Coping with Algae in Garden Ponds

Coping with Algae

There are many different species of aquatic algae, but for practical purposes, all can be divided into two groups: free floating or suspended and filamentous. The former are often minute, single-celled species that cause an algal bloom in the pool, turning the water like pea soup; the latter include silkweed, blanketweed and mermaid’s hair. Although rarely a sign of an unhealthy pool, algae can cause the pool owner considerable distress, totally ruining the visual effect.



Free floating and suspended algae, of the kind that turn the water green, are most prevalent in spring, but may be found at other times if the pool is too rich in nutrients. At the start of the growing season, even a well balanced pool may suffer from a temporary algal bloom as the water begins to warm up. Algae will appear very rapidly in such conditions and proliferate until the higher plants, such as the submerged aquatics, begin to grow actively and the waterlilies and other floating foliage plants develop to produce surface shade. Once all the aquatics are established in the pool and growing freely, the nutrients that cause the problem will be mopped up and the algal bloom will disappear almost as quickly as it arrived.

Coping with Algae in Garden Ponds It is essential to be patient during the spring and to wait for nature to take its course. Provided you have a suitable balance of plants, eventually all will be well. Whatever you do, resist the temptation to empty the pool of water and refill with fresh. This may produce a temporary alleviation of the problem, but after a few days, the water will be greener than ever. Fresh water, even from the tap, is loaded with mineral salts, which will take much longer to be absorbed, especially as the higher plants that resolve the problem will have been disturbed and, therefore, checked in their growth. Never change pond water that is green if you have a balance of other aquatic life in the pool. Just be patient.

However, it is a good idea to use a flocculating product, which will temporarily sink the algae out of suspension, allowing light to reach the submerged plants and thus accelerating their growth.



Every Pond owner will encounter green water at some point. Algaecide products are available that will kill green water algae, but care must be taken to remove the dead algae, otherwise it may decompose, harming fish and plants. An alternative way to deal with green water algae is to use a flocculating product that will clump the single-celled algae together, making them heavier and causing them to sink. This will allow light to reach the submerged plants in the water. You may need to repeat the treatment several times, as flocculating products will not prevent the green water from reforming.

If you suffer an ongoing problem of green water, you should consider installing an ultraviolet (UV) filter. Essentially, this comprises a fluorescent tube that emits ultraviolet light rather than visible light. Because UV light is harmful to the human eye, such a filter will be enclosed in a dark tube. The water passes through this and around the UV tube, and the UV light kills the algae. A UV filter should be used in conjunction with a mechanical filter to collect the dead algae.

The methods of removing filamentous algae, such as blanketweed, are completely different to those used to combat green water. Algaecides are available, but as with green water, the dead algae should be removed to prevent water pollution. Alternatively, a pond balancing product, which adjusts the balance of nutrients in the water, can be used. Although these products tend to take longer to work, they increase the nutrients available to the desirable pond plants, while restricting nutrients to the algae.

Whatever product you use, remember that if you kill a large population of filamentous algae, it will start to decompose and contaminate the water. Therefore, after treatment has proved effective, it will still need removing from the pond, although it is safe to leave any small pieces that break.



Filamentous algae come in several different forms. The most persistent and troublesome is blanketweed, a thick, mat forming kind that is coarse and fibrous, and which tangles itself around all manner of aquatic plants. In bad cases, it often forms large floating mats or colonies, the lower parts frequently becoming dense accumulations that begin to decompose through lack of light and create an unpleasant smelling brown or black mass. One of the most irritating aspects of blanketweed is that it often grows lustily in a pond that otherwise is completely clear. Rarely does it seem to prosper in green water.

Silkweed is a similar proposition. This is a more slimy growth that tends to cling around submerged aquatics and the leaf stalks of waterlilies and other deep-water plants. Usually, it is a very dark green. Unlike blanketweed, which can be lifted easily from the water by hand, it tends to slip through the fingers and is more difficult to remove.

Mermaid’s hair, the hairy, pale green algae that clings to planting baskets and to the walls of the pool, is quite innocuous and can actually help to give the pool a natural look. Provided it does not start to invade the stems of waterlilies or become tangled among submerged aquatics, it should not present a serious problem.



The ideal method of algae control is a biological one: the creation of a balance that will not permit it to develop freely. This has been outlined already and embraces the principle that if there are sufficient submerged plants to utilize the major plant foods in the water, and enough surface shade provided by floating foliage to reduce the light penetrating the water to a level that only the submerged plants can tolerate, clear water will occur quite naturally.

This is a very good theory, and for the most part it works, although occasional setbacks will occur. A pond is a natural and evolving environment, so inevitably changes take place, and constant monitoring will be required if a natural balance is to be maintained. Simply introducing a fresh container of aquatic plants in compost, which may add a few more nutrients to the ecosystem, can be enough to tip the balance.

The natural way is certainly the best method of coping with algae. Having the right plant balance is the key, but this must he linked with the sensible introduction of fish. An over-population of the latter can quickly tip the balance back in favour of the algae, especially the green water discolouring kinds. If fish become a significant interest and you want to steadily increase your stock, it may be worth abandoning natural algal control in favour of a filter.


18. March 2011 by admin
Categories: Gardening Ideas, Water Gardening/Water Features | Tags: , , , | Comments Off on Coping with Algae in Garden Ponds


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