There are many people who will tell you not to put Japanese Koi and plants together. Pet store and internet articles suggest that koi will destroy any plants you put in your pond. I have kept Japanese Koi in my water gardens for six years without any Koi destroying any of the plants. The plants provide shade and shelter for the koi when they become frightened, and contribute to the clarity of the water.
Japanese koi are by far the most beautiful and desirable fish you can have in an outdoor water garden. In Japan, koi farming has held a position somewhere between a hobby and an art form for more than two centuries. To keep Japanese koi in a water garden, you need to understand that they are bottom feeders and are most comfortable burrowing around, in, and through debris at the bottom of their natural environment. In fact, the purpose of the famous whiskers is to help them dig through rocks/debris more efficiently. Putting a layer of gravel in your pond will give the koi something to root in and will also add to the natural biological environment. Plants will also benefit from being directly on the gravel. I always take the plants out of their pots and rinse the roots before planting them in a bed of gravel, which adds to the natural beauty of the water garden.
It is true that koi eat plants. It is also true that in the hot summer months they are tireless collectors. I describe them as eating machines. That is why it is important to keep plants that they can eat without destroying them. My two favorites are watercress and bog bethony planted in a shallow area with a high water flow. Watercress is an excellent plant food source for koi. Watercress contains significant amounts of iron, calcium, and folic acid, along with vitamins A and C, and koi love it. It is best to plant on a shallow ledge with rocks around it to prevent the koi from pulling out the roots. You can buy it in the produce section of your supermarket. Just put it in some shallow water on the gravel and it will grow. Trust me, you will need the koi to help keep these invasive plants in check and you may still need to thin out at the end of the season. These plants are great filters that will help keep the water crystal clear and the koi happy.
Floating water hyacinth can cause koi to eat the roots. I usually plant in the shallow creek just below the waterfall. Its roots burrow deep into the gravel to provide excellent filtration. They tend to grow more and flower less. Mine have grown 3 feet tall and extend 12 feet downstream and seem to benefit from the splashes from the waterfall. This is another plant that will need to be pulled up in the fall. Pickerel Rush is another plant that enjoys splashes from a waterfall. I use them to hide siding under the waterfall and they have bloomed consistently all season.
