Great Lakes Hydrilla Collaborative

Early Detection

Best Management Practices for Hydrilla Detection, Management, and Monitoring

Early Detection

Train professionals to detect hydrilla early, especially in areas where there is heightened concern. Provide these individuals with information on who to contact if they find hydrilla or a plant suspected to be hydrilla.

Include signage at boat ramps to help aid early detection and provide outreach to lake associations, lake user groups, and marina owners focused on how to report the presence of hydrilla or a plant suspected to be hydrilla.



Timing of Herbicide Treatment

To maximize use of available management resources, conduct surveys for hydrilla when water temperatures reach 62.6 °F (17°C) for at least two weeks.

Conduct pre-treatment plant surveys beginning in mid-July to inform the annual treatment plan, as surveys will determine plant locations and will provide input to determine duration and dosage of treatment.

Use contact and systemic herbicides to treat hydrilla. Contact and systemic herbicides are associated with different treatment timing windows and address different aspects of the hydrilla plant. Employ chemical treatment using contact herbicides after tubers have sprouted (late June to July) but prior to the formation of new tubers (late August to November). Tuber sprouting has been documented to be synchronous in the Great Lakes and Northeast and, therefore, should allow for consistent annual treatment timing. In the Great Lakes Basin, time systemic chemical treatments, such as fluridone, which targets vegetative tissues (leaves, stems, or roots) to occur no earlier than mid-June, as systemic herbicides should be applied when tubers are sprouting.

Treatment (General)

Use bathymetric data to facilitate an accurate determination of water volume. Using this data will help generate treatment plans that will achieve more consistent and evenly distributed herbicide concentrations which is more efficient and cost-effective effective.

Provide herbicide applicators with GIS shapefiles of the areas to be treated that can be downloaded into their GPS systems. Doing so will help to keep herbicide applications in the target area of interest.


Monitoring is critical to assess the rate of plant expansion, inform the components of a treatment plan, and evaluate the efficacy of a treatment plan. Monitoring is critical to obtain an estimate of remaining hydrilla populations after each year of treatment. Monitoring data are also used to inform the development of annual treatment plan, to determine locations, durations, and dosages of herbicide treatments. Annual monitoring should include:

  • Assessment of tuber presence and density: Conduct annual fall tuber sampling at established locations via taking sediment cores, which provides an indication of tuber presence and density. Conduct this intensive monitoring for the first couple of years of a treatment plan because, as multi-year management projects progress, tuber numbers get so low that the sampling effort outweighs the benefits of collecting the data to demonstrate significantly reduced tuber values. During the later years of a project, perform assessments of plant species diversity and abundance as described below.

  • Assessment of plant species diversity and abundance: Annual rake-toss data provide an indication of hydrilla abundance (estimated biomass) as well as overall plant species diversity. Conduct surveys pre- and post-treatment each year using grids established using the point-intercept method. Pretreatment surveys should be performed mid-to-late July and post-treatment surveys should be performed in late September/early November. Increase grid size if new locations of hydrilla are found to facilitate a larger search area for detection. Assess the rate of plant expansion to inform the control strategy. Record the following metrics: native and rare plant species presence and abundance; and hydrilla plant status, including whether plants are injured, and whether there is re-growth or formation of tubers or turions.

1Systemic herbicides are absorbed by vegetative tissues (foliage, roots) and translocated to other parts of the plant.


Hebebrand, K., and J. Bossenbroek. 2017. Potential Spread of Hydrilla verticillata in the Great Lakes Basin, University of Toledo – Hydrilla Risk Assessment, Final Report. Prepared for Ecology and Environment, Inc.

New York Sea Grant. 2014. New York State Watercraft Inspection Steward Program Handbook: A Guide for Starting New Watercraft Inspection Programs. Prepared for the Cornell University Statewide Invasive Species Outreach Program Publication ID: NYSGI-H-14-001. Accessed online at:

Rothlisberger, J.D., W.L. Chadderton, J. McNulty, and D.M. Lodge. 2010. Aquatic invasive species transport via trailered boats: what is being moved, who is moving it, and what can be done. Fisheries, 35(3), 121- 132.