Characteristics: (WDNR) Hydrilla is a prolific, rapidly-growing submerged aquatic plant that can thrive in water from a few inches to 20 feet deep. Leaves are small (1/2 - 3/4 inches), triangular-pointed and occur in whorls of 4 to 8 leaves along the stem. Unlike many native water plants, hydrilla leaves have serrated edges and one or more protruding barbs or bumps along the midrib on the underside. They are usually green but may bleach in the sun to yellow or brown. Stems are heavily branched near the surface and grow horizontally, forming dense mats of vegetation. Small tubers are present at the rooted base of the plant.
Hydrilla has several methods of reproduction. Within a water body, branch or root fragments from broken plants can drift to new areas. Also, It can spread to new locations from plant fragments attached to boats and trailers. Turions - tiny, compact buds which form in leaf axils along the stem - break free and drift to new areas. Studies at the University of Minnesota have indicated that the turions of the monoecious form are likely to survive in northern climates. The dioecious form appears to be less cold tolerant. Tubers, which form on the roots and can lie dormant for several years, can propagate new plants. Hydrilla can grow in a wide range of conditions, including low light, flowing or still waters, shallow or deep. It out-competes the widespread invasive Eurasian water-milfoil with its even more rapid growth and reproduction. It is a serious threat to lakes and streams everywhere because of its adaptability.
Hydrilla is often confused with the native Waterweed (Elodea canadensis). Hydrilla leaves are slightly serrated and in whorls of 4 to 8 around the stem. Elodea leaves are smooth edged and occur in whorls of 3 around the stem.
Sketch from University of Florida
Methods of control: While eradication is unlikely, control is possible. Options for effective control vary with size and duration of the infestation, water quality conditions, lake bottom conditions, water flow, willingness to use permitted herbicides and financial resources. All mechanical and chemical control measures require permitting by the WDNR and development of an aquatic plant management plan.
Hand pulling can be effective when the colony size is small, water clarity is good and the lake bottom is soft enough to enable divers to completely remove plant roots. Care must be taken to avoid releasing plant fragments into the water column.
Mechanical harvesting is generally not permitted. Extreme infestations might qualify for harvesting if no other control methods are feasible. Mechanical harvesting requires a WDNR permit which will likely require an aquatic plant management plan.
Chemical control with WDNR permitted aquatic herbicides applied by licensed applicators is expensive and often controversial in terms of effectiveness and environmental safety. Repeated applications, often long term, are necessary for effective control. As with other control methods requiring WDNR permitting, chemical control programs require an aquatic plant management plan.
Links to PDF documents
Hydrilla - Have you seen this plant (DNR)
Links to other sites
Wisconsin DNR
University of Florida
Hydrilla - "The Perfect Aquatic Weed"
Stop Aquatic Hitchhikers
Hydrilla in WI: Press release
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