Water lilies and pond marginals benefit from being established during the warmer days of late spring into summer as the water is warming up nicely so allowing the plants to settle in quickly. Plants are placed into specific groups including marginals, deep water plants, floating and submerged aquatics to help you decide what you may be looking for.
One of the main reasons for adding plants to the pond is to help pond balance. Reducing the amount of light reaching the water by establishing floating plant cover will also reduce the issue of the ‘pea soup effect’, which can be a curse in new as well as established ponds. As algae depend on sunlight, carbon dioxide and dissolved mineral salts for survival, reduce the amount of sunlight they receive by growing plants such as water lilies. Grow sufficient numbers of plants so that their floating leaves cover more than 50 per cent of the water surface.