Striking a Deal with the Weed from Hell

The scene at Florida’s Crystal River National Wildlife Refuge in Kings Bay last October would have been familiar to anyone who has ever engaged in the battle to control the spread of invasive plants. Eager volunteers scurried about the shoreline of this manatee wintering ground, carting large plastic bins stuffed with water hyacinth, a notorious aquatic weed that’s caused headaches on five continents. Closer inspection, however, would have revealed the activity to be anything but business as usual: instead of hauling water hyacinth out of the bay, the conservationists were putting it back in—almost 4,300 gallons’ worth by day’s end.

Those volunteers were taking part in a bold pilot project that is the latest chapter in a half-century-long ecological story that reads like a fable. It starts with a well-intentioned campaign to rid Kings Bay of the water hyacinth, an aggressive nonnative species. Next come decades of additional control measures and a tragic downward spiral that transformed these crystal-clear waters into an unpleasant soup of slimy green algae. Then the story takes an unexpected turn, back to its original antagonist. Only this time, Bob Knight, the wetlands restoration ecologist leading this pioneering project, has recast water hyacinth as the unlikely hero. He believes this South American native, if controlled, could help solve the algae problem and return the bay’s ecosystem to a more desirable state. The irony in this approach is not lost on anyone involved.

Sometimes referred to as the “weed from hell,” water hyacinth is a free-floating plant with an explosive growth rate. After being introduced to Americans at the World’s Industrial and Cotton Centennial Exposition held in New Orleans in 1884, water hyacinth seduced gardeners with its beauty and hardiness. From the ponds and lakes where it was intentionally planted, it quickly spread—due in part to floods. But a bigger boost came from post–World War II development. As humans dammed rivers, drained wetlands, and filled water bodies with sewage and fertilizer, they created ideal water hyacinth habitat. At its worst, it can form impenetrable mats, boost mosquito numbers, and deplete the water of dissolved oxygen—effectively suffocating fish. Many ecologists have spent their careers trying to eradicate water hyacinth, and the idea of encouraging its presence, especially in an idyllic spot such as Kings Bay, leaves them almost speechless.

Supporters of the initiative, on the other hand, insist there’s nothing to fear. For one thing, the plant has persisted in the backwaters of Kings Bay—for the most part innocently—for decades. It’s almost impossible to eradicate. However, it is easy to contain and control, especially compared to algae. In addition to shading out algae, water hyacinths can provide habitat for fish and other wildlife. Their dangling root masses filter algae and promote denitrification. What’s more, they’re veritable salad bars for manatees. “If you ignore the label ‘exotic’ and focus on function,” says Jason Evans, a University of Georgia ecologist and advisor on the Kings Bay project, “this plant is almost exactly what you would think this ecosystem needs.”

The adversaries could hardly have chosen a more emotive battlefield. Covering more than 600 acres and feeding into the Gulf of Mexico through the six-mile Crystal River, Kings Bay is actually more lake than bay. Its constant inflow of underground water from more than 30 major springs keeps it at a steady 22 degrees Celsius. It provides an ideal habitat for fish and other wildlife and is a warm wintertime haven for sensitive manatees. Each year tourists come by the tens of thousands, many to experience what it’s like to swim amidst the bay’s passive sea mammals. By anyone’s criteria, it’s a magical place.

Or at least it was. Trouble began in the late 1950s when the initial spread of water hyacinth became a nuisance for boaters. Following chemical control measures, the hyacinth presence was reduced to acceptable levels, but a new problem soon arose in the form of hydrilla. Another fast-growing nonnative, hydrilla grows from the bottom, making it harder to manage. Initial control efforts in the 1960s included dousing parts of the bay with thousands of gallons of sulfuric acid, which was later recognized as harmful to other life forms. Copper-based herbicide treatments in the 1970s were also halted when high levels of copper were detected in sediments and in the organs of dead manatees. Managers then tried harvesting, mechanical shredding, and applying a new battery of herbicides that included diquat, endothall, and fluridone.

As the battle against hydrilla raged into the 1990s, more frequent explosions of algae, particularly Lyngbya wollei, arrived on the scene and filled the water column with fragments and long, slimy ribbons. Eventually, higher salinity and increased murkiness overcame the hydrilla and the native eelgrasses that once nourished the manatees, leaving mostly algae soup. Longtime area activist Helen Spivey recalls when the water was so clear that nighttime strollers would walk into it by accident. Now, says the 85-year-old former state representative, “the algae get belched off the bottom and float across the shore and into the small trees along the bank, where it hangs and dries out and ends up looking like toilet paper. We call them Lyngbya Monsters.”

It’s not clear why the ecology of the bay flipped. One theory is that growing pressure on underground aquifers from the surrounding population reduced inflow from the springs, raising salinity and lowering the bay’s ability to flush away floating algae. Another idea is that decades of invasive plant control inadvertently gave algae a competitive advantage. Such unintended consequences have been observed elsewhere. During an eight-year study at a reservoir in São Paulo, Brazil, researchers documented periodic single-species algal blooms while water hyacinths were present. After hyacinth eradication, there was total domination by multiple algal species, some potentially toxic.

Either way, the situation in Kings Bay is now a cause of widespread concern. For some, it’s merely unsightly. For others, there is fear for the area’s lucrative tourism industry. And even though the bay’s manatee numbers have increased in recent decades—a trend that may be due partly to conservation measures and partly to the loss of wintering habitat elsewhere—others are concerned for the long-term health of the bay’s charismatic residents. For one thing, manatees are not overly fond of grazing on algae. More worrisome is the threat of toxicity. There were 829 reported manatee deaths in Florida in 2013, the highest total ever, and nearly one-third of them have been attributed to contact with algae toxins. While most were marine algae, analysis of a manatee corpse from the Crystal River area found toxins associated with L. wollei.

Managers have run out of options. Attempts in recent years to replant eelgrass have failed when manatee grazing outpaced planting. Desperate volunteers now periodically extract loads of algae using hand rakes, a backbreaking and costly approach that’s as permanent as a haircut.

Which brings the story full circle—back to water hyacinth. Research since the 1970s has shown it to be something of a wonder plant when it comes to ridding water of everything from excess nutrients to heavy metals and other contaminants. In China, where water hyacinth has been a nuisance for decades, authorities recently enlisted the plant in their long-running struggle to resuscitate algae-choked lakes that serve as water sources for millions. In Lake Dianchi near Kunming, major nitrogen reductions were recorded in 2012 after water hyacinth was grown for a year in floating pens spanning more than 1,000 acres, a project that included harvesting the plants for fertilizer and for use in biogas production. (1) And then there’s the Tominé Reservoir, part of the Bogotá River in Colombia. After years of declining health precipitated by the usual suspects, including excessive nutrient input, water hyacinth invaded this major source of regional drinking water in 2003. Recognizing the opportunity to test the plant’s water purification capabilities, researchers corralled the invader at one end of the reservoir representing 7.5 percent of its surface area. Then they took ongoing measurements of various water quality parameters at different locations and compared them with pre-invasion data. They found that water quality had improved so much by 2006 that it had reached standards originally set for 2020. What’s more, water quality increased with proximity to the hyacinth coverage zone. Manuel Rodríguez Susa, an environmental engineer at the University of the Andes in Bogotá, confirmed in a recent email that things continue to go well: “Today Tominé Reservoir is still in good health thanks to water hyacinth.”


At Kings Bay, where some residents have been advocating for a return of water hyacinth for decades, Knight thinks the plant’s powers can be utilized in a more direct fight against algal dominance. For starters, water hyacinth mats attract a rich diversity of organisms that includes snails and other important algae grazers. But more importantly, they provide shade. Having worked with aquatic plants for 30 years, Knight thinks that blocking growth of free-floating algae over a portion of the bay could reduce overall turbidity and help eelgrasses and other submerged plants regain a competitive edge. He aims to find out just how much hyacinth coverage is needed to achieve good water clarity.

Matt Cohen, an aquatic ecologist at the University of Florida, is skeptical that acceptable levels of water hyacinth would have useful impact on the entire bay. So Cohen is taking a wait-and-see stance regarding the Kings Bay experiment. “We don’t know what prompts this conversion to an algae state,” he says, “and that’s part of why I think doing this experiment in a careful way is totally warranted.”


It took Knight a year to acquire permits from six different government agencies in order to set up a small experiment with a treatment area less than a quarter-acre in size. Once the first floating pens were in place in March 2012—stocked with hyacinths as well as with native frog’s-bit and water lettuce—Knight discovered just how much manatees love water
hyacinth: The big sea mammals quickly emptied the corrals each time they were filled. As a result, he designed and built cage-like pens that restricted access without endangering the tenacious grazers.

With the new cages stocked, Knight is set to collect water quality data in the upcoming growing season. So far he’s been able to show that water hyacinth is a capable candidate. The plants are growing in the bay’s current salinity conditions, something critics had doubted and something that cannot be said for the two native floating plants in the experiment.

Ultimately, project supporters hope that yesterday’s enemy could be tomorrow’s friend. They believe the hyacinths can play an important role in a conservation strategy that also includes reducing nutrient runoff and restoring spring flows. “Since these invasive plants are here and we can’t get rid of them,” says the University of Georgia’s Evans, “I think it’s counterproductive to be killing them and not taking advantage of their functions. These are important tools. We should be using them.”

—Garry Hamilton

1. Wang, Z. et al. 2013. Chemosphere doi:10.1016/j.chemosphere.2013.03.014.

Top photo ©Peter Chadwick/DK Images; hyacinth experiment photos courtesy of Kings Bay Springs Alliance



  • Andrew March 17, 2014 at 6:39 am

    This is blasphemy in AIS (aquatic invasive species) circles. The problem is that so many of them are here to stay and we need better ways to live with them. Yes we still should eradicate small pioneer populations when possible and prevent their spread, but waging a never ending chemical war on thousands of acres of invasive plants in a no win situation. We can also find ways of harnessing their power which in many aquatic systems could be nutrient removal, or in wetlands biofuel production.


  • patrick a shea March 17, 2014 at 12:55 pm

    Land and water management on a micro or macro scale required rigorous science consistently applied. All too often people start on a tangent, have initial successes publicized and that is the end, regardless of whether the particular application proves to be a success or failure. On the environmental side there are too many purist who would have all flora stop in their present place and not move beyond. Ecosystems and evolution are dynamic. When, and hopefully after scientific consideration, if humans choose to intervene there are rarely, if ever, 100% pure solutions. The greatest and wisest note of caution from Greece two thousand years ago, still applies today -DO NO HARM.


  • Kathleen Reed March 17, 2014 at 2:38 pm

    Has anyone tried the hyacinths in wastewater and stormwater retention ponds? Whatever use the hyacinths are put to, once they maximize their population, the hyacinths need to be harvested for food or fuel. Spraying them with herbicide just puts the stuff they cleaned up BACK into the system, and the decay process removes oxygen from the water.

    Also, it sounds like we need a LOT more eel and other aquatic forage grass planted, if the manatees are eating up all that is planted. Is there a way to grow eel grass so that grazing does not destroy the plant?

    Final thought: Is the Hyacinth going to provide a healthy, balanced diet for the manatees? If not, and assuming they are in short supply, then other missing feed stuffs have to be planted, too.


  • Joel Merritt March 19, 2014 at 6:38 am

    This is something that has always been near and dear to me since the influx of people moving here from places where they are not used to seeing much vegetation on the water. ( colder climates) This all began as a good idea to clean up our waterways but as ideas go some good ideas go bad and in my opinion this is one. People complained about the hyacinth which is non native and so here comes SWFMD to the rescue and started spraying and spraying and spraying. I myself being a fisherman and a hunter have spent countless hours on the water near where I live and I have a completely different take on this so called nuisance. Most people have only seen the top side of a hyacinth and not the bottom side. The top side may have a menacing appearance while the bottom side is filled with small worms, grass shrimp, minnows and crawfish of which is the life blood of our delicate eco system. Now in my view since the eradication of water hyacinths began we have had an influx of hydrilla and some other viney weed that not only grow prolifically but attach themselves to the bottom and choke out every thing that lives there and can only be controlled and never eradicated by chemicals that are harmful to our eco system. While hyacinths can filter water, shade out and eliminate hydrilla and never attach to the bottom of our rivers and lakes which makes it much easier to control with the types of equipment we have today as opposed to the time when killing hyacinths became a mission and a never ending job for swfmd with their massive budget. With the way that the hyacinth floats it can be moved easily by wind or the ever changing water levels we have in Florida. After the start of the eradication I have seen a decline in the grass shrimp, worms, crawfish and the like which like I mentioned before is the life blood of this eco system. I knew hyacinths could not be all bad once I saw the plant used in sewage treatment plants to filter and clean the water. I do believe with the population in Florida that we could use a good natural filtering system that does not put its roots down in our waterways. In my personal opinion I believe the benefits of a good water filter and medium for animal life far out weigh the inconvenience of a floating and not anchored vegetation. I feel that the water hyacinth is no more non-native than the manatee and we nurture them! Also food for thought is that when the eradication of the hyacinth began the population in Florida was approximately 2 million and now near 20 million so I believe the more filter systems we have the better off we will be.


  • Andrew May 26, 2014 at 7:07 pm

    Unfortunately we have to live with some of these invasive plants and animals. Sometimes biocontrol measures work and the invader’s invasiveness is reduced and the ecosystem can return to something “normal”. In other cases we can wage a never ending war on the invasive species. Perhaps sometimes it is worth it to save entire ecosystems. I think in many cases we will just have to learn to live with, and even take advantage of invasive species.


  • Carsten January 26, 2015 at 5:56 am

    A must read classic primer for all on this very subject is the 2000 reissue of The Ecology of Invasions by Animals and Plants by Charles S. Elton (1958) with a forward by David Simberloff. Water hyacinth like all well established alien Invader plant populations are here to stay. Attempts to fully eradicate are futile and an absolute waste of tax payer resources. Elton knew this and advised on this course of action when he addressed the issue in 1958. And I agree with Andrew that society and municipalities need to learn to live with them. Many horticulturists and arborists in the northeast have already taken this approach lead by pioneers like Peter Del Tredici


  • Eileen K. Straughan January 26, 2015 at 6:22 am

    Great to read this article and learn that the water hyacinth is being used for this purpose. Back in the early 1980s (at the beginning of my career) I had a project with EPA to study the benefits and implementation potential of natural and created wetlands for wastewater treatment. During the study I visited Walt Disney World’s secondary wastewater treatment plant where Disney engineers were culturing water hyacinth in the final clarification basins to improve the quality of the plant effluent. They found that growing the water hyacinths as a final step in the wastewater treatment process produced tertiary level (nutrient removal) treatment. The plants were harvested, composted and used as an amendment in landscape maintenance. Sustainability at its finest way back in 1982!


    • Bob Himschoot January 27, 2015 at 8:20 am

      I remember the Disney project but I believe it was considered too expensive to harvest and compost as they weren’t really set up to do a full compost program. Having said that I know that a company today sells a product called “Bee Mats” that are set up in pods for nutrient removal in polishing ponds, lagoons and lakes on golf courses. like everything else they do need maintenance, Pruning and removing old growth.


  • Sylvan Kaufman February 23, 2015 at 8:00 am

    The article mentions that water lettuce and frog’s bit are also grown in the pens. Those are both rapidly growing native plants and it would be interesting to know if they could do the same job as the water hyacinth without the risk the water hyacinth poses. Although there has been controversy over the native status of water lettuce too.


    • Maureen McConnell August 30, 2015 at 1:53 pm

      If water hyacinth accumulates toxins removing them from the water, what happens to the toxins when the water hyacinths are eaten by manatees?


  • Psych Nairo May 30, 2016 at 4:51 am

    Indeed it is “from hell”! It is threatening to cover one of the largest sources of fresh water lakes in my region. People around the lake have been trying to combat it by making crafts from it but it spreads rapidly!! Thanks for this insightful post!


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