Plant Story--The Beautiful Horrible Water Hyacinth

Floating water hyacinth, Eicornia crassipes

The water hyacinth is one of the plants I treasure because I learned about it before I knew what it looked like. Therefore, one day I had a moment of joy when all things I knew about it came together.

You see, there was Eichhornia crassipes with the complex breeding system, water hyacinth the terrible weed of subtropical lakes and streams and that handsome aquatic plant for sale at garden shops.

What? They're the same plant?

Yes indeed!


I studied biology at University of Michigan and genetics at the University of California, Berkeley. I'm sure someone was growing water hyacinth in both those places, but since I was trying to learn common and native plants as fast as I could, I had no time for tropical aquatics.

But my advanced classes at Berkeley featured unusual plant reproductive systems. Eichhornia crassipes is tristylous. Tristylous species have three style lengths (for receiving pollen) and also three lengths of stigmas (releasing pollen). Since they are pollinated by insects, the insects visiting the flowers will get the pollen on their bodies in two of three places that are both different from the position of the style of the flower producing the pollen. That means if they visit other flowers of the same clone there will be no pollen deposited but if the insects visit plants that are different genetically, with a style of a different length, pollen will be deposited. This promotes crossing between individuals and genetic diversity while allowing every individual to produce seeds. (It is a more complex version of the distyly I described for English primroses see blog link).

Tristyly is easier to understand with a sketch. Three flower types:

The three water hyacinth flower forms are given
above. Black arrows indicate how pollination occurs.
The fourth picture gives definitions and is how style and
anthers are arranged in most plants.

I was impressed--it is not quite having three sexes, but close. Tristyly is rare, being known from only water hyacinth's close relatives in the Pontideriaceae, some species of Oxalis (wood sorrel),  species in the Lythraceae, including Lythrum salicaria purple loosestrife, a few flaxes (Linaceae), onion relatives (Amaryllidaceae) and in the tropical Connaraceae (see references in Weller review p.250  link) out of all 400,000+ flowering plants.

water hyacinth, Eichhornia crassipes
You can see in the flower
on the right that the style is in the middle position
(white dot, shorter than dark-tipped anthers)

In classes about ecological issues, I heard about an aggressive weed of southern waterways, water hyacinth. Water hyacinth was brought to the U.S. South from South America because it is so attractive but it got away and became a major pest.

Looking up water hyacinth, the weed, it turns out that it was introduced to the U.S. on December 16, 1884 at the World's Industrial and Cotton Centennial Exposition (=World's Fair) in New Orleans. Within 50 years, it was out of control. It is believed to be the most important--worst--aquatic weed in the world, because, like the U.S., this pretty plant was introduced to tropical and subtropical areas all over the world, where the same story played out: it multiplied dramatically.

floating mat of water hyacinth

The statistics are impressive. It is the fastest growing plant known. A plant or population can double in six days. The plant forms dense floating mats of plants, kept on the surface by round air-filled leaves. An acre of water hyacinth can weigh 200 tons, a huge mass of plant material. The plants spread to cover ponds, lakes, and rivers, shading out everything in the water. The mat of water hyacinth cuts off oxygen circulation between the air and the water, suffocating fish. The dense stands are impenetrable to boats of all kinds. All over the world it is a hard-to-control serious weed. An aquatic menace.

I was in Costa Rica and somebody said, "that's water hyacinth, Eichhornia crassipes" and all those ideas came together.

water hyacinth, Eichhornia crassipes.

Water hyacinth is South American, native to wetlands in Brazil and adjacent countries. The common name compares the flowers to the familiar spring bulb, hyacinth (about hyacinths). The scientific name Eichhornia honors Prussia diplomat J. A. F. Eichhorn (1779-1856). The specific epithet, crassipes, means "with a thick stalk." There are six species in the genus Eichhornia and at least two others are unwelcome weeds in Florida (see USDA map). To distinguish it, Eichhornia crassipes, the international weed, is also called floating water hyacinth or common water hyacinth.

In its native range floating water hyacinth has all three style types. However, if you pull flowers out of a river in California or Florida, usually they are all the same style type. You'd think that if only one of three forms needed for reproduction was present the plant wouldn't be able to spread very well. After all no seeds would be produced. Wrong. This adaptable weed spread over all that area by cloning. That record growth rate is for cloning. (In addition it will self-pollinate if pollen is transferred to the style. Studies in its introduced range suggest germination conditions prevent seed recruitment, not absence of seeds.)

The State of Florida estimated that they had 120,000 square miles of waterways blocked by water hyacinth in the 1960s. Today, after a long push to eradicate it, they report it is down to about 2,000 square miles. (link) That is still too much --and the plant quickly expands if ignored. Across states in the southern United States, it is illegal to own or transport water hyacinth. Here's the map (scroll down) link

Frost kills water hyacinth, so northern states have not banned it. And yet...it is on Rhode Island's prohibited species list because they have been finding lakes full of it. So far those have not repeated in subsequent years, suggesting irresponsible dumping of live plants, not a self-sustaining population, but they are concerned that it will develop frost-resistance or that the climate will warm enough to let it survive the winter, and have banned it. Wisconsin bans it for the same reasons. The description on Indiana's website takes a tack similar to Rhode Island's: we see it in summer, so far it hasn't survived the winter, but that is an ongoing risk. (link)

Water hyacinth is beautiful and I've been buying it for my tiny pond, artificial and not connected to any waterways, most years. Frost turns the water hyacinths in my pond into miserable green glop.

But experts report water hyacinth regrows after mild frosts.

It is easy to find it for sale, often with a "cannot ship to" list. Home Depot called it the "number one floating aquatic plant." And said nothing about it being illegal in many states. (link) Possibly some of that is the subtle regional-ness of the internet, which gives you local sources. Water hyacinth is not illegal in Colorado but it is on the noxious weed Watch List because of its record elsewhere (link). Reading for this blog post, I think it may be a bad risk anywhere winter is mild. (And that might include Colorado since 2017-8 was the mildest winter on record for Colorado link and the third warmest year recorded in Colorado link and the U.S. link).

It has neat floats, inflated leaves, that keep it on the water surface.

Ecology is complicated though. While most North American animals aren't interested in eating water hyacinth, manatees eat it. Perhaps the difference is that they are big generalist vegetarians and most aquatic animals in North America are smaller or carnivorous. link

Raw, water hyacinth is poisonous to dogs and some people react badly to it. However, cooked it is generally edible. (See Plants for a Future link). Green Deane of the blog Eat the Weeds likes water hyacinth very much and he has eaten all kinds of wild plants. (link) He further lists a lot of water hyacinth's positive aspects. It certainly can be used as fodder for livestock and poultry though large amounts served fresh are not recommended (Feedipedia, Farm Radio)

The floating mats provide habitat for small fish and many invertebrates eaten by fish.

Water hyacinth filters nitrogen and other nutrients out of the water, reducing pollution as it turns organic pollutants into water hyacinth biomass. It will remove heavy metals from water so can be dangerous to eat if it was growing where it can absorb toxins. As a result of taking up pollutants and metals, it can clean up--purify--polluted water. That is one of its uses in aquaria. Florida's Crystal River National Wildlife Refuge is using water hyacinth to clean up Kings Bay which has been over run by algae. Note the photo at the end of the article: the water hyacinth is penned. link

Water hyacinth growing in a pot, probably a nice way to enjoy its flowers,
if you can prevent breeding mosquitos in the water. (This may have had small fish).

Water hyacinth is thus a plant of many faces--botanical curio, major international weed, beautiful ornamental, edible plant and more. I recommend you watch for water hyacinth. You might see it in an indoor water garden or a northern pond or in a tropical waterway anywhere in the world.

Comments and corrections welcome

References (in addition to those linked in the post above)
Gannon, M. Water Hyacinth - In and out of Your Water Garden. 2014. Fullserviceaquatlics.com
University of Florida. Center for Aquatic and Invasive Plants. Water Hyacinth. Eichhornia crassipes, http://plants.ifas.ufl.edu/plant-directory/eichhornia-crassipes/
Zhang, Y.-Y., D-Y. Zhang and S.C. H.Barrett. 2010. Genetic uniformity characterizes the invasive spread of water hyacinth (Eichhornia crassipes), a clonal aquatic plant. Molecular Ecology. 19 (9): 1774-1786. link (from Spencer Barrett's home page at University of Toronto. Often researchers post their publications online).

Kathy Keeler, A Wandering Botanist

More at awanderingbotanist.com

Join me on Facebook: https://www.facebook.com/AWanderingBotanist