Koi Ponds and Poisonous Plants
Question ... What plants are most poisonous to Koi? Are there any that you absolutely avoid around or near a Koi pond?
Answer ... Lets start with trees, since they can shed their leaves, pollen and berries into pond from a great distance especially if a pond is downwind from them. Laburnum and Yew have to be at the top of the danger list since every part of these trees including the pollen is poisonous. Alder Buckthorn too has poisonous leaves, bark and berries.
Any tree that sheds any amount of leafage or berries into the pond is liable overload the biological filter system, but the leaves of willow contain salycilic acid the basic ingredient of asprin, which is highly poisonous to fish. Oak leaves contain tannin, which in large volume can also be poisonous.
Smaller trees and large shrubs include the guelder rose (Viburnum opulus) that is poisonous in all its parts and to a lesser extent the Rhododendrons, Oleanders and the wood spurges i.e. the daphnes. Holly berries can be poisonous in quantity. I have always been told to avoid planting elder near ponds but cannot find any written evidence for avoiding it.
Dodgy climbers include the Wisteria, the Solanums (avoid all members of the potato and nightshade family), Honeysuckle and sweet peas, all of which have poisonous berries and seed. If you let Ivies flower and seed, these too can be poisonous in quantity.
Large perennials often found in the boggy regions around a naturalistic pond that may cause problems are the Giant Hogweed, Foxgloves, Larkspur, Lupin, Monkshood and Rhubarb. Even bluebell seed, buttercups, celandines and hellebores are best avoided.