Using ecological data to determine the best management practices for controlling the invasive aquatic plant, flowering rush

Background/Question/Methods

Invasive species are a threat to ecosystems and human use of these ecosystems.  In the context of climate change, habitat fragmentation and pollution, these threats are even more pronounced.  Managing the impacts of invasive species, under the auspices of our environmental challenges requires the careful consideration of organism ecology and phenology.  When a new invader enters a region, lake managers often move quickly to treat the problem.  This may involve a quick analysis of effective and acceptable herbicides and appropriate use.  However, a clearer understanding of the invading plant’s ecology and phenology and interactions with native species may ultimately lead to more rapid control of the invader.  The invasive plant flowering rush (Butomus umbellatus) is a growing problem around the Great Lakes, northwestern United States and several sites in Minnesota.  Flowering rush has been a pervasive problem in the Detroit Lakes chain of lakes in northern Minnesota for over 45 years.  Lake managers have tried mechanical and chemical controls with little success.  We assessed the growth, biomass and carbohydrate allocation of flowering rush and the native hardstem bulrush (Schoenoplectus acutus) to determine whether a better understanding of phenology and ecology could be used in to develop a more effective management plant for flowering rush.

Results/Conclusions

We found both the native and invasive plants emerged and senesced within one week of each other, with hardstem bulrush becoming significantly taller by June (F=224.32, df=1, P<0.0001).  Flowering rush reproduced predominantly by rhizome bud in the Detroit Lakes area (northwest Minnesota) and produced an average of one bud per two grams dry mass of rhizome or 400 buds m^-2.  Bud production remained high throughout the year, even under the ice.  Roots and rhizomes accounted for about 80% of the plant biomass.  Rhizome biomass remained consistently high in all lakes throughout the year, though some significant differences were observed by sampling date (F=10.42, df=17, P<0.0001) and sampling site (F=35.01, df=3, P<0.0001).  Starch content was significantly higher in rhizomes than in the leaves (df=3, F=7.72, P<0.001).  Management options for flowering rush will need to include spot treatments or treatment of small beds in order to minimize impacts to hardstem bulrush.  The allocation pattern to rhizomes and the high bud production will necessitate a multi-year treatment plant.  Best management practices must also consider climatic conditions such as wind, which results in rapid dissipation of herbicides in these lakes.