The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
If you are creating a pond from scratch, site it with at least part of the pond in slight shade and aim to plant up at least one third of the surface with aquatic plants, while ensuring there are also sufficient submerged oxygenators.
Ideally there should be a minimum pool depth of 60-75cm (2-2.5ft) to prevent water warming up rapidly in sunshine, which speeds up algal growth. Pond liner colour is likely to be of limited significance when it comes to reducing algae; a pale liner might reduce temperatures but increase photosynthesis and thus algal growth, whereas a dark liner may decrease light reflectance but increase water temperature which is known to promote algae.
Where blanket weed and algae are already a problem in an established pond, consider the following;
- Blanket weed and other floating algae are easily removed by twirling them out of the pond with a cane but this will only be a temporary solution
- All pond algae can sometimes be discouraged by floating a mesh bag of barley straw on the pond – about 50g of straw per sq m (1½ oz per sq yd) of water surface area is ideal. Add the straw in spring and remove it when it has turned black (usually about six months later). Sometimes lavender is added to the barley straw for additional effectiveness
- Various treatments have been promoted by manufacturers in recent years. Some appear to be effective only in settling suspended debris, others may give short-term control of free-floating algae. Some have been claimed to be effective in controlling the most troublesome types, such as the filamentous algae or blanket weeds. There are, however, several different groups of algae collectively called blanket weed. These individual treatments only affect certain algae, so you might have to try several products to find one that works in your pond. Treatment needs to be repeated at intervals
- Do not be tempted to change the water in the pond, as it will bring only short-term relief, the problem becoming worse subsequently
- These methods should all be treated as quick fixes, and the conditions that caused the algae to grow in the first place should be improved
For algae and aquatic weed control options in large ponds and lakes, professional help may be required. Suitably qualified operatives can be obtained using the Find a Contractor section of the National Association of Agricultural Contractors website or Complete Weed Control. There are currently no chemical control treatments available to amateurs.
Dyofix Pond Black pond dye claims to help prevent algae. It works by colouring the water black and reflecting ultra violet rays away from the surface of the pond so that the algae cannot photosynthesise. Surface plants such as lilies and reeds will still benefit from the sunlight. Once applied, Pond Black will last for several months, and can be used all year round. Pond Blue is paler than Pond Black, enabling fish and other aquatic life to be seen beneath the surface of the water.
Large ponds and ultrasound
Trials at the Centre for Ecology and Hydrology found promising results in the use of ultrasound to control pond algae. A device in the water emits ultrasound waves and works by shaking apart the algae cell wall, leading to algae death within 7-21 days.
There is some evidence that it can even be tuned to different frequencies to target different types of algae, though currently it does not appear to be effective on free-floating algae (e.g. Chlamydomonas and Euglena blooms). There appears to be no adverse effect on pond wildlife but more research is being conducted in this area.
The equipment is not cheap to buy, but may be worth considering for larger ponds or hired for shorter periods where other methods of control have failed. Professional installation is needed for it to be effective. The device is left permanently in the water and runs on electricity or solar power.
Algae control options for larger ponds including ultrasound: WaterLand Management.