Pond green and murky? Here’s how to fix it

Pond water often turns green in spring. This is caused by microscopic green algae, which thrive on minerals that occur naturally in water. Spring sunshine stimulates algal growth, and the water can quickly turn green. If blanket weed develops, you may spot green filamentous strands, rather like green cotton wool. While blanket weed is often viewed as a problem, it can be beneficial to wildlife as it provides spawning opportunities for amphibians, as well as shelter and food for tadpoles and aquatic invertebrates.

Ponds containing aquatic plants usually don’t need any action apart from patience because, as soon as the plants in the pond are growing strongly enough to create shade and to take nutrients from the water, the balance of nature will be restored and the water should become clear again. However, if you want clearer water, faster, there are some things you can do to speed things up.

Sometimes pond water can become polluted or foul-smelling. This may be caused by a lack of plants growing in and around the pond, excess nutrients getting into the water from nearby farmland or other artificially fertilised area, or if there has been a build up of plant debris such as leaves from a nearby tree. In this case it may be necessary to empty and clean out the pond, making sure disturbance to wildlife is kept to a minimum.

Ponds containing fish are more likely to develop problems because fish faeces and excess fish food boost nutrient levels, which leads to more algal growth.

How to stop your pond turning green in spring

If you have a fish pond, take care to only give the correct amount of food to avoid a build-up of excess food. Also, consider installing a pond filter to help keep the water clean.

Avoid applying fertilisers close to a pond where rain could wash them into the water. When topping up the water level in a pond, use collected rainwater wherever possible, because tap water is high in nutrients.

If there are insufficient plants in the pond, spring is the ideal time to add new ones. There are four loose categories of pond plant: submerged oxygenators, deep water plants like water lilies, marginal plants which tend to grow at the edge of the pond in shallow water, and floating plants which literally float on the water’s surface. For a healthy pond, aim to have a mix of all four types, and include emergent plants like irises, which provide stems for dragonfly nymphs to emerge from in summer. Aim to have two-thirds of the water surface covered by plants. Take care to buy plants from a reputable supplier to avoid introducing invasive non-native species, and use aquatic potting compost, which has fewer nutrients than soil or multi-purpose compost.

How to fix a green pond

As long as there are plants in the pond and no pollution, your green pond should naturally turn a normal colour within a couple of weeks of new spring growth. However, there's a range of products available if you want to speed up the process. The easiest solution is to stuff some barley straw (available in pet shops) into an old pair of tights and drop this into the pond. As the barley straw breaks down it releases a chemical that inhibits algae growth, so can help speed up the rebalancing of the pond water. Other products mimic the actions of barley straw, but are available in liquid form.

Buy pond algae solutions from Thompson & Morgan, Crocus, Suttons