Eurasian watermilfoil

Myriophyllum spicatum

Background

Identification

• Grows submerged, rooted in mud or sand, with branching stems 20 to 30 in. long that widen towards the root.
• Finely divided, feather-like leaves, 1/2 to 1­1/2 in. long, in groups of 4 around stem.
• Spike of flowers, 1­1/2 to 3 in. long, extends up from water surface, typically pink.

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

Growth and spread

• Grows rapidly, creating dense mats on the water surface.
• Can reproduce from stem fragments.

Habitat and local distribution

• Freshwater lakes, ponds, and slow-moving waters.
• Present but uncommon in Bay Area ditches and lake margins (Hickman 1993); also found in the Delta (BDOC 1994).

Impacts

• Dense mats at water surface can exclude native plants and related wildlife, shade aquatic habitat, and impede boating, swimming and other water recreation.
• Clogs irrigation pipes and canals.

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).

Manual or mechanical control

• Mechanical harvesting—expensive; results in further spread of plant due to the sprouting of plant fragments left behind (Spencer, pers. comm.); not recommended unless infestation covers significant portion of the water body.
• Estimated costs: harvesting costs range from $500-800 per acre, with additional costs for mobilization and equipment ($35,000-110,000) (WSDE 2001); there may be additional fees for disposal of plant material.
• Manual removal—time-intensive but viable method for infestations of less than one acre; all fragments and roots must be removed to prevent resprouting (Bossard 2000).
• 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.
• Raising water levels—limits plants' access to light; may be used in conjunction with light-limiting dyes or shade barriers (Spencer, pers. comm.).
• Lowering water levels—dehydrates plants or freezes them in winter (Spencer, pers. comm.).
• Estimated costs: minimal costs if outlet structure in place, with additional potential costs due to loss in tourism or recreation (Gibbons et al. 1999).
• Suction dredging—removes underwater roots and plant material on water surface; expensive, but good for small infestations (Spencer, 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

• No insect biological control agents approved by USDA for this species in California (Bossard 2000).
• Sterile (triploid) grass carp—may work, although Eurasian watermilfoil is not a preferred food (WSDE 2001). Stocking permit for aquatic plant management is required by the California Department of Fish and Game; 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—fluridone applied in low concentrations early in season to reduce impacts on native vegetation's active growing period (Anderson 1981).
• Estimated costs: costs per acre for materials and application by a contractor may range 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