The favoured approach to control algae in your pond is to starve them out by ensuring the pond is well balanced i.e. has enough higher order plants thriving in it. This approach is easier and cheaper than employing chemicals. As in any other part of the garden if the gardener does not put in his own plants then nature takes over and the ground becomes overrun with weeds. Algae are the equivalent of weeds in water. They are single-celled plants. To inhibit them we need to encourage the growth of competing plants.
Plants produce oxygen and together with Nitrisomonas bacteria, this breaks down any organic material in the pond. The decomposing material is converted into nitrates. Nitrates are the perfect food for plants. Without plants the pond would soon become stagnant. The trick is to have enough plants so that the algae do not get any chance to feed on the nitrates. At the beginning of the season there may be a flush of green water but do not rush to poison the algae as it is necessary for the growth of the other plants.
To keep the water clear there must be two thirds of the pool surface covered with plants or oxygenators just below the surface of the pond. Most of the plants that will do the job effectively will grow rapidly so it is important to get the right varieties for your size pond and maintain them on a regular basis.
For a pond to flourish it is essential that it contains four types of plants:
Oxgenators
Deep water aquatics
Marginals
And floating plants
The quantity depends on the size of your pool.
1 bunch of oxygenators for every 0.2sq.m of surface area
1 deep water aquatic for every 2.3sq.m of surface area
1 floating plant for every 0.9sq.m of surface area
1 marginal for every 0.5sq.m of surface area
Oxygenators
The most efficient oxygenator is Elodea crispa, also called Lagarosiphon major and commonly called ‘curly pond weed’. It is easy to keep under control even in small ponds. You are advised NOT to get Elodea canadensis or Canadian pondweed as it is extremely vigorous. A bunch will consist of four to six strands of plants 10-13cm long, tied together with lead wire. Most people will throw these in the pool and they will root in the accumulated mud. However when they need cutting back it can be tricky to find the central point of growth to cut back. Instead, push the bunches into an aquatic planting basket full with pea gravel. Initially, place the basket so the oxygenator cuttings lie at not much more than 30cm below the surface. Once they seem to be growing strongly you can move them down to depths of up to 60cm below the surface.
Deep water aquatics
Lilies are the most popular deep-water plant but they resent splashing and would probably struggle to survive in your pond. Most require a lot of space. The pygmaea varieties although small need calm water. The odorata forms are also small and fragrant. They are happiest at depths of 20-50cm but I don’t know how they stand up to moving water.
Another deep-water plant is the water hawthorn, Aponogeton distachyos. It is very popular as it flowers from early spring to the end of November. Planting depth up to 75cm.
Nymphoides peltata or Villarsia ,commonly known as the ‘yellow fringe lily’ is sometimes included in the marginal plant list. It has a strong creeping habit, and makes excellent cover with its small, lily like leaves. It produces a profusion of small, yellow, star shaped flowers in summer. Planting depth is 10-60cm. It can seem rampant but is easy to control.
Floating Plants
Floating plants provide invaluable pool cover and prove to be highly efficient competition to algae but can be a nuisance themselves. Definitely AVOID duckweed (Lemna minor) and fairy moss (Azolla filiculoides).
You could try water hyacinth (Eichornia crassipes), water lettuce (Pistia stratiotes) or water chestnut (Trepa natans) but these will probably need replacing yearly.
Hydrocharis morsus-ranae commonly known as Frogbit and Stratiotes aloides (water soldier) are native plants and will over winter at the bottom of the pool.
To plant these just throw them in! They may sink to the bottom but will re-emerge when they recover.
Marginal Plants
Most of these are spring flowering and there is a wide choice. Look for plants with an interesting leaf or upright linear form to provide a contrast to all the other things. Look carefully at their spread unless you want to be continuously pulling them out! These ones don’t get too big.
Alisma plantago-aquaticum is very dainty. It flowers in July/August. You will have to prevent it from setting seed to control its spread. Height 50-80cm Spread 50-80cm. Depth of water over the soil 0-20cm
Calla palustris has arum-like flowers in June or July followed by red seed heads. Height 15-25cm Spread 1m. Depth of water over the soil 2.5- 25cm
Caltha palustris var. alba (white marsh marigold) Height 25-40cm Spread 50cm. Depth of water over the soil 0-10cm
Carex riparia ‘Bowles Golden’ attractive grass Height 30-50cm Spread 50cm. Depth of water over the soil 5-20cm
Dichromena colorata is an unusual looking plant with white star shaped flowers from July until September. Height 40-60cm Spread 60cm. Depth of water over the soil 2-5cm
Iris laevigata would look good as you have other irises in the garden. Rich blue flowers in June. Height 60-90cm Spread 30cm. Depth of water over the soil 0-20cm
Iris laevigata ‘Variegata’ green and white foliage for most of the year. Blue flowers in June. Height 50-60cm Spread 60cm. Depth of water over the soil 2.5-10cm
Iris sibirica Will grow anywhere. In your borders as well. Height 60-100cm Spread 60cm. Depth of water over the soil 0-10cm
Lobelia siphilitica This has bright blue flowers from July to September Height 45-90cm Spread 45cm. Depth of water over the soil 0-5cm
Myositis palustris or Myositis scorpiodes blue flowers in May (have already?) Height 20-30cm Spread 20-30cm. Depth of water over the soil 5-40cm
Pontedaria cordata (have already?) heart shaped green leaves blue flowers August to September Height 60-80cm Spread 60-80cm. Depth of water over the soil 10-30cm
Sagittaria sagittifolia var. leucopetala ‘FlorePleno’ double form of arrowhead flowers from July to August Height 50-80cm Spread 50-80cm. Depth of water over the soil 10-20cm
Sisyrinchium ‘E.K.Balls’ semi-evergreen linear leaves. All summer it produces a succession of star shaped, mauve flowers. Height 25cm Spread 25cm. Depth of water over the soil 0-5cm
Sisyrinchium californicum yellow flowers self seeds prolifically. Height 20cm Spread 20cm. Depth of water over the soil 0-5cm