Giant salvinia

Salvinia molesta

Background

Identification

• Free-floating aquatic fern, with no true roots.
• Leaves in sets of three, two floating, one submerged and root-like; oval floating or standing leaves are arranged in opposite pairs, each typically 3/4 in. long. In dense mats, leaves take a nested appearance, arranged on-edge in chains (see photo).
• Upper leaf surface covered with minute hair-like structures (see photo).
• Typically forms mats on water surface up to several feet thick (see photo).

Growth and spread

• Grows very rapidly, plant population can double in a week.

Habitat and local distribution

• Freshwater ditches, ponds, lakes, calm rivers.
• First seen in US in 1998; spreading through southern US, including the lower Colorado River on the CA/AZ border. Found and possibly eradicated in San Diego River (WAPMS 2003; USGS 2003).
• Potential invader of Bay Area freshwater sloughs.

Impacts

• Crowds out native plants.
• Thick mats shade shallow habitats.
• Depletes oxygen in water.
• Clogs irrigation and water supply structures.

Prevention and Control

Prevention

• Invasive plant awareness and regular monitoring is critical to identify and stop a new invasion before it takes off.
• Inform the public and any boat launch area staff of the need to remove all plant debris from boats and equipment at the ramp area after each use. The California Department of Boating and Waterways (CDBW) has developed educational resources for boaters (CDBW 2003).
• Preventing new infestations is particularly important with Salvinia due to the plant's extremely rapid growth and reproduction.
• Local plant management agencies (e.g., CDFA, CDBW, and USFWS) should be contacted if Salvinia is discovered in the Bay-Delta watershed.

Manual or mechanical control

• Manual removal—CDFA uses this method in small ponds (Leavitt, pers. comm.); however, method not effective for larger areas.
• Estimated costs: vary depending on if volunteers conduct removal and on the plant density; if divers and dive tenders need to be contracted, costs may range from $500-2,400 per day (Gibbons et al. 1999). There may be additional fees for disposal of plant material.

Biological control

• Sterile (triploid) grass carp—stocking permit for aquatic plant management required by CDFG (Leavitt, pers. comm.); however, not permitted in many San Francisco Bay-Delta waterways and not suitable for water bodies with inlets and outlets.
• Estimated costs: costs per fish range from $7.50-15.00 (Gibbons et al. 1999); quantity dependent on plant species, density of plant, and water temperature (WSDE 2001).
• Salvinia weevil ( Cyrtobagous salviniae)trials are currently underway in the lower Colorado River (Olsen 2003).

Chemical control

• Application of herbicides—CDFA applies fluridone at low rates in water bodies where there is little to no flow to maximize the herbicide's exposure time (Leavitt, pers. comm.).
• Estimated costs: costs per acre for materials and application by a contractor range may from $900-1,400, depending on size of treatment area, scale of treatment, and dosage. It is recommended to contract a licensed professional for herbicide applications (Gibbons et al. 1999).

References and more information