Ponds and water gardens add a new visual dimension to any garden, but when the water turns green and thick strands of algae cover the surface, threatening to choke all the other plants, the sight is far from a pretty one.
It can sometimes seem an endless battle to keep the stuff at bay – but fortunately there are a few useful approaches to getting it under control and giving you back the attractive feature you always intended.
Both forms of algae – the “green water” variety and the long strands of blanket weed – commonly appear as the days get sunnier and are nourished by excess nutrients in the pond provided by left-over fish food, fish faeces and rotting plant material. Simply avoiding over-feeding your fish, clearing out fallen leaves and making sure that perhaps as much as a half or two-thirds of your pond is covered with vegetation – these surface plants will compete with the algae for light – will make a big difference.
Caused by a bloom of single-celled algae suspended in the water, this problem is typically encountered in newly built ponds, where the water chemistry and planting has not yet settled down properly and become balanced – although well established ponds can still be affected. Natural methods of controlling green water include using aquatic plants or providing trees or bushes to give shade to control the levels of sunlight reaching the pond – though green water may still happen early in spring, until such time as the protective plants develop new foliage.
Chemical treatments and ultraviolet (UV) sterilization provide the two other main ways to deal with this problem. Chemical treatments can be successful – particularly in a new, unbalanced pond – but they tend to be expensive, of short duration and must be chosen carefully if the pond already contains plants and fish.
However, as a routine approach, the use of UV is probably the most common and effective. As a physical treatment, it does not affect the water itself and this, coupled with the fact that it takes place outside the pond, means that it has no potential to harm the fish or aquatic plants. It is also very easy to install and in combination with a good biological filter system and adequate planting, makes the best long-term solution to avoiding green water. A variety of units are commonly available in garden centres and specialist shops, made by a number of manufacturers and in a range of sizes and prices to suit most ponds and pockets.
As with green water algae, competitive planting can prove effective in controlling blanket weed, though a variety of other methods – natural, chemical and electronic – can also be used. Some people simply remove it by hand – either with a rake or with a purpose made tool – a distinctly low impact method. Even if it is not going to be your main control strategy, pulling out as much as possible is always a sound first move before starting any other treatment.
A wide range of products are available to deal with blanket weed which need to be added into the water. One natural and environmentally friendly additive in particular – barley straw – has received growing publicity in recent years as something of a miracle cure, though not all pond keepers are converts. An organic, enzyme-based approach which works by inhibiting algal growth, it takes a month or two to work its magic and is administered either as pouches of straw or doses of straw extract, which need to be repeated at regular intervals throughout the year. It is also effective against green water.
More traditional treatments are available, often offering the cheapest and quickest way to deal with the problem but, as before, they need to be selected with care if the pond has already been stocked and planted. Some types are algaecides – algal weed killers – while others work by binding up and removing the nutrients from the water, stopping them being available to fuel the algae’s growth. Conventional nutrient removers required the addition of various chemicals, but the latest generation are microbial treatments, harnessing the power of beneficial bacteria, which are supplied either freeze-dried or “dormant” in a solution.
For those preferring a more high-tech approach, a variety of easily installed electronic systems are now available, designed to keep blanket weed under control without harming other pond life. Some units put an electrical pulse through the water, disrupting calcium ions and breaking down the algal cells, while devices others use ultrasonic waves to cause the same result.
No two ponds are exactly alike, so it should come as no surprise that no one treatment is 100 per cent effective all of the time and often the best solution to algal problems is a combined strategy. However, with a bit of patience and careful planning, algae should never take the shine off your water feature for too long.
Last Modified: May 24, 2022