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Giant Salvinia spp.

(Salvinia molesta, Salvinia herzogii)

Photo of giant salvinia
Photo credit: Forest and Kim Starr

Giant salvinia are a species of floating water ferns that are often grown as ornamental plants but can escape into the wild and become noxious weeds. They have also been used to extract nutrients and pollutants from water. These species can overgrow areas of still water, causing major disruptions to recreational activities. They are extremely similar in appearance and habit, and very difficult to distinguish from each other. Both are considered invasive in Wisconsin and share classification.

Overview

Other names for this plant 
  • Common names: Kariba weed, African pyle, butterfly weed, aquarium watermoss
Ecological threat
  • In good conditions, this plant can double its dry weight every two and a half days, quickly overwhelming other plant species and forming floating mats of vegetation one to three feet thick.
  • After plants die off in autumn/winter, organisms that decompose the plant material use up the oxygen in the water. In addition, the thick mat of vegetation on the water’s surface can limit the exchange of gases between air and water. These processes lead to water stagnation, killing fish and other organisms.
  • Thick mats of vegetation can restrict boating, swimming and affect drainage or flood mitigation.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for giant salvinia species was based upon this literature review [PDF] developed by the department.

Identification

Leaves: The plant is made up of small spongy green leaves along a stem, in whorls of three with two floating and one submerged. The submerged leaves look like roots, and the floating leaves are covered with rows of hair that repel water and allow the plant to float.

Flowers: None.

Fruits & seeds: A nutlike sporocarp can be produced underneath the plant, but reproduction is primarily through vegetative fragments.

Roots: No true roots – submerged leaves can appear feathery and root-like.

Similar species: There are several other species of Salvinia spp. all of which share similar appearance and growth habit. From a distance, the floating vegetation may appear similar to other aquatic invasives such as water lettuce (Pistia stratiotes) but upon close examination, water lettuce lacks the hairs giant salvinia has covering the leaves.

Distribution

Currently, there have been no reports of giant salvinia in Wisconsin. Have you seen it? Send us a report.

Control

Mechanical: Giant salvinia reproduces from plant fragments, so mechanical removal must be done carefully and thoroughly, removing all parts of the plants. This is usually only achievable with small populations, so eradication of large populations is unlikely. Once removed the plant must be dried, burnt, bagged and placed in a landfill, ensuring it will not reenter the waterway.

Chemical: Certain aquatic herbicides such as diquat or fluridone have shown effectiveness against giant salvinia infestations. Large applications of herbicide can cause a rapid loss of oxygen in the water due to dead material sinking to the bottom and decomposing. Aquatic applications of herbicide require a permit.

Photos

View giant salvinia pictures in our photo gallery.

Resources

Sources for content
  • University of Nevada Cooperative Extension: Identification and Management of Giant Salvinia, Factsheet-02-069.
  • University of Florida Center for Aquatic and Invasive Plants: Factsheet [exit DNR]
Links for more information