Category Archives: Invasive Aquatic Plants

What Happens to Lake Plants in the Winter?

What happens to aquatic plants when there is ice on our lakes? Most of our native plants die back in the late fall and then sprout new growth in the spring from seeds, roots, or “winter buds”. However, a few plants, like native coontail and invasive curlyleaf pondweed, tough it out under the winter ice. These plants have the ability to endure long periods of low light and are adapted to living in cold water. However, the conditions under the ice in some winters  can challenge even these winter survivors.

Curlyleaf Life Cycle
Invasive curlyleaf pondweed out-competes native plants by sprouting in the fall, growing slowly under the ice, and then exploding toward the surface shortly after ice out.


Sunlight has no trouble getting through lake ice. Snow on top of the ice blocks out more sunlight, but plenty of light still gets through when there is less than a foot of snow.  However, things get dark quickly when there is more than 14 inches of snow – this can block out most of the light, making the under-ice world very challenging for plants and fish. Low light can lead to reduced oxygen in the water (less photosynthesis by plants), which can stress fish or even lead to winter fish kills.

Snow, Light, and Curlyleaf

For a long time, lake dwellers have noticed that invasive curlyleaf pondweed seemed to grow more densely in places where snow was plowed away for ice roads or where the wind blew snow off of the ice. This strongly suggests that the extra light that passed through these areas with little snow allowed curlyleaf sprouts to persist or grow slowly through the winter. Come spring, these curlyleaf plants would already have been a few feet tall and ready to explode toward the surface as water warmed.

Without sufficient light under the ice, curlyleaf sprouts are not able to grow or send out runners to make more stems. Come spring, this light-stunted curlyleaf may be more sparse and only a few inches tall. When the ice finally does disappear, it will likely take several weeks for the curlyleaf plants to catch up, meaning that if the spring weather does not allow them to grow quickly, they may have to compete for light with other plants, and may not reach the surface of lakes as quickly. This is the current thinking about why curlyleaf growth is noticeably less dense following snowy winters.

SCIENTIFIC SUPPORT: A recent study looked at the amount of curlyleaf in some Minnesota lakes over several years and found that snowy years did in fact coincide with reduced curlyleaf (Valley and Heiskary 2012; Lake & Reservoir Management)

Spring Curlyleaf Treatments

When treating curlyleaf with herbicides, the current strategy is to treat in the early spring when curlyleaf is actively growing, but before most of the native plants have sprouted new growth. This helps to protect the native plants from herbicide damage and kills the curlyleaf before it can produce new turions (reproductive buds). Right now, the Minnesota DNR issues permits for curlyleaf treatments that limit treatments to the period when water temperature is between 50°F and 60°F. The thinking behind this temperature range is that lab studies have shown that curlyleaf begins to grow actively at about 50°F (making it susceptible to herbicide damage), while some native plants sprout and begin to grow at around 60°F (only a few plant species tested).

However, this use of this 50° to 60°F water temperature range is less than ideal. I have tracked lake water temperature using automated probes for several years and have found that water temperature can change by more than 10 degrees over the course of one day in near-shore areas. Furthermore, water in shallow areas warms more quickly than in deeper areas, and springtime temperatures can fluctuate wildly from day to day.

So the question becomes – Where and at what time of day do we measure the temperatures? Should we focus on measuring temperatures in near shore areas, deeper spots, mornings, evenings? Each of these could give very different results. What if the lake has been cold for a long time, but then rapidly warms up to 65° due to a few days of very hot weather, only drop below 50 again in the following days? We need to remember that the temperature is being used as an indicator for what the plants are doing. Warmer waters are almost certainly driving the sprouting of native plants, but given the variability of spring water temperatures, that 50°-60° window seems like a moving target that may not be as practical in real lakes as it was in controlled laboratory experiments. I think there is an old idea that might just give us a better way to time these treatments.

What Can We Learn from Farmers

Long ago, farmers recognized that weather and soil temperature affected when different plants sprouted, grew, and reproduced. Over the years, scientist developed a simple method to track this cumulative effect of temperatures on different plants. This method uses weather records to calculate “Growing Degree Days” – really just a simple way to track the amount of heat that plants have experienced (one hot day would be the same as several warm days). This method is also used to calculate “Heating Degree Days” which tells us how much we had to run our furnaces to keep our house warm. A similar calculation for lakes may help to clarify the optimal timing for curlyleaf treatments and would allow for smoother planning by using weather forecasts to predict when key Growing Degree Day thresholds would occur.


Lake Minnetonka Pilot Study

DCIM100GOPROI am currently conducting a small research project on two bays of Lake Minnetonka (Hennepin Co., MN) to track early-spring sprouting and growth of native aquatic plants (with financial backing from the Minnehaha Creek Watershed District and Dr. Mike Netherland from the University of Florida – Gainesville). In addition to monitoring plants, I have installed electronic probes that will collect temperature and light data (~1 foot off of the bottom and in the top few inches of sediment) across a range of water depths. I plan to correlate plant activity with water and sediment temperature. My hope is that we will find clear patterns between native plant activity and a calculated  “Lake Growing Degree Day”. If we do find a clear pattern, this study may lead to additional data collection on more lakes next year to help refine how we time curlyleaf treatments and ultimately change how the DNR permits such treatments.



Does My Lake Have Cancer?

OLYMPUS DIGITAL CAMERAIn our bodies, cancer is marked by out-of-control cell growth that can eventually disrupt key bodily functions and lead to a downward spiral of health impacts that degrade our well-being.

Similarly, in lakes, infestation by invasive aquatic plants typically leads to expanding areas of dense plant growth that displace native plants, impair recreation, and can lead to a downward spiral of impacts that reduce the ecological and recreational quality of a lake.

Given these similarities, it shouldn’t be surprising that the strategies we use for controlling invasive aquatic plants are pretty similar to those that we use to treat cancer.

Early Detection
Cancer screening is touted as the best way to find new cancerous growth before it has spread.  If not caught early, treatment options are often limited, treatment must be more aggressive, and the chances for success are lower. Similarly, in lakes, the chances for controlling new infestations of invasive plants are best when new introductions are caught early. Once a new infestation has established in a lake, it is nearly impossible to eradicate.

Physical Removal 
When cancer is caught early, doctors often opt to surgically remove the growth to prevent spread. Similarly, in lakes, new infestations of invasive aquatic plants may be removed from small areas by using hand-pulling or very localized “spot” herbicide treatments. The success of such “surgical” plant removal strategies have the greatest chance for success when used on new infestations that have been detected shortly after the initial introduction to the lake. During this early stage of a new infestation, more aggressive plant removal strategies (like cutting and harvesting) can actually speed the spread of invasive plants by creating fragments that drift to new areas and begin to grow. Consequently, harvesting and cutting should only be used in lakes with established infestations.

When treating cancer that may have spread throughout a patient’s body, localized removal is not feasible. In such cases, doctors use aggressive drugs that are designed to attack the fast-growing cancer cells while leaving healthy cells untouched. Although this sounds like an ideal strategy, these powerful drugs often impact healthy cells to some degree, leading to side-effects like hair-loss and fatigue. The best chemotherapy drugs are those that are better at targeting the bad cancer cells while being gentler on good cells.

Similarly, using herbicides to control invasive plants over large areas in lakes sounds like a perfect strategy; kill the bad plants, keep the good plants. However, herbicides almost always lead to some negative impacts on native plants. Just as some minor chemotherapy side-effects can be tolerated by patients (hair loss and fatigue), minor impacts to native plants can be tolerated in lakes. However, overly aggressive or inappropriate use of herbicides can severely damage the native plants in a lake – potentially leading to a downward spiral of negative effects on water quality and fish. Herbicides are powerful tools that must be used carefully; more herbicide does not necessarily mean better results.

Palliative Care
When it becomes clear that cancer treatments are not working, doctors may move their focus from curing the disease to easing the suffering of the patient. In lakes with well-established infestations, it  is nearly impossible to eradicate invasive plants given the tools we currently have. In such cases, managers should consider shifting their focus from getting rid of the bad plants to controlling areas where the invasive plants are causing the greatest nuisance. In such situations, carefully planned harvesting and herbicide application can help maintain acceptable recreational or ecological quality in the lake.

A Cancer Staging Model for Lakes?
Over the last few years, I have been thinking more and more about how to develop a system similar to “cancer staging” for infested lakes. We have all heard of doctors diagnosing things like “stage II lung cancer”, but what does that mean? Current cancer staging (Stage I, II, etc.) uses a combination of characteristics (form of cancer, tumor size, tumor location, patient age, etc.) to classify specific cancer situations, thus making it easier for doctors to review the treatment of similar cases. This greatly simplifies the doctor’s treatment decisions, maximizes the chances for a successful recovery, and helps to set realistic expectations.

In contrast to the current state of cancer staging and treatment, aquatic plant management strategies are usually judged by whether they have worked in a fairly small number lakes. Too often, the results are not very clear – sometimes a strategy that worked well in one lake failed miserably in another lake. Obviously, this suggests that different lakes behave differently, but how does this help us plan a treatment in our lake? “Staging” of lake infestations would help clarify which past cases were most similar to our lake, thus simplifying our management decisions, maximizing our chances for successful control, and helping to set realistic expectations.

Infestation Staging would need to incorporate such things as:
• Which invasive species has invaded
• Degree of establishment (bed size/extent/turions)
• Native plant community
• Water clarity/nutrients
• Mean lake depth & % littoral
• Sediment characteristics

I would love to talk about collaborating with others on developing this idea further. Please contact me if you have any interest.

–James A. Johnson, MS