Koi vs Plants
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The koi and plants discussion is a topic that has been debated strongly among fish keepers.
The majority of fish keepers that have experimented with plants and koi do agree that koi at some point or another will test the plants to varying degrees from nibbling to all out ravaging the plants.
Most ponders notice it's not so much the leaves of plants as it is the tender roots of plants the koi will seek out.
There are many plants which have koi proof roots. Water Iris, Umbrella Palm, Sword Plant, Cattails, Carex to name a few, can withstand some of the worst koi grazing. The fish also seem to prefer young plants.
A trick to get your plants off to a strong start is to create a area of the pond separate from the hungry mouths of fish. Blocking off that section with rocks or bricks can keep your cherished plants wet and growing, but away from the fish.
Potting your plants in a solid container is also a trick that will keep those tender roots of plants safe and protected. The perforated black plastic pots the aquatic plants come in are of no use, as the shoots soon come out the holes and into exposed water.
Large koi often have the power to make a real mess out of lily pots. Their size and power can tip pots and uproot entire plants.
To keep the koi out of the plants... plant in heavy or large bottomed pots. You can use wire mesh or plastic grate on top to allow the new Lily stems to come up,... but won't allow the fish to root in your planting media.
Experienced keepers have found introducing young koi in a well planted pond often will familiarize the fish with the plants and help to keep the odds in favor of the fish not eating the plants.
If you have older fish, supplying them with a regular feeding of vegetable and greens will often take their attention away from plants.
In my experience, I have never had any issues with my koi and my pond that has over 60 types of water plants. I used large, solid walled pots, and heavy 2 inch gravel on top to keep out medium and small fish out. The only plant they ever like to graze on was the prolific penny wort that grows along the ponds edge. The fish would nip the roots and stems alike. These pieces would then float away and start new plants all over the ponds edges. So instead of killing the plant, they helped it reproduce!
Carlos Flannery