Larry Maupin is a freelance garden writer and photographer from Dallas, Texas. A frequent contributor to gardening and water gardening magazines, he is a member of the Garden Writers Association. This excellent article and photos (Copyright Larry Maupin 2003 All Rights Reserved) are reprinted here with permission.

Salvinia molesta
"possibly the worst weed in the world"


By Larry Maupin - Click images to enlarge

They are arriving quietly everyday. Alien invaders are coming, not from distant worlds across the universe but from foreign countries and climates to invade our garden worlds. They're not little green men, but rather little green plants, seeking sun, soil and sustenance. This silent invasion has been going on for centuries, but with the acceleration of modern travel and our demands for "new" plants, the aliens are arriving at an increasingly alarming rate.

Many of our favorite garden and aquatic plants and tropical fish originated in tropical countries with climates very different from our temperate United States. That¹s much of their charm for us, for their tropical beauty adds greatly to our garden experience. However, there is a growing concern among those who have taken up the responsibility to protect our natural environment from harm. Various fisheries biologists, aquatic plant researchers and others have observed and reported the damaging effects that some fast-growing exotic plants can have if they "escape" into the wilds of our country.

While the vocabulary used in the invasive plant debate continues to evolve, some terms are commonly used and understood. 
Native - A plant or animal that is naturally found in wild areas within a certain region, which can be a small local area or an entire continent. For example, "Giant salvinia (Salvinia molesta) is native to South America." 
Indigenous - A plant or animal whose natural habitat is found only within a defined area. For example, "Purple loosestrife (Lythrum salicaria) is indigenous to Australia, but is banned in 23 of our 50 United States." 
Exotic - Any plant or animal imported from another area. For example, "Except for their native plant displays, most nurseries offer only exotic plants, ranging from the amur maple tree (Acer ginalla) to zoysia grass (Zoysia tenuifolia)." 
Introduced - Any exotic plant or animal that is brought in from another area and placed in the wild. For example, "Kudzu (Pueraria lobata) was introduced to the South by railroad barons to control erosion along newly built railroads." 
Escaped - A cultivated plant or animal for use in gardens and landscapes that has propagated itself and flourished in wild areas, often becoming invasive. For example, "English Ivy (Hedera helix) is a popular hardy vine for the landscape except in the Pacific Northwest, where it has escaped into
the wild and become classified as a destructive weed." 
Naturalized - An escaped plant or animal that becomes established in the wild but is not invasive. For example, "Giant reed grass (Arundo donax) was introduced on the West Coast but now is naturalized from California to Kentucky." 
Invasive species - According to the Executive Order 13112, signed by then-President Bill Clinton on February 3, 1999, an "invasive species" is defined as "a species that is 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health." 

A popular definition of a weed is any plant that is growing where you don't want it. Most folks would not consider the southern live oak (Quercus virginiana) to be a weed, but if you have hundreds of deeply rooted acorn seedlings in your groundcover bed, you would definitely call them weeds.

So it is with many plants on the various state and Federal "invasive species" lists. One man's treasure becomes another man's trash. One blooming water hyacinth is lovely in a small pond; thousands in a lake or stream can destroy an ecosystem or a livelihood. Schefflera (Brassaia actinophylla), which most people know as a popular houseplant, is a Category I (worst type) invasive in Florida. Butterfly bush (Buddleja) is often hard to maintain in the Southwest U.S., but it is taking over disturbed soil areas in the East. English ivy (Hedera helix) is the pride and joy of the American Ivy Society, "Dedicated to the Education and Promotion of the genus Hedera," which is located in Naples, Florida; the same plant is very unpopular in the Pacific Northwest due to the destruction it is causing there.

Besides plants, exotic fish and shrimp pathogens and parasites have been introduced into the United States on infected stock for aquaculture. Crates and containers can harbor snails, slugs, mollusks, beetles and microorganisms. Military cargo transport also brings in harmful species, such as the Asian gypsy moth and brown tree snakes. Some invasives even make the evening news, including zebra mussels, imported fire ants, and Africanized honey bees. We've grown familiar with dandelions and crabgrass - two that were imported with the best of intentions.

This is not a new problem for America or for the rest of the world. To keep this in perspective, understand that not all introduced or exotic plants have proven to be harmful. In fact, most are beneficial, providing food, medicine, colorful flowers or attractive foliage. Of an estimated 50,000 plant introductions to the USA, less than five percent have become established in the wild. Of these only three-tenths of one percent are considered to be causing harm. Despite the relatively small percentage of introductions becoming problematic, the extreme severity of the problems that they can lead to should not be under-estimated. For example, in the sub-tropical state of Florida, over 240 million taxpayer dollars have been spent in trying to control invasive plants over the past 20 years.

Providence, Rhode Island's Brown University Office of Technology Assessment has analyzed the economic impact of invasive species in the United States; they report that "invasive species have cost $97 billion dollars in the United States from 1906 to 1991." In 1999 Dr. David Pimentel et al reported that annual losses due to invasive species now cost us over $138 billion dollars per year. Pimentel et al's figure only includes economic damages and control costs, not environmental and health costs. That translates to costing roughly $575 per year for every man, woman and child in America!

While Florida's warm climate and long growing season makes it ripe for the ravages of invasive species, no state is immune. In fact, every state has developed its own list of invasive species, including Alaska.

The worst weeds are classified as "invasive," but what makes one plant invasive and another not? Most non-native plants introduced to new areas by humans do not cause environmental problems. And yet, this one question fuels much of the controversy surrounding the invasive plant issue.

Another definition describes an invasive species as one that is capable of reproducing offspring rapidly at considerable distance from the parent and offers the rule of 6 miles in three years and 100 miles in 50 years as a guide. We know that floating aquatic plants in river systems can far exceed this guideline, which makes them some of the worst weeds in the world.

 Salvinia molesta

Colonizing Stage
It appears as just another water plant, floating loosely and freely (it is a true floater). It multiplies very rapidly in warm, nutrient-rich, still water.

Intermediate stage
As the population grows, it spreads over the water until the surface is
covered. At this point is begins to increase in depth and may become up to two feet thick due to wind and wave action.

Mat-forming stage
As the population becomes more dense, the form of the plant changes from a flat set of leaves to an upright form with undulating layers of foliage, as shown in the close-up photo at the top of the page.

 Consider the effects of just one invasive plant, giant salvinia (Salvinia molesta). There are other "baddies," like water lettuce (Pistia stratiotes) and water hyacinth (Eichhornia crassipes), but giant salvinia is the newest and baddest bully on the block. It is technically an aquatic fern native to the river systems of southern Brazil. From there it has spread to South Africa, Australia and the United States. The U. S. Army Corps of Engineers' Aquatic Plant Control Section in Jacksonville, Florida believes the plant "was most likely introduced through the aquarium and landscape trades." Many folks enjoy their plants and fish until they are no longer able to care for them, then they decide to "set them free." Dr. R. Michael Smart, head of the U. S. Army Corps of Engineers' Aquatic Ecosystem Research Facility in Lewisville, Texas provides a succinct alternative: "Don't dump what you are tired of - destroy it." With no native predators or pests to check their growth, giant salvinia and other aquatics can spread quickly.

Invasive species use up nutrients that other species rely on, and their rampant vegetation blocks sunlight and destroys fragile environments. Fish won't eat it and it forms dense mats of foliage that choke out other aquatic plants. Wind and wave action can cause it to form mats up to three feet thick; then when the thick mats die of starvation, the decaying plant material robs the water of oxygen, killing all aquatic creatures.

Excess vegetation also ruins the waterway for recreation, fishing and navigation, while the pond or lake takes on the look of newly mowed pasture - acres of greenery.

Giant salvinia has been called "possibly the worst weed in the world" due to its aggressive growth -- it can double in quantity every 2.2 days under ideal conditions. That means that one plant becomes 8000 in 30 days! (Don't believe it? Do the math.) In two months the number is in the millions - clearly not a plant you would want roaming free in your waterways.

Thriving ecosystems can be reduced to monocultures of giant salvinia. In much of the world it is known as Karaba weed, for it inundated the entire Karaba Lake in Zimbabwe, Africa, within only a few years. Can you imagine a single species of floating plant covering 96 square miles of water? So, throwing an aquatic plant into a stream is akin to tossing a Styrofoam cup out of a car window, except that the cup can grow to 8000 cups in a month. And each of them can produce 8000 more!

Typical methods of controlling invasive aquatics in the wild include mechanical harvesting, pesticide application and biological controls. Floating equipment designed to remove vegetation is slow and expensive to operate, and the plant material must be disposed of somewhere - another expense. Pesticide applications are expensive and controversial, and they are not always 100 percent effective.

The last option offers some hope, at least for controlling giant salvinia. Researchers seek insects, fungi or other organisms that will infect or damage the offensive species and cause its death. Possible biological controls for giant salvinia include the use of a Brazilian native called salvinia weevil (Cyrtobagous salviniae); initial tests have been encouraging.

Because of the extensive environmental damage and huge cleanup expenses involved, legislation has been developed to reduce or prevent more damage. Executive Order 13112 also created the Invasive Species Council. Its goal is to reduce or eliminate the opportunity for exotic, or "non-indigenous," plants to escape. Part of the effort to reduce damage involves restricting the possession, sale or shipment of certain plants in certain states.

There is a federal invasive plant list, maintained by the U.S. Department of Agriculture, that covers all 50 states, and each state can prepare its own list as it sees fit. In fact, every governing body in the country, including counties and municipalities, seems prone to publishing their own invasive species list with accompanying laws and fines. Included on the Federal list are a number of woody trees and shrubs, herbaceous and aquatic plants and many types of fish, birds and other animals. Even some insects, snails and fungi are listed. While the invasive plant lists are constantly evolving, they do represent the worst known offenders - the ones that cause the most damage.

The National Aquatic Invasive Species Act may be law by the time you read this. Its sweeping guidelines will have a major impact on safeguarding our waterways, or restrict your access to new varieties, depending on your opinion in this matter. If you think this invasive plants stuff is nonsense, consider this possibility: you arrive at your favorite park to find a new sign: "Permanently closed to the public due to infestation of the venomous Brown Tree Snake."

In December 2001 experts from across the globe met in St. Louis, Missouri, to explore and establish workable voluntary approaches for reducing the introduction and spread of non-native invasive plants. The Workshop on Linking Ecology and Horticulture to Prevent Plant Invasions was convened by the Missouri Botanical Garden and the Royal Botanic Gardens, Kew. This historic conference brought together for the first time some of the most respected leaders in their fields.

The fruits of their labor are significant. Rather than furthering the science of invasive plant management and control, which is occurring in various scientific communities all over the world, they took a broader view -- looking at the problem from the viewpoints of government, educators, researchers, landscape professionals and gardeners. Their joint efforts resulted in these findings:

1. People are the major distributors of plants.
2. The magnitude of this distribution is unprecedented and has allowed dispersal of species that manifest aggressive traits in new areas.
3. Plant introduction and improvement are the foundation of modern agriculture and horticulture, yielding diversity to our supply of plants used for food, forestry, landscapes and gardens, medicinal and other purposes.
4. A small proportion of introduced plant species become invasive and causes unwanted impacts to natural systems and biological diversity as well as economies, recreation, and health.
5. Plant species can be invasive in some regions, but not in others.
6. The impacts of invasive plant species can occur at times and places far removed from the site of introduction.

Then the workshop members laid out plans of action for each of the involved segments of the ornamental horticulture/gardening community. They determined that, "Codes of conduct for specific communities of interest are an essential first step in that they encourage voluntary initiative, foster information exchange, and minimize the expense of regulation."

A draft for the voluntary code of conduct for nursery growers dealing with invasive species has been nearly finalized. It was drafted at the meeting in St. Louis, and is being OK'd by university, environmental group and governmental representatives.

The preliminary codes they adopted for themselves include evaluating the invasive potential of introduced species, developing and promoting suitable alternatives, phasing out existing stocks of invasive species in their regions, following all laws on importation and quarantine of plant materials and encouraging customers to use noninvasive plants.

The Workshop on Linking Ecology and Horticulture to Prevent Plant Invasions also formulated a Voluntary Code of Conduct for gardeners:

1. Seek information on which species are invasive in your area. Sources could include botanical gardens, horticulturists, conservationists and government agencies. Remove invasive species from your land and replace them with non-invasive species suited to your site and needs.
2. Do not trade plants with other gardeners if you know they are species with invasive characteristics.
3. Request that botanical gardens and nurseries promote, display and sell only non-invasive species.
4. Help educate your community and other gardeners in your area through personal contact, and in such settings as garden clubs and other civic groups.
5. Ask garden writers and other media to emphasize the problem of invasive species and provide information. Request that garden writers promote only non-invasive species.
6. Invite speakers knowledgeable on the invasive species issue to speak to garden clubs, master gardeners, schools and other community groups.
7. Seek the best information on control of invasive plant species and organize neighborhood work groups to remove invasive plant species under the guidance of knowledgeable professionals.
8. Volunteer at botanical gardens and natural areas to assist ongoing efforts to diminish the threat of invasive plants.
9. Participate in early warning systems by reporting invasive species you observe in your area. Determine which group or agency should be responsible for reports emanating from your area. If no 800 number exists for such reporting, request that one be established, citing the need for a clearinghouse with an 800 number and website links to information about invasive plant species.
10. Assist garden clubs to create policies regarding the use of invasive species not only in horticulture, but also in activities such as flower shows. Urge florists and others to eliminate the use of invasive plant material.

Successful control of the green invaders can restore degraded, invaded habitats. For example, volunteer workers in Oregon eliminated diffuse knapweed from wooded hillsides and open grassland areas that are home to over 300 native plant species. Meanwhile, removal of alien grasses from Lanphere Dunes Preserve in California naturally restored native plant cover! Early reporting and quick response resulted in the complete removal of giant salvinia from a pond in north Texas. Conservation takes time, effort and good planning, but it works!

Garden clubs and educators are getting the word out. For example, The Invasive Weeds Awareness Coalition held a week-long series of events and activities in Washington, D.C. during the fourth annual National Invasive Weeds Awareness Week in February of this year. Education is a huge part of the solution.

As this topic evolves, it will be increasing important for responsible gardeners to find and follow the latest information and recommendations of acknowledged experts. It is important to get information specific to your area, because, as we've seen, general blacklists don't always work. Two of the best sources include your local Agricultural Extension Agent's office. The other is your local native plant society. Every area has its native plant groups where the natives versus exotics issue is always hot.

Discuss invasives with your aquarium and pond supplier. Water hyacinth and water lettuce are popular for biofilters, but virtually any aquatic plant can perform a similar function. Ask about alternatives -- there are many. A great place to start is with native species. You might be surprised at the diversity of native aquatics in your state.

Good sources for info on the web include the Federal sites, and Don¹t miss the University of Florida's Center for Aquatic and Invasive Species at The Missouri Botanical Gardens' site is another good resource:

Over zealous legislation may blacklist species that are vigorous but not truly invasive; so more research, if funding is provided in our weak economy, can help the decision-making process for legislators. We're already spending the money on controls - let's spend it more wisely on prevention. As Dr. Smart said, "I'll give you water hyacinth if I have to, but we've got to stop giant salvinia." More plant thugs are surely on the way. Now is the time for informed aquarists and water gardeners to study the issues in their states and get involved in the solutions.

The keyword is LOCAL. What's best or worst in your region may be the opposite in another. "Think globally -- act locally" was never truer than with the invasive species issue.

1. Learn about the problem plants and animals in your area. Your state's parks and wildlife department keeps an official list, and your favorite fish dealers and aquatic nurseries should know the list also.  
2. Be especially careful if building a pond near natural waterways to prevent aquatic plant and critter escape. 
3. Do not forget that wind, streams or even rainwater drainage systems can send weed seeds far from your home, into wild habitats! 
4. Never release plants or fish into the wild to "set them free." Give them to a water gardening friend, pond dealer or aquarist. They'll appreciate the gift.  
5. Lastly, be aware - fines are stiff. Keeping or releasing the wrong plants can be bad news for you, for the environment and for the government agencies that have to round up the escapees. 

Copyright Larry Maupin 2003 All Rights Reserved

Aquatic & Marginal Plants Index of Images
With Invasives Noted

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