Among the most popular are water lilies. Plant enough to cover 50 to 75 percent of the pond surface, about one for every 10 square feet of surface area (dwarf varieties are available for barrel-type gardens). Floating leaf plants will cover the water surface to a point that, if done properly, will limit the amount of light reaching the depths of the pond where algae growth is controlled; there are two types of water lilies (Nymphaea spp.), tropical and hardy. Tropical water lilies are in turn divided into day and night water lilies. Rustic water lilies have flowers that bloom all day long. Some rustic water lily flowers change color during the flowering period, which adds to the character of these plants.