Hydrilla

Hydrilla verticillata

Background

Identification

• Grows submerged, rooted in mud or sand.
• Small spear-shaped leaves 1/2 to 3/4 in. long, 1/16 wide, with toothed edge, arranged in a whorls of 4 to 8 leaves, many whorls along each stem.

Identification key in: Hickman, J. ed. 1993. The Jepson Manual: Higher Plants of California. University of California Press.

Growth and spread

• Can reproduce from fragments of stems, rhizomes (underground stems), or roots.

Habitat and local distribution

• Canals, ponds, lakes (Hickman 1993).
• Native to Eurasia.
• No verified presence in the Delta as of summer 2006.

Impacts

• Can form huge masses throughout the water column that block water flow, shade habitat, deplete oxygen, and interfere with boating, swimming and other water recreation.

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 2003a; CDBW 2003b).

General control notes

• Essential to control is removing the vegetative and reproductive matter (Leavitt, pers. comm.).
• CDFA has a hydrilla eradication program statewide and should be contacted to discuss control efforts.

Manual or mechanical control

• Manual removal—effective for small infestations, but fragments must also be removed (Leavitt, pers. comm.).
• 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.
• Suction dredging—CDFA uses this method on a small scale, however fragments must also be removed to prevent resprouting (Leavitt, pers. comm.).
• Estimated costs: vary depending on plant density, equipment used, and transport fees for the removal of dredged material; costs for contract divers and dive tenders range from $1,200-2,400 per day, with additional fees for dredged material removal (Gibbons et al. 1999).

Biological control

• Sterile (triploid) grass carp—permit required by CDFG; currently authorized only in six counties in Southern California (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).

Chemical control

• Application of herbicides—CDFA applies copper-based products which provide rapid control, and fluridone which provides slower control of the above ground vegetation in the water column (Leavitt, pers. comm.).
• Estimated costs: costs for materials and application by a contractor may range from $900-1,400 per acre for fluridone, 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