A Pretty Problem 

If you’ve spent time in Florida’s lakes and rivers, chances are you’ve seen a water hyacinth (Eichhornia crassipes). These floating plants are beautiful, with vibrant green leaves and large purple blooms. Unfortunately, water hyacinth is also one of Florida’s most problematic invasive plants. 

This South America native was first introduced to North America at the 1884 World’s Industrial and Cotton Exposition in New Orleans, Louisiana. Reports indicate that water hyacinths were given out to exposition attendees, and a woman named Mrs. Fuller from Edgewater, Florida brought her hyacinths home as ornamentals for her property. The water hyacinths grew quickly, and Mrs. Fuller discarded excess plants into the nearby St. John’s River—unknowingly triggering a radical expansion. 

Unchecked Growth 

From this initial introduction, water hyacinths grew at a frightening rate. Current research estimates that water hyacinths can double in 6 to 18 days. To put this in perspective, a single water hyacinth plant might grow into over 30,000 plants in just three months, and over one quintillion plants in a year.  

Just over a decade after Mrs. Fuller introduced water hyacinths to the St. John’s River, reports indicated that 200 miles of the river were covered with the plant. In her 1933 short story “Hyacinth Drift,” Florida ecological author Marjorie Kinnan Rawlings described how “water hyacinths had filled the channels” along the St. John’s River. In the same story, a fisherman on the river asserted the same: “Where they got a channel marked on your map it’s plumb full o’ hyacinths.”  

By the 1950s, water hyacinths covered an estimated 126,000 acres of Florida’s waterways, significantly impeding commerce and recreation. Individual plants are often connected with horizontal stems called stolons, allowing hyacinths to form huge floating mats that can entirely block boat traffic.  

Due to the floating nature of water hyacinths, these mats can also move on wind and water currents. This means that boat ramps or docks that are completely clear one day might be packed with impassable hyacinths the next day, creating inconveniences on the recreational level and economic catastrophe at a commercial scale. These mats can also accumulate substantial inertia, causing structural damage to dams, bridges, and other water-adjacent structures upon impact. 

Ecological Impacts 

Aside from economic and recreational hazards, water hyacinths also contribute to considerable ecological consequences. As with many other invasive plants, the presence of water hyacinths threatens native plant populations, reducing aquatic biodiversity. For example, mats of hyacinths can uproot native plants as they move. Additionally, these mats can completely shade rooted aquatic plants, preventing these plants from photosynthesizing and causing them to die. This limitation on photosynthesis also reduces the dissolved oxygen concentration in the water body. Dissolved oxygen is essential for the survival of fish and other aquatic wildlife, and it is not uncommon for limited dissolved oxygen to contribute to mass events of fish death.  

The giant mats of water hyacinth can also block rivers and canals, forming similar effects to a dam. When water cannot flow past these mats or is severely restricted, flooding worsens and can cause additional ecological and human impacts.  

A Solution is Found 

The management of water hyacinths has been a long, complex process of trial and error. The Rivers and Harbors Act of 1899 mandated that the United States Army Corps of Engineers maintain the country’s navigable waterways. In the context of the St. John’s River, the Corps was authorized by Congress to use “any means necessary” to clear the river of its water hyacinth problem.  

Preliminary attempts to control the plant included the application of chemicals like arsenic and sulfuric acid, which were toxic to water hyacinths, but also to other plants, wildlife, and livestock. Plant managers also attempted to mechanically remove hyacinth mats, but the plants grew much faster than they could be removed, making this method alone economically ineffective. 

However, in the early 1940s, a promising solution was finally developed with the creation of the synthetic herbicide 2, 4-Dichlorophenoxyacetic acid (2, 4-D). 2, 4-D is a selective broadleaf herbicide that is applied to the leaves of target plants, and it works by causing plant cells to divide and grow without stopping, which destroys these tissues and causes the plant to die.  

According to the Environmental Protection Agency, 2, 4-D is completely safe when used in compliance with labeled use instructions. However, it is incredibly effective at managing water hyacinth, and was used extensively in the mid 1900s to reopen waterways that had been blocked for decades by the plant. 

The introduction of 2, 4-D was accompanied by the introduction of a practice known as maintenance control. Though water hyacinth populations have been brought down to manageable levels, herbicide applications continue across 20,000 to 30,000 acres annually, with a goal of keeping the statewide coverage below 5,000 acres. Instead of allowing hyacinth numbers to return to crisis levels, frequent, small-scale treatments, also known as maintenance control, has proven a more efficient technique. 

What’s Next for Water Hyacinths? 

Unfortunately, there has been no magic cure for the water hyacinth issue in its entirety. Plants still expand at the same high rates, and maintenance control is necessary to limit the problematic nature of water hyacinths. 

However, there have been promising advancements in mechanical and biological control methods that, used alongside chemical control, can help make fighting the water hyacinth a little more manageable.  

Modern mechanical control methods include cutters or shredders, which grind up water hyacinth biomass, and harvesters, which collect the intact plants and deposit them away from the water.  

Biological control methods have also been successfully implemented. Biological controls are species that decrease the competitive advantage of target invasive plants over their native counterparts, which slows their growth rate. These agents must be carefully studied to ensure that they will not have adverse impacts on non-target species, but several biological controls have been proven effective in reducing the growth rate of water hyacinth. Examples include the mottled and chevroned water hyacinth weevils, water hyacinth moth, water hyacinth planthopper, and water hyacinth mite. 

None of these methods can completely eradicate water hyacinth when used alone, but research has shown that when biological control agents are used, herbicide rates may be drastically reduced without losing efficacy of control. Wildlife agencies are actively working to determine how best to combine different control methods for the most economically and environmentally sound practices that allow for continuous maintenance control. 

One of the most helpful steps that the public can take to slow the spread of water hyacinth is to help prevent it from entering bodies of water. Water hyacinth can reproduce via fragmentation, meaning that a small piece of water hyacinth can start a brand new population if it is accidentally transferred to a new body of water. This makes it crucial to clean and dry all boats, gear, and clothes after recreating on the water.   

To quote Dr. Jason Ferrell, Director of the UF/IFAS Center for Aquatic and Invasive Plants, “The only way to keep our waterways open and clear is to manage them year-round and never expect them to play nice.” However, with careful maintenance control, reduction of the spread of the plant, and continued research on the best management practices, the dominance of water hyacinths in Florida will remain in the past. 

 

Info from UF/IFAS, Florida Department of Agriculture and Consumer Services, Florida Aquatic Plant Management Society, Florida Fish and Wildlife Conservation Commission, U.S. Fish and Wildlife Service, Environmental Protection Agency, National Pesticide Information Center, Aquatic Biologists, Inc., and Wang et al. 2012. Cover image by Wikimedia Commons user Wouter Hagens, Public Domain.