Photo credit: © Kim and Forest Starr, https://flic.kr/p/DJ9ziG
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Species at a Glance
Water hyacinth is a free-floating flowering perennial that forms dense “rafts” of plant material in the water that can cover a lake surface from shore to shore. Its beauty makes it a popular ornamental plant for ponds; however, its fast growth makes it one of the worst aquatic weeds in the world, as it can double its population in as little as six days.
Leaves: Oval, rounded, circular, or elliptical leaves are arranged in rosettes on stalks that can rise 0.9 m (3 ft) above the water’s surface. They are thick, glossy, waxy green, waterproof, and typically up to 15 cm (6 in) wide. The sides are gently incurved and often undulate, and leaf bases are heart-shaped, square, or rounded with dense veins.
Flowers: Multiple flowers (8-15) form in a single showy spike that can be up to 30 cm (12 in) long atop a thick stalk. Each flower in the spike has six lavender-blue petals. The uppermost petal is somewhat larger with a bright yellow, blue-bordered, central spot.
Fruits/Seeds: A three-celled capsule contains many seeds.
Stems/Roots: Spongy, inflated stems can grow up to 0.5 m (1.5 ft) long. The stems are filled with air spaces, which gives them a spongy appearance and allow them to stay afloat. Fibrous roots hang submersed beneath the floating leaves. They are dark purple to black and have a feathery appearance.
Water hyacinth can be confused with American spongeplant (Limnobium spongia) because of its similar looking leaves; however, American spongeplant has white roots instead of black and has slender, ridged stems instead of bulbous, inflated stems. It can also be confused with water lettuce (Pistia stratiotes), which has large ribbed leaves and doesn’t have the showy flowers characteristic of water hyacinth.
This species grows in shallow temporary ponds, wetlands, marshes, sluggish flowing waters, lakes, reservoirs, rivers, and ditches of temperate climates. It can tolerate extremes in water level fluctuations and seasonal variations in nutrients, pH, temperatures, and toxins. It is not winter hardy and needs temperatures above 12°C (54°F) to survive.
Because of its attractive purple flowers, water hyacinth is a popular plant among ornamental pond and water garden enthusiasts. As a result, escape from water gardens as well as deliberate releases have been major modes of dispersal. Once introduced, its high growth rate has allowed it to quickly establish and spread. It reproduces both by fragmentation and by forming plantlets at the end of a shoot that grows from the base of the stem.
Native to South America in Brazil, water hyacinth is now found in more than 50 countries on five continents. It is believed to have been introduced to the United States in 1884 for the Cotton States Exposition held in New Orleans. Because of its beauty, it was given as gifts to attendees who took it home to add to backyard ponds. By 1900 it had escaped cultivation to become a serious pest. In the Mid-Atlantic, this species is found in New Jersey, New York, Delaware, Virginia, North Carolina, and South Carolina.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Water hyacinth forms thick mats that block waterways and limit boat traffic, swimming, fishing, and other recreational activities. In drainage and irrigation canals, it impedes flow which can result in flooding and clogged pumps. Dense mats prevent sunlight and oxygen from reaching the water column and the submerged plants below, shading out native species and reducing biodiversity.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).