AIS Spotlight – Hydrilla

Every month, we will put a spotlight on an aquatic invasive species (AIS) in a re-occurring monthly article. This month, Hydrilla! Check it out!

Hydrilla (Hydrilla verticillata) is a submerged rooted plant that grows vertically toward the water surface and, like Eurasian watermilfoil, can then grow horizontally across the water surface. The leaves of the plant are arranged in whorls of 4 to 8 leaves along the stem. One of the ways to distinguish hydrilla from the common look-alike native plant common waterweed (Elodea canadensis) is to look for serrations along the edges of the leaves of hydrilla, as well as spines on the underside of the leaves.  Common waterweed also typically has leaves in whorls of 3, while hydrilla has 4 or more leaves in a whorl. Hydrilla is native to India and Korea but was imported to the United States for use in aquariums. Wild populations of the plant were first seen in Florida following illegal aquarium releases.

Hydrilla prefers habitats with slow moving water in lakes, rivers, and other freshwater systems. Once it is established in a waterbody, pieces of the plant can break off from the mother plant and start a new plant somewhere else in the lake. Hydrilla can also be transported on boats, trailers, and other recreational equipment to new waterbodies. This invasive plant creates tubers that allows the plant to survive in the sediment of waterbodies during winter months. These tubers can survive out of water for several days and remain viable in lakes for up to four years.

Once established, hydrilla has been documented to cause significant disruptions in invaded waterbodies. Similar to Eurasian watermilfoil, hydrilla can grow faster than many native plants. Since hydrilla can grow horizontally across the water surface once it reaches the surface, it can form mats that can shade out native plants. The dense mats of hydrilla can interfere with boating, fishing, and other recreational activities on the water. Dense mats can also slow and change water flow in systems. Research on the impacts of hydrilla on native wildlife varies widely especially when comparing between lakes. As with most invasive species invasions, impacts will vary in intensity between different lakes. Hydrilla is classified as Prohibited in all counties in Wisconsin.