Koi are carp and carp are omnivores - meaning that they will eat a wide range of foods including insects, crustaceans, other (small) fish and of course, plants. So where does that leave the pond enthusiast with koi, a pond and a hankering for some pond plants? Well, it’s not an easy square to circle but we’ll give it a go.....
Protect the roots of the plant. Even if they don’t eat the plant, koi will sift through the pond substrate, or aquatic soil in plant pots, looking for tasty morsels and so will often uproot plants. Therefore, planting in aquatic pots and putting a deep layer of larger, heavy gravel around the base of the plant can work.
Choose large plants. Opting for larger, more robust plants are more likely to survive the attentions of your koi.
Make full use of marginal plants. Plants that have their leaves in the air but their roots in water have an advantage over fully aquatic plants because they can access higher levels of carbon dioxide in the air (its much lower in water) to grow faster and more vigorously – just what you’re looking for to out compete algae. So marginal planting in baskets (as described above) or if protected by rocks or very shallow margins / planted in water courses, can be a godsend. Particularly robust marginals include Yellow Flag Iris (Iris pseudacorus) and Pendulous sedge (Carex pendula). Koi may nibble at any exposed roots that grow through the baskets but not enough to cause the plant a problem.
Pond plants. Most 'oxygenators’ (typically those plants that are fully submerged under water) will be greedily consumed by koi of all sizes. Our native hornwort (Ceratophyllum demersum) may survive (its high silica content makes it less palatable and it has no roots to be pulled up) and is used by fish for spawning but it can be quite brittle so may not survive the attentions of large inquisitive koi.
Floating planters are occasionally available. Usually these are an outer rim made of a very buoyant material into which you can slot a smaller aquatic plant pot into. If planted with suitable plants these can become little islands of green bobbing around your pond. They may even provide some interest to your koi who may mouth them and push them around. These little islands will follow the water flow and you're likely to keep finding them lodged in the same places. Your koi may chew at any exposed roots but that's just what they do...
Vegetation filters can be incorporated into the return route from your filter. It's basically just a container or section through which water return from your filter is channelled and is planted up. Vigorous marginal plants are perfect for this - none more so than watercress (Nasturtium officinale) which will remove nitrates and other compounds from the water leaving your filter.
Finally, feed your fish well before you introduce any new planting that your fish can access. Koi are intelligent and inquisitive and just like children they learn much about their world by using their mouth, not only tasting but judging textures, size and shape. A good feed will reduce their appetite and may temporarily reduce their curiosity, giving your pond plants a change to establish... but koi are koi, so no promises!