Common water hyacinth (Pontederia crassipes, Mart. 1823)

Identity

Common water hyacinth (Pontederia crassipes Mart., 1823) is a floating aquatic in the Pontederiaceae family, long known as Eichhornia crassipes. Warmth above 25 °C (77 °F) strongly stimulates multiplication: biomass doubles in 10–18 days, helping place it among the 100 most damaging invasive species listed by IUCN. A phylogenetic revision published in 2018 showed that Eichhornia was not a natural group distinct from other Pontederia and included it in that genus. The old name nevertheless remains dominant in scientific literature and regulatory texts. ^{3 1 2}

Its epithet crassipes means ‘thick-footed’, referring to the inflated leaf stalk, a spongy, air-filled structure. The former genus name Eichhornia honored Prussian minister Johann Albrecht Friedrich von Eichhorn. The common comparison with garden hyacinths concerns the appearance of the flower spike, not a close botanical relationship. ^{10 8}

Recognition

Glossy, leathery, oval to nearly round leaves form a floating rosette, with no visible stem between foliage and roots. The most reliable identification feature is the bulbous leaf stalk, swollen into a spongy float. Neither Pistia stratiotes, with velvety leaves and flat stalks, nor bottom-rooted Pontederia cordata has this structure. ^{6 7}

Beneath each rosette hang feathery roots, blue-black to purplish, which can reach 1 m (3 ft) long in deep water. They serve both as ballast and as organs for absorbing dissolved nutrients from the surrounding water. ⁷

An upright flower stalk 20–50 cm (8–20 in) tall carries a spike of 8–15 purple to violet flowers, each with a yellow patch on its upper petal. Each flower opens for only one day, but staggered opening keeps the whole spike attractive for several days. ⁶

Useful taxonomy

The former genus Eichhornia, now included within Pontederia, dominated older classifications of this plant and generated numerous redundant names. Important synonyms include:

• Eichhornia crassipes (Mart.) Solms
• Piaropus crassipes (Mart.) Raf.
• Oshuna crassipes (Mart.) A.Haines
• Eichhornia crassicaulis Schltdl.
• Eichhornia speciosa Kunth
• Heteranthera formosa Miq.
• Pontederia crassicaulis Schltdl. (historical scientific names)

The name Eichhornia crassipes remains more widely used than Pontederia crassipes. European restrictions in force since 2016 under Regulation 1143/2014 still refer to the old name, as do most publications before 2018. The accepted scientific name is nevertheless Pontederia crassipes Mart. ^{1 4}

Habitat and range

South America’s Amazon and Río de la Plata basins form the native range, spanning Brazil, Argentina, Uruguay and Paraguay. Introduced as an ornamental from the late 19th century, notably at an 1884 exhibition in New Orleans, the plant spread through pond and aquarium trade into almost all tropical and subtropical regions worldwide. ^{3 10}

Every continent except Antarctica now has records of the species, which IUCN includes among the 100 worst invasive species. Well-documented populations include Lake Victoria in East Africa, the Nile Delta and numerous South Asian irrigation systems. In mainland France it is an introduced species monitored for its risks. Since 2016, it has been listed as an invasive alien species of Union concern under EU Regulation 1143/2014, prohibiting import, keeping, growing and sale throughout the EU to protect uninfested aquatic habitats. These restrictions describe the EU context rather than a worldwide rule. ^{3 9 4}

Life cycle and reproduction

New water bodies are colonized mainly through vegetative reproduction. Horizontal stolons extend from rosette bases and produce daughter plants at their tips, allowing biomass to double in just 10–18 days in warm, nutrient-rich water. ^{3 9}

Sexual reproduction also occurs. One plant can produce several thousand seeds annually, and buried seeds can remain viable in mud for more than 28 years. This dormant seed reserve explains why populations can return years after plants have been removed. ¹⁰

Temperatures of 25–30 °C (77–86 °F) favor optimal growth. Cold below 5 °C (41 °F) and frost threaten the rosette; in temperate climates, the plant is consequently treated as a seasonal pond annual, reintroduced in spring only where keeping it is permitted. ⁶

Uses and risks

ASPCA classifies the plant as non-toxic to dogs, cats and horses. Its principal documented danger is ecological and economic rather than poisoning, a distinction that does not reduce the consequences of releasing it into waterways. ⁵

Dense mats block light and reduce dissolved oxygen in the water below, sharply reducing aquatic biodiversity, obstructing navigation and fishing, and providing breeding sites for disease-vector mosquitoes. Conversely, the species is studied as a hyperaccumulator of heavy metals and dissolved nutrients for wastewater phytoremediation. Harvested biomass is also used for biogas, compost and fiber crafts. ^{3 7}

Biological control mainly uses two Neochetina weevils and a defoliating moth, Niphograpta albiguttalis. These insects have been introduced in several countries to slow proliferation without relying heavily on herbicides. ⁹

Varieties, look-alikes and related plants

Botanically stable cultivars are not recognized for this species. Nursery names such as Major, referring to larger rosettes, describe size variation without a taxonomic status separate from Pontederia crassipes.

Other floating freshwater plants are frequent sources of confusion:

• Water lettuce (Pistia stratiotes) forms a velvety rosette without inflated leaf stalks or showy flowers;
• Common duckweed (Lemna minor) is tiny, without an individually conspicuous rosette;
• Pontederia cordata roots in the bottom, with lance-shaped leaves and no bulbous floating leaf stalk.

When comparing Pistia stratiotes, Lemna minor and Pontederia cordata, look for inflated floating leaf stalks on a free-floating rosette. When flowering, the purple spike with yellow-marked flowers provides an additional reliable distinction. ⁶

Main sources

• Plants of the World Online / Kew, Pontederia crassipes Mart.
• International Plant Names Index, Pontederia crassipes Mart.
• IUCN Global Invasive Species Database, Eichhornia crassipes
• EPPO Global Database, Pontederia crassipes (EICCR)
• ASPCA, Water hyacinth toxicity entry
• Missouri Botanical Garden Plant Finder, Eichhornia crassipes
• USGS Nonindigenous Aquatic Species, Eichhornia crassipes species profile
• Wikipedia FR, Water hyacinth
• OFB Invasive Alien Species Resource Center, Water hyacinth: nuisance or resource?
• Bugwoodwiki, Eichhornia crassipes species account