A recent news update highlighted the spread of the highly-invasive aquatic plant hydrilla (Hydrilla verticillata). Sometimes called “waterthyme,” this plant has invaded the southern United States, some Atlantic states and part of the Great Lakes, including Ohio’s bodies of water. Now being recognized as a major problem for native vegetation and fish, it disrupts recreation on lakes.

Native to Asia and Africa, its introduction was thought to be connected by the aquarium trade, easily dispersed by small fragments and eventually introduced into our waterways.

According to the University of Wisconsin, hydrilla has long branching stems with four to eight serrated leaves and teeth under the midvein, growing rapidly underwater and spreading undetected. It is now found in our lakes, rivers, wetlands and even ditches, and it tolerates a variety of conditions such as full sun to complete shade.

Crowding out native species, dispersal of seeds is by water currents. Even recreational equipment such as boats, trailers, anchors or motors that are moved from lake to lake can spread their range. Because of its dense, matted growth, fish populations become limited, water becomes stagnant, and the lake becomes the perfect breeding area for mosquitos.

These aquatic plants have turions — specialized buds that can separate from the parent plant and drop to the lake floor, attaching itself, becoming dormant until spring. These monoecious plants have reproductive organs on the same plant, with the female flowers whitish and the male flowers greenish in color.

Controlling invasive plants is always a challenge, and this plant is no exception. Before a decision is made, surveys must be conducted to evaluate new and established infestations. Implementing a treatment plan and monitoring it annually are key to success.

According to the Department of Natural Resources, one of the control options is physical. Hand pulling, suction harvesting, surface barriers, dredging and chaining are only partly successful and require considerable time and effort. Another option is biological — implementing herbivorous fish such as grass carp, and herbivorous insects as leaf-mining flies or weevils, midges and the leaf-cutting moth. Biological control is inexpensive, selective and not labor intensive, but the tubers are not killed, and this method must be repeated often. Lastly, chemical control is always a last option because of its toxicity to fish, native plants and cost. Chemical control is non-selective and can affect more than the target plant.

Control of hydrilla will be a challenge for many years but monitoring and control is necessary to keep our waterways clean.

Kane Shipka is an Ohio State University Extension Master Gardener Volunteer.