When it comes to keeping koi in a pond healthy and safe, plants are vital -- particularly those that float on the pond’s surface. Floating plants are beneficial in a koi pond because they not only make the pond more visually appealing, they also provide overhead protection from common koi predators such as cats and raccoons. Aside from their protective qualities, floating plants in the pond are a key component of the nitrogen cycle, converting koi waste into harmless nitrates. They produce nitrogen and provide shade for fish on hot summer days. Floating plants such as water lettuce (Pistia stratiotes), the water lily (Nymphaeaceae), water lotus (Nelumbo nucifera) and water hyacinths (Eichhoria crassipes) are often found in koi ponds.
Water lettuce is a free-floating plant that forms dense mats on the water’s surface, offering a lot of protection and shade for koi fish. Water lettuce grows in U.S. Department of Agriculture hardiness zones 9 and 10. Water lettuce has large, light green leaves that look a lot like an open lettuce head, thus their name. They also produce small white and light green flowers that are visually appealing in a backyard pond. Water lettuce grows rapidly and should be monitored so it doesn’t grow out of control. It’s important that you don’t allow the plant mat to grow too large or all the light will be blocked.
Water lilies are a backyard pond favorite because they produce large, fragrant flowers. These flowers grow best in U.S. Department of Agriculture hardiness zones 3 through 11. Aside from being visually attractive, water lilies produce large lily pads which provide shade and protection from predators for koi fish. The benefits of a plant that produces shade are two-fold. Shade not only cools the temperature of the water but also helps prevent the overgrowth of algae, by blocking out the light. Because koi often eat water lilies, they are a bit harder to incorporate into koi ponds, but it can be done. Make sure they are located in an area of the pond that is blocked off with rocks, and plant the lilies in solid containers that can't be dug into by stubborn, hungry koi.
Water lotus plants provide much the same appeal in a koi pond as water lillies -- they have large, attractive flowers and large leaves which provide shelter and shade for koi. Water lotus grows in U.S. Department of Agriculture hardiness zones 5 through 10. Some favor the water lotus over the water lily in koi ponds because it can withstand koi trying to eat it a bit better. Water lotuses, unlike lilies, grow in shallow water and only need a few inches of water covering their pots. They also don't have long stems for koi to eat. This makes it a lot more difficult for koi to dig into the pot and eat the roots.
While water hyacinths are native to South America, they have become a staple floating plant for ponds in the southern United States. Water hyacinth grows best in U.S. Department of Agriculture hardiness zones 8 through 10. These plants are free-floating and grow up to a height of 3 feet. Water hyacinths are perennial plants that grow dark green leaves and pastel colored flowers on a central stalk. Because water hyacinths are known to grow vigorously, their growth should be monitored in koi ponds. If a water hyacinth is allowed to grow rampant, it will cover the entirety of the pond’s surface, thus depleting the oxygen supply and killing the koi.