfoil). They will also help support the algae-eating animals, such as water snails and tadpoles.

Most oxygenating plants grow well, but depending on your local conditions, you may have to try several to establish which grow best. Most are easy to control so they shouldn’t get out of hand. However, do bear in mind that some oxygenating plants are invasive. Things like parrots feather, have the potential to escape and overwhelm native plants.

A fountain or a waterfall makes a lovely water feature. They look good and serve a practical purpose – adding more oxygen to the water. Installing a waterfall or fountain will require a pump which can be combined with a filter and a UV clarifier. These also help to keep the water clearer.

If planting water lilies, remember that they don’t like being splashed, so arrange fountains accordingly.

Other tips to reduce algae in a pond

It’s part of the natural pond cycle that early in the season there will be an algae bloom, when the water first warms up. Then the oxygenating plants start to work, the vegetation grows and the lily pads will spread over the pond surface. Within a week or two the green bloom fades and the water becomes clear. The period of green should be limited to a couple of weeks early in the season. If it continues beyond a few weeks, here are some other things to bear in mind.

The size of a pond can affect its natural balance. A larger pond will maintain its natural balance more easily while small, shallow, under-planted ponds will heat up faster and suffer more from algae.

I have found barley straw effective, although it doesn’t seem to work for everyone. The straw decomposes in the water inhibiting the growth of algae.

A pond filter can be very helpful to remove algae. If you have a significant number of fish, a filter is essential to maintain good quality water and to ensure that fish excreta doesn’t feed the algae. It is important to buy the right size of filter for the volume of water (determined by the size and depth of the pond), and number of fish to be stocked in the pond. Specialist suppliers offer advice on this.

There are chemicals which can be added to the water, but I’m not happy to add them to a pond which is full of wildlife. It is a matter of personal choice. The sustainable way forward is to build up the ecological balance in the pond so that it naturally takes away the algae.

With a little effort, it is possible to have an algae free pond. Apart from the early spring bloom, algae is not inevitable as long as you have a few tricks up your sleeve to keep it at bay.