Wildflower is published quarterly by the Wildflower Center. Its content is national in scope with articles about the conservation and use of native plants as well as news from the Wildflower Center. A subscription is provided to Wildflower Center members as a benefit of membership.
The famous lake does, however, have something in common with other wetlands in Texas and across much of the South. In summer, a floating plant with thick, glossy, round leaves and very showy lavender flowers forms dense mats that clog waterways, making fishing, boating and all other water activities impossible. Infestations of water hyacinth (Eichhornia crassipes) can be many acres in size, and an acre of the noxious plant can weigh more than 200 tons.
Water hyacinth reduces biological diversity, impacts native submersed plants, alters immersed plant communities by pushing away and crushing them, and also alters animal communities by blocking access to the water and/or eliminating plants the animals depend on for shelter and nesting.
The problems and expense the state of Florida, for one, has faced in getting this South American native plant under control are well-known, but the plant is a threat nearly everywhere it is encountered. The water hyacinth joins plants like narrow-leaf cattail (Typha angustifolia) in the Great Lakes region, water chestnut (Trapa natans) in the Northeast, and salt cedar (Tamarix spp.) in the West to disrupt the health of wetlands across the country.
Considered to be one of the greatest threats to biodiversity, there are an estimated 7,000 invasive plants and animals in the United States today. Invasive species cost the United States $120 billion a year in damages and losses, according to studies by Cornell University researchers, and the cost of invasive plants accounts for nearly $35 billion of that figure.
These plants are particularly troublesome in aquatic habitats, which are relatively stable environments that can lack widely fluctuating temperatures or inconsistent chemistry. This provides the perfect habitat for invasive plants to become established, reproduce and, eventually, take over the system in some cases.
"Aquatic systems are a little more susceptible to invasion," says Flo Oxley, director of education and conservation at the Wildflower Center.
Wetlands - whether they are bogs, fens, wet meadows, marshes, swamps, ponds, rivers or lakes - play a vital environmental role. They control floods, purify water, and store it for drinking and irrigation. They provide a home and food for wildlife, as well as recreational places for people to fish, swim and boat.
Among the most widespread invasive aquatic plants are phragmites - also known as giant reed grass, which is found in every state except Alaska and Hawaii - and purple loosestrife, which has been called a semi-aquatic plant and is found in every state except for seven, according to the U.S. Department of Agriculture. Both phragmites (Phragmites australis) and purple loosestrife (Lythrum salicaria) are native to Eurasia. There is also a form of phragmites that is native to North America, rare and not believed to be invasive.
Opinions differ on which aquatic invasives are the worst. In the southernmost part of the United States, from south Florida westward through Texas and even to California, botanists and land managers often name the water hyacinth as the most troublesome aquatic plant. In the Great Lakes region, the narrow-leaf cattail is often named the worst.
"There isn't a universal worst," explains Holly Crosson, interpretation coordinator at the University of California at Davis Arboretum and past coordinator for the university's RIDNIS (Reducing the Introduction and Distribution of Aquatic Non-Native Invasive Species) Project. Several years ago she served on a committee of invasive aquatic plant experts from around the country charged with making a list of the top 10 worst aquatic plants. The list included Eurasian watermilfoil (Myriophyllum spicatum), which is a huge problem in the Northwest and the Australian native melaleuca (Melaleuca quinquenervia) because it is such a huge problem in Florida, particularly in the Everglades. They could never narrow the list down to fewer than 20.
"Yes, it's possible to get rid of this stuff, but there are economics," says Chetta Owens, a plant ecology contractor at the U.S. Army Corps of Engineers' Lake Lewisville Aquatic Ecosystem Research Facility in Texas.
Herbicides can be used, but of course they have their own dangers, and mechanical harvesters work for some plants in some locations but can only harvest one or two acres per day. "Water hyacinth might actually grow faster than that," says Owens.
There are biocontrols for some invasives. Weevils have been used with success against purple loosestrife and a South American native, giant salvinia (Salvinia molesta). Grass carp, a fish, will mow down hydrilla (Hydrilla verticillata) - a native of Asia that clogs drinking-water intake pipes in the Rio Grande between Texas and Mexico - but has been known to gobble desirable native plants as well.
Texas Parks and Wildlife Department has successfully used grass carp that cannot reproduce to combat hydrilla in many lakes, says the department's Aquatic Habitat Enhancement program director, Earl Chilton. It has not been able to use the fish to fight hydrilla in Caddo Lake because grass carp releases are regulated differently in Louisiana, which has jurisdiction over half the lake.
Conservationists across the country face similar challenges when trying to eliminate invasive aquatic plants. Many states emphasize early detection and rapid response when a plant known to be invasive appears in a natural area. However, more and more experts are turning to prevention.
Water gardeners are seen as the key to preventing both the spread of known invasive aquatic plants as well as the introduction of new invasives. "One of the major pathways of invasive species is actually through horticulture," says Barry A. Rice, an invasive-species specialist with The Nature Conservancy's Global Invasive Species Initiative.
Crosson says this makes sense. "People who have aquariums and water gardens are people who love nature," she says. "They want to bring nature closer to their homes and backyards. What is someone like that going to do with a massive amount of plants they can't keep? The tendency is to not want to kill them, so they release them out into the wild."
The water gardeners who do this are not being bad, says Oxley, but they need to educate themselves. "If you are going to create a water garden, you need to be a responsible steward of the ecosystem you are creating in your backyard," she says.
The safest thing for the environment is to kill unwanted aquatic plants by freezing or drying out the plants completely, then throwing them in the garbage. Composting is usually not recommended because the seeds can survive.
It's even more effective not to have invasive plants in your water garden in the first place. Some invasive plants are illegal to sell or own by federal or state law, but the laws are rarely enforced. Thus, illegal plants are readily available in nurseries, through catalogs, on the Internet and through plant swaps.
It helps gardeners to know the scientific name of invasive aquatic plants and to use that name when buying or ordering plants. Planting only natives is a good start but isn't an absolute protection, because some native plants become invasive when moved into an environment even slightly different from their native habitat. Fanwort (Cabomba caroliniana), for example, is a native aquatic plant that can become invasive.
The narrow-leaf cattail that is causing so much trouble in the Midwest has been found in salt marshes on the East Coast from the time of the earliest botanical records, which is why it was long thought to be a native plant. Today, however, many experts believe that it is originally a Eurasian plant that was transported by the first European explorers and settlers. Great Lakes region scientists are troubled by the plant's ability to hybridize with the native broadleaf cattail (T. latifiolia) to create T. x glauca, a hybrid even more aggressive than the narrow-leaf cattail in its ability to disrupt freshwater wetlands.
Another unfortunate situation is the frequency with which invasive plants are inadvertent stowaways in orders of other plants. Kristine Maki, a county aquatic invasive-species coordinator in Wisconsin, conducted a study and found that 93 percent of orders for aquatic plants included plants - and even animals - that hadn't been ordered. Ten percent of the stowaway plants were on the federal noxious weed list.
Before putting a newly purchased plant in your water garden, Maki recommends taking a look at it in a white bucket. Remove any plants or fragments of plants that you didn't order and wash the plant well. Maki has found that dirty plants carry more stowaways - and a garden invasion can start with a single seed.
The experts urge gardeners not to create a water garden near a natural body of water so there is less of a chance that seeds or plant fragments (which especially in invasive plants can start a new plant) will accidentally travel from the garden into nature. They also urge gardeners not to build water gardens in places that can be expected to flood, since floodwaters can carry the invasives out of the garden and into nature.
As for how much caution to take, Rice says that people living in rural areas have to be more exacting than those in urban areas. He is a water gardener himself, but he grows his aquatic plants indoors. "It's like a good neighbor thing. I don't play my stereo late at night, and I don't let my plants get out." Few people are as careful as Rice. The main message, says Crosson, is not to release your water garden plants into nature, either on purpose or accidentally.
The final step is to get the word out. When your local nursery sells a plant that you know is invasive, Crosson says, point it out. Encourage public water gardens to post signs to educate the public about invasive plants.
As dangerous as invasive aquatic plants can be outside of their natural environment, many invasive-plant experts admire them and appreciate their beauty. "I wish I could see purple loosestrife in its native habitat so I could say, 'What a beautiful plant,' because it is beautiful," says Rice. It's unfortunate, he says, that here in the U.S. purple loosestrife is wiping out native plants and making life difficult for native animal species such as bog turtles.
"You see a lot of stuff demonizing these plants, describing a 'war on weeds,'" he says. It's not the plants that are bad. It's what they are doing to the environment they grow in. In the proper environment, or in the abstract, the plants can be appreciated for their toughness and tenacity, says Rice. Just don't let them out of your garden.