Dear Lady of the Lake,

I have a timeshare on a channel on Clear Lake. This year with the low water, there is a bright green vine-like plant that is growing across the channel and taking over everything. I have enclosed a photo. Can you tell me what this is and will it go away when the water comes back?

— Paulo


Dear Paulo,

Seems like you are pretty peeved about Primrose and I don’t blame you! Based on the description and photo you provided, this nuisance invasive plant you are asking about is commonly called creeping water primrose, yellow water primrose, or marsh purslane. it’s scientific name is Ludwigia peploides. There is another variety of primrose that is almost identical called Ludwigia hexapetala, also commonly just referred to as marsh purslane, creeping primrose, Uruguay water primrose, or six-petaled primrose (although not all specimens have six-petals). Both of these have been found in abundance in Lake County and around Clear Lake.

Both creeping water primrose species are labeled by Cal-Flora and California Invasive Plant Council (Cal-IPC) high risk rating for invasiveness.

This plant is definitely a nuisance, but it’s origin and status is unclear depending on the source you are using. Some sources, such as the Jepson Manual, consider these plants “naturalized” as they are found all over the Americas, both north and South. This source also considers this plant an invasive weed. The US Department of Agriculture's Invasive Species Compendium (ISC) labels this plant a noxious weed. In many lakes and wetlands Primrose is just another member of the aquatic plant and wetland plant community. However, in other places, like Clear Lake, primrose is a very competitive, aggressive and destructive invasive that displaces native, beneficial wetland plants like tules, smartweed, spike-rush, and native sedges.

Primrose, when it grows densely in a water-way can clog water intakes, restrict boat access and navigation, prevent recreational access, trap fishing lures and lines, and in some cases dense primrose creates the stagnant water conditions that promote West Nile Mosquito habitat.

Description

Invasive primrose is a herbaceous perennial (comes back every year from the same roots). Commonly found in wetland or riparian areas such as shorelines, mud flats, creeks, river edges, wetlands, and marshy areas. This plant will creep along the water surface, hence the “creeping” reference in its common name, but the plant has to root from the shoreline and creeps over the surface of the water. It doesn’t rise up from the water like tules or water lilies do.

The leaves are bright green around 3- 5 inches and can be slender and pointed like a lance with early or younger leaves round or egg-like. The leaves are hairless, and found on a stalk 1-2 inches long. The stem of primrose can grow indefinitely through the summer; I have pulled stems from the water up to 3 meters long.

These plants creep along and grow like strawberries; they send down shoots from the creeping stems that go into the ground so when the plant dies at the end of the season, those roots are ready to grow new growth the next season. It’s perfectly engineered to grow very successfully. This makes it the perfect invasive.

According to Cal-Flora, primrose has a heat tolerance of 95 degrees, but it’s very evident that it can tolerate the months of 100+ degree weather we have been having here. In fact, even without aquatic shorelines this year around Clear Lake due to the drought and record-low water levels, the creeping water primrose seems to be very resilient!

During the summer with low water levels and extreme heat, the tules, as well as other shoreline species, are showing signs of heat and drought stress. However, the creeping primrose seems to be doing just fine! In fact, as you observed Paulo, the primrose is using the drought and low water to its advantage and growing in the empty space provided by the low water levels. Researchers have found that overall warmer climates favor the seedling viability, the germination success, and the biomass of seeds produced in introduced primrose in California and France (Gillard et al. 2018). Again, this is a life history characteristic that makes this plant and excellent and effective invasive species.

So while the drought seems to be benefiting this species, it also grows very well in wet and high-water conditions. I think the primrose growing in the middle of a channel, for example, will not necessarily grow when the channel is under 5-10 feet of water next year, but the shoreline primrose and it’s roots are very much still viable along the shoreline and very capable of creeping into and across the channel throughout the next summer season.



Management and mitigation options

As a lake manager, primrose is one of the most frustrating species that I am trying to manage on Clear Lake. Complete eradication is probably not a feasible goal, so control and maintenance is the more likely strategy for management. This strategy is very expensive and requires consistent and regular effort. Every year more primrose can grow, and every year it has to be actively managed.

Invasive plant management science is a broad and burgeoning field, as new species and newly observed characteristics are constantly adding to the knowledge and field of research. Basically, there are more species and more issues than there are available research funds and researchers. Every system and ecosystem responds differently to the introduced invasive species and potential treatments. Sometimes, a treatment shown to be highly effective in the lab setting is less than successful in the field in real-life applications. That makes management difficult, especially when the options are all very expensive, such as herbicide chemicals or labor intensive manual removal.

For private properties and homeowners that live along Clear Lake shoreline and connected channels and canals, there are management options through the Clear Lake Integrated Aquatic Plant Management Program. With the right permits and instructions, you can manually remove the primrose from your shoreline zone, or you can hire a professional licensed herbicide applicator to chemically treat the primrose on your shoreline.

There is also a local diver company that can manually pull the primrose from your shoreline by hand, but this is most effective when the water level is high enough that the plant is actively growing in the water. This last option is very effective, but most effective for smaller shoreline properties.

There are pros and cons to every treatment option, but for an effective removal that can last, both chemical or manual treatments need to be implemented for several years consecutively in a row to remove the seedbank and any remaining fragments or roots.

If you have property on Clear Lake or just want to learn more about primrose ecology and management, the County of Lake Water Resources Department is hosting a free, virtual webinar on primrose at noon on Friday, Aug. 27. The event flyer is provided below.

Sincerely, Lady of the Lake

Angela De Palma-Dow is a limnologist (limnology = study of fresh inland waters) who lives and works in Lake County. Born in Northern California, she has a Master of Science from Michigan State University. She is a Certified Lake Manager from the North American Lake Management Society, or NALMS, and she is the current president/chair of the California chapter of the Society for Freshwater Science. She can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it. .