Latin names: Most algae belong to the following: Single-celled, free-floating algae (often Chlamydomonas, Chlorella or Euglena species) or filamentous algae such as blanket weed (usually Cladophora species)
What is it?
Algae can cause 'pea soup', blanket weed or floating scums. As well as looking unsightly, excessive algal growth, encouraged by surplus nutrients, can lead to de-oxygenation of water with a consequent detrimental effect on aquatic life.
Symptoms
Depending on the type of algae, the pond water may appear pea-green or have dense growths of hair-like green strands known as blanket weed floating in it.
Causes
Excessive algae forms when there are too many nutrients in the water, due to a build-up of organic matter (sludge) on the bottom, fertilisers leaching into the pond or regular topping up with tapwater. Chemical treatments are available but practical solutions are simpler, cheaper and usually longer acting.
Control
Prevention
Depth of water Ensure a minimum pool depth of 75cm (2.5ft) as shallow pools warm rapidly, sunshine speeding up algal growth. Use black liners to minimise heat reflecting into the water.
Minimise nutrients Remove any soil from the pool before filling, and only immerse plants in containers of aquatic compost, surfaced with shingle. Apply fertilisers to nearby lawns carefully, as run-off can occur after heavy rain.
Plant up the pond Ensure oxygenating plants are present in sufficient quantities. Aim to have at least one third of the pool's surface area covered with oxygenators or other aquatic plants. Avoid over-stocking with fish, which produce nutrient-rich waste.
Leaves and debris Net the pond in autumn to prevent leaves (nutrient source) from falling into the water.
Changing water Changing water upsets the natural pool balance. Remove blanket weed with a rake; duckweed can be skimmed off or removed using a net. Remove sunken debris every few years. Clean ponds in early summer, after aquatic life has bred. Divide aquatic plants at this time.
Barley straw Submerge mesh bags of barley straw (or lavender clippings in larger ponds) just below the water's surface in early spring, replacing them when they turn black. Chemicals released through straw decomposition inhibit algal growth. Roughly 25-50g of straw per sq m of water surface area is required, depending on the severity of the problem.
Pond filter Pond filters can be installed, removing algae and nutrients through mechanical and/or biological methods. Consult specialist suppliers regarding you requirements.
Ultrasound Trials at the Centre for Aquatic Plant Management found promising results in the use of ultrasound to control pond algae. A device left permanently in the water emits ultrasound waves and works by shaking apart the algae cell wall, leading to algae death within 7-21 days. Currently it appears to be most effective on blanket weed. 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 (see below for suppliers details). Professional installation is needed for it to be effective.
Treatments Chemical algaecides can be used for short-term control but may not be effective against all types of pond algae.Further information
Straw/lavender pond pad suppliers:
www.green-ways.co.uk (pond pads)
www.norfolk-lavender.co.uk (pond pads)
www.aquahydrotech.com (barley straw extract – liquid)
Professional control (details of qualified contractors):
British Association of Landscape Industries