Starry Stonewort
Aquatic plant management: starry stonewort
What is starry stonewort?
Starry stonewort (Nitellopsis obtusa) is a bushy, bright green macrophyte. It looks like many native, beneficial grass-like algae, like other stoneworts and muskgrasses found in inland lakes. It can be set apart based on its production of star-shaped bulbils. These white, star-shaped bulbils are about the size of a grain of rice. They form on clear threads at the base of the lake weed. Starry stonewort is a macro algae. It does not have a vascular system like true aquatic plants. Each branchlet or stem is a single cell. Starry stonewort can spread from stem fragments and/or bulbils. This lake weed may form a dense carpet of material in shallow areas. This may impede recreational activities and ecological values such as fish spawning areas. Yet, the severity of these ecological and recreational impacts are still under investigation.
Aquatic weed control
Many researchers and state agencies have tried a variety of pond algae control strategies to reduce starry stonewort. Most of these algae control strategies have not significantly controlled starry stonewort. While many treatment service providers use copper, diquat, and other lake weed killers to control starry stonewort, these aquatic herbicides and algaecides have to be used repeatedly to be effective. Copper based algaecides, especially chelated copper products, can reduce starry stonewort biomass, and remove the majority of it from the water column. At water accesses, reduction of starry stonewort stops it from getting in boat motors and spreading to other water bodies. While they may provide reduction in the short-term, the use of copper-based aquatic herbicide or algaecide is not recommended for long term lake weed control of large established populations. Copper based algaecides have non-target impacts on native aquatic plant species. Examples are native macroalgae such as muskgrasses and stoneworts. Copper can be toxic to a range of wildlife, including fish. These are serious impacts from copper on beneficial lake plants and animals for limited and short term reduction in starry stonewort. Therefore, physical or mechanical methods might be better. Hand-pulling, lake weed cutters, or mechanical harvesting can provide quick reductions in starry stonewort populations. There can be potential reductions in following years. For example, hand-pulling of starry stonewort was done annually in Grand Lake (Minnesota) from 2017-2021. In 2017, around 450 pounds were collected. In 2021 less than 15 pounds were collected.
Where do I find out more?
Starry stonewort is still spreading throughout inland lakes in the region. Whether or not your lake has starry stonewort, aquatic plant monitoring is an important control strategy. It detects starry stonewort early and sees how it is spreading (see “Monitoring” fact sheet for more information). Early detection may not lead to eradication. Yet, it will make next mechanical/physical control efforts cost less and labor less. If your lake has nuisance levels of lake weeds or pond weeds, you should contact a lake management service provider to help see if control efforts are needed. If so, they help decide how to proceed (see the “Management Plan” fact sheet for information on finding a lake management service provider). The state’s natural resource agency is also a good resource for advice on nuisance and invasive aquatic plants.
Factors for deciding control include:
- How much of the lake has starry stonewort?
- Are the patches only starry stonewort?
- Are there native lake plants?
- Has the invasive aquatic weed area been staying the same or increasing?
If you’d like to start small-scale aquatic weed control efforts (such as hand-pulling), contact your local Extension agency. They help confirm if you have starry stonewort and help determine if there are other native aquatic plants mixed in with the starry stonewort that should not be pulled.