Giant Salvinia

Salvinia molesta

Class A Noxious Weed

This plant is new to the state and has been designated a Class A Noxious Weed by the New Mexico Department of Agriculture.

Commercially Available

Despite being classified as a noxious weed, this plant is still widely sold in stores! Keep an eye out, and remember that it is illegal to plant and transport noxious weeds in the state of New Mexico.

Riparian Threat

This plant is known to invade riparian areas or otherwise damage the health and abundance of our water resources.
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Wildlife & Ecosystems

This plant is detrimental to native ecosystems, negatively impacting the quality of forage for wildlife and reducing biodiversity in plant communities.
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QUICK FACTS

Giant salvinia is a fast-growing aquatic fern that forms dense mats called a “sudd,” blocking sunlight, depleting oxygen, and disrupting aquatic ecosystems. It spreads quickly via fragments and is often unintentionally transported through human activities like boating and aquarium hobbyists.
This invasive plant damages ecosystems by displacing native vegetation, reducing oxygen levels, and creating stagnant water conditions that promote mosquito breeding, increasing disease risks. It also affects agriculture by obstructing waterways and infesting crops like rice.
Management is difficult due to its ability to reproduce solely through vegetative fragmentation. While early detection, mechanical removal, and water flow management can help, chemical treatments are often ineffective in large bodies of water.

1. Overview

family	Salviniaceae – Watermosses	origin	Brazil and Argentina
life cycle	Annual or Perennial	other names	Kariba weed, Aquarium watermoss, African pyle

The aquatic escape artist that suffocates native ecosystems and turns healthy waterways into a tangled mess.

Giant salvinia is a free-floating aquatic fern that wreaks havoc on waterways, even in its native range. The plant forms dense mats that restrict water flow and choke out sunlight. Dead plants decompose slowly and siphon the water’s absorbed oxygen, killing aquatic creatures. The thick foliage stagnates the water and becomes a haven for mosquito larvae, increasing populations of the disease-carrying insect in regions where the plant has invaded. Giant salvinia spreads easily via fragmented leaves and stems, making control of the weed extremely difficult.

History of Giant Salvinia

Native to parts of South America, Giant salvinia is a growing threat, particularly in the southern United States. The weed has become a problem due to the plant being sold in aquariums and water gardens. Aquatic hobbyists often need to change the water in tanks and ponds, which can allow contaminated water, including that with leaf fragments, to enter public waterways. It subsequently spread via boats, trailers, and streams. [2]

In 1981, giant salvinia was added to the federal noxious weed list, but it had already escaped into the wild. An infestation of the weed was first identified in 1995 in a small private pond near Walterboro, South Carolina. It was subsequently exterminated from the pond by the South Carolina Department of Natural Resources. A few years later, in 1998, a new infestation popped up on the Louisiana-Texas border, where it had grown large enough to interfere with boat traffic. By 2010, the weed had infected an additional 34 bodies of water in the state of Louisiana. [2]

Being a relatively new weed in the U.S. and, thankfully, confined to water bodies, giant salvinia is still not as widespread as many other species that have existed here for over 100 years. Some task forces have been successful in completely eradicating the weed on a statewide level. [2]

2. ID Guide

What does it look like?

Giant salvinia is an aquatic fern. It is a free-floating plant that does not require soil to grow. The fronds (leaves of the plant) are about 1/5 of an inch long and broad and green (with brown edges in maturity). The fronds form in whorls of three, with two above water and one submerged, modified frond that acts like a root in the water. The modified frond is usually darker in color, typically brown-gold or white, and stringy (highly divided). These modified fronds are often mistaken for roots. It has a slightly similar appearance to the native common duckweed (Limna minor) but is distinguished by much larger leaves and notable “egg beater” hairs. [2,3]

Having trouble identifying a weed? Contact us.

Key Features

Plant: The weed begins as free-floating leaves or fragments with horizontal stems that sit just below the water surface. Brown leaves grow on the stem below the water and become relatively long to help stabilize the plant. [1]
Leaves: The frond (leaf) blades are mostly ovate to oblong and are relatively flat in young plants. When the plant matures, the fronds tend to be slightly folded along the midrib, with a shallow notch at the tip and a cordate (heart-shaped) base. The fronds are covered in bristly, egg-beater-shaped hairs. The upper surface has rows of cylindrical papillae. Each papilla has four hairs that join at the tips to form an egg-beater shape. This basket-like structure acts as an air trap, providing the plant with buoyancy in water. [1]
Seeds: The plant does not flower and reproduces only through vegetative fragmentation. When the plant is mature, it will fragment spontaneously. Its egg-shaped spore sac develops neatly along modified stems below the water. These spores (if present at all) are deformed, infertile, and do not facilitate propagation. [1,2]

Robert Vidéki, Doronicum Kft., Bugwood.org

The fronds are covered in bristly egg-beater-shaped hairs.

Mic Julien, Commonwealth Scientific and Industrial Research Organization, Bugwood.org

The weed starts as free-floating leaves or fragments with horizontal stems that sit just below the water.

USDA APHIS PPQ – Oxford, North Carolina , USDA APHIS PPQ, Bugwood.org

The frond (leaf) blades are mostly ovate to oblong and are relatively flat in young plants, folding with age.

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Spore sacks are deformed, infertile and do not facilitate propagation.

3. Infestation Basics

“It is no exaggeration to say that Giant salvinia is one of the world’s worst weeds.”

-Dr. Randy Westbrooks, 2010

Impacts

Where does it grow, and how does it spread?

Giant salvinia grows in freshwater areas, particularly in warmer climates. While cold-tolerant, it will not survive icing over the water surface unless it has grown deep enough for plant fragments to escape freezing. As spores are sterile, propagation only occurs through the fragmentation of stems and foliage. [4]

It only takes a small fragment to start the colonization of an entire area. Anything that comes in contact with multiple water sources is at risk of facilitating the spread of giant salvinia. This means boats, trailers, animals, waterscape equipment, off-road vehicles, and connecting streams are all capable of unknowingly spreading the weed. Giant salvinia can still be easily purchased through aquarium and pond retailers, meaning uninformed hobbyists could introduce the plant almost anywhere. [5]

Giant salvinia undergoes different stages of growth in order to adapt to its surrounding environment. When experiencing adverse conditions or low nutrient levels, the weed will grow much more slowly with smaller leaves. When colonizing, it will move to open water areas and grow larger, more numerous leaves. When the plant is thriving, it forms dense, interlocking mats to sequester light from competing plants and dominate the area. At the matting stage, growth slows down to conserve available nutrients. [4]

The weed reproduces only through aggressive vegetative reproduction. As soon as a plant fragment is introduced to a new area, it enters the colonization stage and begins to expand rapidly. At this point, it develops with purposefully weak stems, allowing it to break and scatter easily, thus forming new colonies. [4]

Key takeaway: Giant salvinia spreads rapidly through small plant fragments, with various human activities and retail sales unknowingly facilitating its spread into new freshwater areas.

Why is it so invasive?

This weed is adept at invading large areas in a very short amount of time through rapid vegetative growth. This leads to the strangulation of waterways, endangering wildlife and the health of the waterway itself.

Giant salvinia creates large, floating mats of plant matter called “suds” on the surface of any standing fresh water. These mats sequester all available sunlight, effectively killing any other aquatic plants that grow below them. These mats can be up to one meter thick and can grow extremely quickly. Under ideal conditions, plants can double their biomass in just two days. Without a diverse community of aquatic plants, oxygen levels in water bodies will quickly deplete, compounded by a lack of soil disturbance due to the plant’s matting. Dead plant material from the weed decomposes slowly, requiring more decomposer organisms to break down the detritus, which in turn uses up even more dissolved oxygen. Giant salvinia can even impede crops, such as rice, making it difficult for farmers to access their agricultural areas. [3,5]

Because giant salvinia reproduces strictly through vegetative reproduction, it must spread quickly to facilitate the growth of future generations. It exists as either a perennial or an annual, depending on the environment. The plant exists only in freshwater environments or estuaries with salinity below 10% that of seawater. It is tolerant of cold temperatures but cannot withstand the formation of ice, meaning that the plant poses a greater threat in the southern states. [3,4,5]

Key takeaway: Giant salvinia rapidly forms dense floating mats that block sunlight, deplete oxygen, and disrupt aquatic ecosystems, threatening wildlife, agriculture, and waterway health.

Common risk factors for invasion

Human activity: Giant salvinia will spread wherever it has been introduced. Usually, this means it is more likely to infest aquatic areas frequented by humans. Boats are particularly at risk of spreading the weed as fragments can get lodged in propellers or boat trailer parts.
Water bodies: This weed will colonize any standing body of freshwater. This includes small lakes or lake alcoves, ponds, reservoirs, and wetlands. Warm climates allow giant salvinia to have a longer growing season, amplifying its detrimental effect on waterways. Even without human error, the plant can travel via streams and tributaries or through city rain overflows, leading to infestation in bodies of water separate from human activity.

Ecosystem Health

Once established, giant salvinia displaces native plants, resulting in a notable loss of floral biodiversity. This can significantly reduce oxygen and food availability for aquatic creatures, damaging the ecosystem as a whole. Plants can restrict waterways, causing stagnation in otherwise healthy streams. Stagnant waters are more likely to facilitate harmful bacteria and pathogens. [3]

Health and Safety

As giant salvinia form dense mats on the water surface, it can interfere with navigation and human access to waterways. It can damage boat engines and water equipment by clogging up machinery. The mats, combined with the stagnancy they cause, provide ideal conditions for mosquito larvae. This poses a risk to human health as mosquitoes carry harmful pathogens like malaria, encephalitis, and West Nile virus. [3,4]

Agriculture and Food Security

Farmers and ranchers face increased costs associated with managing this weed, including additional labor and mechanical control measures. Giant salvinia can infest rice fields, reducing plant growth and making harvest and care of the crop much more difficult. As it reduces dissolved oxygen levels, fish and other aquatic food sources may be killed. The weed also clogs up fishing nets, further harming the food industry. [4]

4. Management Strategies

The most effective way to control giant salvinia is to minimize its spread. Water recreationists should take measures to thoroughly clean all water equipment before entering and upon exiting any waterway. If possible, avoid thoroughly areas with known existing infestations. Mechanical control has been implemented to some degree of success. In lakes where total removal efforts have been abandoned, plant harvesters will clear paths for boats and watercraft. Small ponds can be drained and left to dry until all plants have withered and died. Chemical control has been effective, but only in water bodies small enough to maintain concentration. [3]

DO’s

Promote water flow. Giant salvinia thrives in slow-moving or stagnant water. Increasing water flow can make the habitat less favorable for the plant.
Keep a close eye on the growth of giant salvinia in your water bodies. Early detection is key to preventing widespread damage.
Use mechanical removal methods: Skimming or physically removing the plants from the surface is effective in reducing biomass and preventing the spread of the infestation.

DON’Ts

Transport plants from one water body to another, as this can spread the infestation. Clean equipment, boats, and footwear before moving between sites.
Ignore small infestations. Even small patches of giant salvinia can quickly grow into major problems. Address infestations early to prevent them from spreading.
Use herbicides in large water bodies. Large amounts of water will dilute the herbicide concentration to the point of ineffectiveness.

** The following information is provided courtesy of the UC Weed Research and Information Center. The Taos Soil and Water Conservation District does not endorse the use of any particular product, brand, or application thereof. **

As this is an aquatic plant, non-chemical control presents unique challenges and is typically complex and expensive.

Hand-pulling	Manual removal of the weed is only carried out to alleviate water blockages in the short term, and not as a permanent solution. This is because reinfestation is certain and the cost of manual control is high. Manual removal is only practical in the early stages of invasion.
Mowing	Typically, floating booms and wire nets have some value in containing salvinia infestations. However, such equipment is subject to breakage under the weight of large windblown mats
Tilling	Habitat modification is a useful strategy in modified areas or areas in which the use of herbicides is unacceptable. Dams, human-made lakes, canals and other water bodies can be drained (or the water level reduced) to strand and dry out giant salvinia.

Boaters and anglers can help prevent spread by removing all aquatic plants from propellers, intakes, trailers and gear before leaving a launch area. Always blow out jet ski intakes and wash boats and equipment land-side before traveling to a new waterway.

A weevil, Cyrtobagous salviniae, has been used successfully in at least 10 countries and has been released in Florida, Texas and Louisiana. It is native to south-eastern Brazil. Feeding and damage by the salvinia weevil is dependent on levels of nitrogen in the plant. In South Africa, Botswana, and India, where the weevils have been introduced, salvinia has been reduced to 1% of its former area. However, in the Northern Territory, Australia, high water temperatures have been associated with the failure of the weevil to control the plant. This may account for its inability to establish in California or Arizona.

The following specific use information is based on published papers and reports by researchers and land managers. Other trade names may be available, and other compounds also are labeled for this weed. Directions for use may vary between brands; see label before use. Herbicides are listed by mode of action and then alphabetically. The order of herbicide listing is not reflective of the order of efficacy or preference.

Glyphosate

Aquamaster, Rodeo

Rate: Broadcast treatment: 3 to 3.75 qt product (Rodeo or Aquamaster)/acre (3 to 4 lb a.e./acre) in water plus an aquatic approved surfactant containing 0.1% v/v non-ionic organosilicone and 0.25% v/v non-ionic spreader sticker. Spot treatment: 2% solution v/v Aquamaster and water plus 0.5 to 2% v/v of a non-ionic surfactant to thoroughly wet all leaves