Ecology and biology of invasive hybrid watermilfoil in northern tier waterbodies

Eurasian watermilfoil is the most widely managed aquatic invasive plant in the United States. Recently in Minnesota, it has started hybridizing with native Northern watermilfoil, resulting in a new hybrid watermilfoil. This is of concern due to reports of increased invasiveness, greater success, and evidence of herbicide resistance of the hybrid. However, these patterns have not been examined in the field.

Therefore, rigorous investigation of the ecology and invasiveness of this species is needed to inform evaluation of impacts and prioritization of management efforts. This study will help identify which invasive hybrid milfoil populations pose the greatest threat to public resources and that may warrant more aggressive responses. The project will address:

  1. How growth of hybrid watermilfoil compares with that of parental milfoil species, and whether performance varies among hybrid populations
  2. What the magnitude and outcome of competitive interactions are when the hybrid co-occurs with Eurasian and native watermilfoil
  3. How the phenology of hybrid compares with Eurasian and native, and whether it varies within hybrid populations


Between May 2017 and November 2018, researchers conducted repeat surveys of watermilfoil beds at nine lakes (3 lakes per species) in Minnesota to examine the timing and amount of surface matting, flowering, and stem growth. They found that in general, hybrid watermilfoil produced more flowers and flowered earlier than Eurasian or Northern watermilfoil, though this varied between years. Hybrid watermilfoil had greater overall surface matting than Eurasian watermilfoil and consistently more than Northern watermilfoil. Hybrid watermilfoil produced an intermediate number of stems compared to Eurasian and Northern watermilfoil.

Overall, this study provides field-based evidence of hybrid vigor in Hybrid watermilfoil. Specifically, earlier and increased flowering may increase Hybrid spread and provided a competitive advantage over parental species. Additionally, increased surface matting may shade out native species to a greater degree than parental species. This study underscores the importance of distinguishing hybrid watermilfoil from either Eurasian or Northern watermilfoil, as some lakes infested with Hybrid will likely require different management strategies.

Project manager: Dan Larkin

Funded by: U.S. Army Corps of Engineers

Project state date: 2016

Estimated project end date: 2019

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