There is a disagreement among owners of koi ponds as to whether they should have plants in them. This is because koi are omnivores and eat plants. This is not so much a problem when putting koi in an established pond with many plants already in it. However, it can be a big problem when adding a few plants to an established koi pond. You can, however, have both koi and plants if you take some precautions as you put your plants in your koi pond.
Density of Plants
A small koi pond needs to have about 70 percent of its surface covered by plants to provide shade to the koi and to keep the water cool enough in the summer for the koi to be comfortable. Larger ponds do not need as many plants because the depth of the pond provides shade and cooler water for the koi.
Types of Plants
Plants provide oxygen for the water and utilize the nitrogen produced by the fish from their food. They also provide shade and cover from predators for the koi. It is good to have a mix of surface, emergent, and submerged plants for your koi. Algae is an unwanted but inevitable presence in most ponds. Koi will eat some of it and shading it with emergent and surface plants will help control it. The desired plants also compete with the algae for nutrients and control it that way.
As their name implies, these plants are completely underwater. They provide oxygen for the water and compete with algae for nutrients. Recommended native varieties include:
- Coontail (Ceratophyllum demersum): Coontail are long plants with leaves all around the stems. They resemble a bottle brush or coontail. They are dark green and provide good forage for aquatic animals and fish.
- Water smartweed (Polygonum amphibium var. stipulaceum): Also called water pepperplant, this is a perennial that can grow to three feet high in dense colonies. It has alternate lance-like leaves in medium green. The underwater portion of the plant provides support for colonies of small invertebrates and vertebrates which are in turn eaten by the koi.
- Waterweed (Elodea canadensis): Waterweed is a perennial plant that grows to be three meters or more in length, depending on the size of its habitat. It is green, translucent, and borne in whirls of three around the stem.
It is important to protect the roots of these plants by covering the bottom of the pond with rocks after planting the plants. This keeps the koi from rooting in the dirt and digging up the roots to eat. Once planted properly in the bottom of the pond, these plants need little care.
These plants float on the surface of your pond without rooting in the dirt at the bottom of it. They provide shade to the koi and help keep the water cool in the summer. Examples of surface plants koi like are:
- Pacific mosquito fern or fairy fern (Azolla filiculoides): This is a free floating small plant about 1 3/8-inch in length. The leaves quilt to hide the stem. This plant can become aggressive and spread over too much of the pond, so should be limited to no more than one third of the pond to prevent a lack of oxygen from making the fish sick.
- Watershield (Brasenia schreberi): Also called dollar bonnet, this is a relatively small leaf plant about five inches in size. The stem and root portion provide areas for small animals and invertebrates to congregate, where they are eaten by the koi.
- Water clover (Marsilea spp.): This plant looks like a four leafed clover either floating on the surface of the pond or held above it on stems.
Koi will eat the leaves of these plants but rarely kill them. These plants need little care other than the removal of the occasional dead or yellow leaves once they are planted.
This is the largest category of plants for your koi ponds. These include the bog or marginal area plants such as cattails as well as lilies and lotus. An issue with these types of plants is that the koi enjoy eating their roots. Plants should be potted in pots two sizes larger than the plant needs, with rocks over the top of the surface. This means that to dig for the root, the koi will have to stand on its head and move large rocks. Most koi will not do this. They will eat some of the leaves, but this should not hurt the plants as long as a third of the plant remains on the surface of the water.
Adding Plants to an Established Pond
It is easier to add koi to an established pond than to add plants to a pond with little vegetation but with koi in it all ready. New plants are novelties for the curious koi and are quickly eaten or tipped over.
The best way to add plants to an established koi pond is to add a group of plants at once so no one plant bears the brunt of the koi's curiosity. Adding plants en masse also makes the pond look finished, while adding one or two plants at a time can simply accentuate the lack of vegetation in the pond.
Plants to Avoid
Not all plants belong in water gardens. Avoid invasive plants such as the Eurasian watermilfoil (Myriophyllum spicatum), giant reed (Arundo donax), hydrilla (Hydrilla verticillata) and water hyacinth (Eichhornia crassipes). These plants can spread into nearby creeks and lakes and cause big problems. In some places, it is even illegal to use these plants. To see if a plant is invasive, check the United States Registry of Invasive Plants.
In addition, some plants may be toxic to fish. There does not seem to be a good list of toxic plants, so some trial and error is required here. Check with experienced koi pond keepers before using an unfamiliar plant. If you cannot find information on whether a particular plant is poisonous, reconsider using it in your pond.
A common cure for sick koi is to add salt to the water. If you do this, you will kill your plants. Do not add salt to water containing plants, but isolate the koi and treat it in a separate container.
The plants in your pond will aerate the water and remove harmful nitrogen from it. This reduces the need to change the water in small ponds and leaves the pond a healthier place for the koi. The plants should not need any extra fertilizer. If you do add aquatic fertilizer, be sure it is safe for fish.
Koi and Plants Can Coexist
With a few simple precautions, koi and plants can coexist. Place large numbers of plants in the pond to spread the burden of the koi's curiosity, place rocks over roots and tubers to protect them, and be willing to occasionally replace a plant or two. Make sure you do not plant poisonous plants in your pond. Do these things and your plants and koi can both inhabit your pond peacefully.