Adding a veggie filter after your bio filter is a good way to use up any remaining nitrates/nitrites in the pond system. Many of us turn to species that grow quickly and absorb any nutrients so that algae has less nutrients to feed on. Water Hyacinth is usually a common choice because it is cheap and reproduces easily. In southern states it is even somewhat winter hardy as long as there are few days below 20* which allows for year round de-nitrification.
Water hyacinth is a member of the pickerelweed family and survives in sun and shade. The plants vary in size from a few inches to over a 3 feet in height. The plant has glossy green, leathery leaf blades attached to petioles that are often soft and inflated. Fibrous roots dangle in the water from the underside of the plant. Hyacinth does flower mid to late summer with bluish-purple petals. Its fruit is a three-celled capsule containing many seeds.
Water hyacinth grows into intertwined mats which may block submerged plants and other floating-leaved plants. Oxygen is reduced below the plant mats and the dense floating mats can impede water flow and create breeding conditions for mosquitoes. Water hyacinths have become an environmental problem and caused economic concerns in southern US climates. It should never be deliberately discarded to lakes, rivers, streams, or drainage ditches. Control and removal from open bodies of water has proved difficult.
Water hyacinth is thought to be unable to survive in temperatures below 20 degrees F.The green leaves may die at temps as high as 32*, but the roots will store enough energy to survive. Longterm exposure to sub freezing temperature has a detrimental impact to its ability to reproduce. Eventually a species may evolve that could be hardy in cold temperatures. In summer months the plant can double in size every 7-14 days.
Excessive plants can be removed manually from the veggie filter and placed in the Koi pond. The fish will eat the roots and shred the leaves leaving little of the original plant behind. The fibrous roots and shredded leaves can clog leaf baskets and cause problems with pump impellers so the pump may require daily inspection or cleaning. The shredded leaves can also help with small leaks as they tend to find and clog the holes.
Excessive plants may also redirect water in the veggie filter such that it finds new avenues to escape in directions other than directly to the pond. The thick mat may also cover problems in the veggie filter that need to be addressed like decaying leaves on the bottom. Pond owners should reduce the plants and inspect conditions of the overall pond system. Disposal should include composting and/or feeding to fish. If you wish to use hyacinth in combination with other water plants you will need to contain it and remove larger plants as they grow.
UPDATE: My hyacinth started turning yellow and brown in Dec as soon as some nights hit the high 30’s, so I removed it on the 10th.
