Phenology is the study of critical life stages in plants in relation to shifts in environmental factors during seasonal changes. Starry stonewort is a green macro alga in the family Characeae native to Eurasia. Much of the Midwestern United States has been invaded by this species. Starry stonewort has been seen to have late season growth patterns in its invaded range. This study found that this growth pattern was dependent upon water temperature and light transmittance both of which affect biomass production and senescence. It was hypothesized that peak biomass would occur in the late summer (August), however we observed biomass peak in November of 2020 and October of 2021 with 230 g m⁻² and 157 g m⁻² of total biomass, respectfully. Bulbil production was similarly hypothesized to peak in late summer (August) as well as early fall (September) and like biomass the bulbil production peak was later in the year. Bulbil production declined in the early summer as sprouting and aboveground growth occurred; but by July of each year bulbil production increased rapidly until the end of the sampling period in autumn. Peak bulbil biomass and density was lower in 2020 with 4.6 g m⁻² and 1,229 bulbils m-2, and in 2021 the biomass and density were 14.7 g m⁻² and 5,211 bulbils m-2. The average annual bulbil density was 1,537 bulbils m-2 and ranged from 0 to 156,944 bulbils m-2. The ability of starry stonewort to grow in dense mats and produce large quantities of bulbils contribute to the difficulty of controlling infestations. Bulbils are a method of spatial and temporal distribution that can allow for recolonization of previously treated areas.

The chemical treatments and combinations of chemical treatments of starry stonewort has been under researched. As such, copper, diquat, and copper + diquat treatments were applied and evaluated on small plots in Lake Koronis, MN during the summers of 2020 and 2021. It was expected that areas not regularly undergoing management would have higher biomass and bulbil densities, but in 2020 the reference areas were not significantly different from areas treated with diquat. In 2020 the copper plots and in 2021 the copper and copper + diquat plots were seen to have less biomass and bulbil densities than the reference plots. Applications of copper had more than 90% reduction of aboveground biomass by 8 weeks after treatment in 2020. Bulbil densities were not affected by copper treatments in 2020. Diquat in 2020 was not effective at reducing aboveground biomass or bulbil density at 4 and 8 weeks after treatment. In the diquat plots, bulbil densities ranged from 33.3 ± 33.3 to 4266.7 ± 3963.3 bulbils m-2 depending upon sample time and site. The lack of diquat efficacy was contributed to water exchange resulting in a half-life of <2 h among all treated plots. In 2021, copper treatments had a 78% reduction in aboveground biomass at 4 weeks after treatment and 27% at 8 weeks. Bulbil density was also reduced by 4 weeks after treatment in the copper treated plots. While diquat alone did not have activity starry stonewort under normal management scenarios in 2020, the copper + diquat treated plots in 2021 had seen a reduction of 76% and 65% in aboveground biomass at 4 and 8 weeks after treatment, respectfully. The combination plots had shown no reductions in bulbil densities. All plots, regardless of the treatment applied, had seen regrowth by 8 weeks after treatment.

While starry stonewort has late season peaks in biomass can imply a potential niche for it to occupy, this notion should be approached with caution as other species such as Hydrilla verticillata also have such seasonal peaks and are aggressively invasive. The biggest obstacle for the management of this species is bulbil production as treatments for aboveground structures have little to no effect on the sediment bound bulbils. The bulbils will then allow for rapid regrowth after treatment or recolonization the year following treatment. Since starry stonewort is such a prolific bulbil producer and can regrow rapidly, new strategies are needed to target bulbil production, induce bulbil mortality, or gain long term control of aboveground biomass. This study found that one application of herbicides was not enough to prevent regrowth via bulbils of starry stonewort in Lake Koronis which was also seen by Glisson et al. (2018). Though, one treatment of copper or copper + diquat did provide short-term reduction of aboveground biomass, or nuisance control; which should be more preferable than the “do nothing option” which will lead to an increased population in the coming years. If left unmanaged starry stonewort will lower the species richness of other macrophytes in the lakes it has invaded and may even limit fish spawning areas and the long-term viability of benthic organisms. Managers and stakeholders should be concerned of these impacts caused by an infestation of starry stonewort.


Ryan M. Wersal

Committee Member

Christopher T. Ruhland

Committee Member

John D. Madsen

Date of Degree




Document Type



Master of Science (MS)

Program of Study



Biological Sciences


Science, Engineering and Technology



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