Plants are considered a valuable element of a pond's natural makeup, providing a valuable source of food for water-dwelling animals, including fish, frogs and turtles. Despite their importance, many types of plants are poisonous not only to fish, but to animals and humans as well. In some cases, ingesting even a small amount of a toxic plant's leaves or berries has the potential to be fatal.
The cardinal flower (Lobelia cardinalis), which thrives in rich, moist to wet soil, grows well in shallower water or at the pond's edge. This plant requires consistently moist or wet soil in order to survive. In the wild, cardinal flower is found growing in marshes and stream banks. All parts of this plant are toxic and fish can be poisoned if the leaves are ingested, according to the Colorado Water Garden Society. If eaten in large quantities, it can also cause distress in humans and other animals. Symptoms may include nausea, diarrhea, convulsions and vomiting, according to the Lady Bird Johnson Wildflower Center. Blossoming begins in May continues for several months, continuing into the early fall. Cardinal flower contains a great deal of nectar. While many insects find it difficult to access these tubular flowers, it is a favorite attractant for hummingbirds and butterflies. Cardinal flower is hardy in USDA zones 4 to 11.
Climbing nightshade (Solanum dulcamara), also known as deadly nightshade, is aptly named for its highly toxic, and potentially deadly, fruit. This vine flourishes in moist soil and continual shade. In its native habitat, climbing nightshade is often found on the edges of moist woods, as well as alongside streams and wet banks. If ingested, the berries of this perennial vine are toxic to both fish and humans. The berries can be fatal if eaten, and symptoms in humans include abdominal pain, nausea, vomiting and respiratory distress, according to the Poisonous Plants of North Carolina website. Simply touching the plant has been found to cause irritation. Drooping purplish flowers with bright yellow stamens begin to bloom in June and July and continue to blossom into early September. If not properly maintained, this vine can quickly become invasive in a pond setting. Climbing nightshade is hardy in USDA zones 4 to 8.
Water hemlock (Cicuta maculata), also referred to as spotted cowbane, is one of the most toxic plants found in North America. Ingesting even a minimal amount can cause poisoning in fish, animals and humans. In people, handling the plant can result in severe skin irritation and a small mouthful is enough to result in fatality. The poison attacks the nervous system, causing dizziness, muscle spasms and convulsions. All parts of the plant, including leaves, stems and flowers, are highly toxic. Water hemlock requires consistently wet soil, and in the wild, it is typically found growing in ponds, marshes or streams, according to the Poisonous Plants of North Carolina website. Beginning in the spring and continuing into the summer, water hemlock blooms tiny white flowers in umbrellalike clusters. The flowers tend to attract birds, bees and butterflies. Water hemlock is hardy in USDA zones 2 to 8.