Skip Navigation LinksWater-Lilies

​Water Lilies


What's the Issue?

Fragrant water lilies are a non-native, invasive Class C noxious weed. They were introduced into Cottage Lake by homeowners as an ornamental plant long before people really understood how fragile the lake ecosystem is, or the impact of non-native species. Water lilies grow in dense patches in water up to 8' deep and crowd out native plants. In addition, they cause the following environmental problems:

  1. The lilies trap sediment at the lake outlet - this creates more shallow water that the lilies can expand into.
  2. They create stagnant areas with low oxygen levels underneath the leaves which impacts fish and other wildlife (including salmon).
  3. This anoxic condition can decrease the water quality of the entire lake by increasing the phosphorous  absorbed from the soil.
  4. The stagnant areas are a perfect breeding ground for mosquitoes
  5. The shade of the lilies provides a hiding spot for non-native fish that prey on salmon.
  6. The lilies increase the water temperature by absorbing sunlight - higher water temperatures impact salmon and cause increased algae blooms.
  7. The lilies out-compete native plants and reduce biodiversity (this impacts the food web and native animals)
  8. They die back in the fall and the resulting decay further decreases dissolved oxygen and adds nutrients to the water that could contribute to increased algal growth and related water quality problems.

In addition to the environmental impact, lilies reduce recreation opportunities on the lake as it is difficult to swim or even paddle through them.

What are we doing about it?

2016 Update:

It had been several years since we treated the lilies and they were starting to spread again (particularly in the south end). In 2016, we scheduled a full lake treatment for the 1st week of August, 2016 (using Glyphosate as with previous treatments). The first treatment left a lot of health looking lilies, so the contractor came back for a second treatment (no additional charge) which seemed to be very successful. ​We are working with


August 2011 Update

This year's treatment is scheduled for the end of August - more details coming soon.

July 2010 Update

This year's treatment is scheduled for early August - more details coming soon.

Aug 2009 Update

The lilies were sprayed on Monday, August 3rd. A follow-on spraying may be needed for the south end where the lilies are still pretty dense. We should be seeing the lilies die back over the next two weeks.

July 2009 Update:

The 2008 treatment (possibly in combination with previous sprayings) was much more effective, and there are many fewer lilies this year. That said, we still want to eradicate the invasive fragrant water lilies completely and will continue the treatment plan this year.

King County was unable to obtain grant funding this year, so Friends of Cottage Lake has contracted with Northwest Aquatic Management to continue the eradication of Cottage Lake’s non-native fragrant water lilies. Treatment of the lilies will occur between August 3 and August 7, 2009, with a possible second treatment occurring later in the summer. As with previous years, the lilies will be sprayed with a herbicide containing Glyphosate and should die back within two weeks of the initial treatment.

2008 Update:

King County was able to secure grant funding to treat the lilies in 2008 as part of the effort to control Eurasian Milfoil (discovered in the fall of 2007). The treatment was the same (spraying with a herbicide containing Glyphosate)

September 2007 Update:

There have been quite a few questions on the water lilies and I apologize for not sending out a report earlier.

As you know, the lilies were sprayed on Thursday, August 2nd - we had expected the crew to come out early in the morning (like the year before), but they came later in the day so we didn't have a lot of people around to watch the work. The reports I got were a bit contradictory, but we do know they had some engine troubles and there was concern they didn't spray as much as we had expected.

Based on these reports, Matt had several conversations with the contractor about the concerns and they agreed to come out a few weeks later to check on the lilies (to judge how well the spraying went). Here is the report from the contractor after that second survey (the Thursday mentioned was Aug 16th):

"We were out on the lake Thursday. I did the 1st treatment and did a
complete analysis of the entire shoreline. Both east and west shorelines
are looking very good. Lily's are still dying. The north end lost about
50 feet and the south end lost about 100, as we went into the patch with
another pass. Judging by the kill, I would say that this is all we can
kill for the year without having the tubers be of issue. The way things are
looking, next year the entire remaining population can be taken care of
aside from the south end which is about 3 years away from complete

I also went out an toured the lake at this time - it is difficult without any maps or markers to determine the extent of the lilies that died back, but based on what I had seen before the spraying and after, I believe that they did kill back a significant section at the north and south ends (although maybe not quite the 100'). Some sections on the west side got very good coverage (no lilies remaining), while others still have a strip of lilies which will need to be done next year.

The "tuber issue" mentioned in the report above is the concern that if too much is sprayed in one season, the dead tubers can float up from the bottom and form islands of rotting plant mass that are difficult to deal with (which we want to avoid).

So, we only had one spraying this year and, with the exception of the south end, should be able to eradicate the lilies next year. This is slower than we had originally hoped, but part of that is due to the fact that we didn't get great coverage the first year. The estimate of another 3 years for the south end is definitely much slower than we had been told when the project started, so we will have to discuss how we want to proceed. The cost of the spraying this year was covered by the grant FOCL members applied for, so we still have some funds for next year (from the original fundraising).

I'd like additional feedback from those of you that live on the lake - how was the coverage in front of your house? Does the report from the contractor match your perceptions of the progress that was made this year? Please feel free to email me if you have comments or questions.


October 2006 Update:

Funding Update:

The eradication of the noxious weeds from Cottage Lake was made possible in part through a WaterWorks grant awarded by the King County Department of Natural Resources and Parks

Friends of Cottage Lake was awarded a $2,500 grant from the "2006 Small Change for a Big Difference" grant program for use in the eradication of the invasive water lilies. The grant was awarded by the King County Department of Natural Resources and Parks with the project being funded by WaterWorks. The $2,500 can be used for costs of permits and filings ($400), legal and public notification costs ($400) and the cost of the herbicide. The grant runs from August 17th, 2006 to Sept 15th, 2007.

Project Update:

See the article in the October newsletter for the latest information or read the web page version of the article here.

August 2006 Update:

The first round of spraying has now been completed around the lake and residents should start seeing the lilies dying back. The original spraying (July 26th) was halted due to the wind, but the crew came back on August 17th and completed the work. Aquatechnex is treating the lilies in strips (furthest from shore first) and will come back for a second round to get the lilies closer to shore. At the south end, it may take three treatments to get all of the lilies.

Frequently Asked Questions (FAQ):

Q: What other options did we consider for dealing with the lilies?

A: Please read the presentation outline from Katie Messick from the April 6th meeting. We discussed aquatic herbicides, manual and mechanical methods at the meeting.

Q: What is the difference between control and eradication of the lilies?

A: Control generally implies that you keep the lilies from expanding further around the lake. Eradication completely eliminates the water lilies from the lake.

Q: Why did the community choose to use an aquatic herbicide?

A: The community choose to eradicate the lilies and using an aquatic herbicide was the only feasible means given the current state of the lake. The lilies already cover about 7 acres of Cottage Lake and they are very dense at the south end. Manual solutions like hand pulling or hand cutting will not eradicate the lilies and probably would have little effect on the dense patches. Mechanical solutions (e.g. harvesting) are very expensive and have other negative impacts (stirring up sediment which can harm or even kill downstream mussels and
other creatures, possible injury to wildlife,etc.) and again are only considered a control solution.

Q: How safe is Glyphosate?

A: Glyphosate is a complex chemical designed to kill plants and, if used incorrectly, it can cause environmental harm. Of the chemicals available, it has a long history of both studies and use on land and for aquatic application. There is a wealth of information on Glyphosate and you can find research that supports our plan, and research that would make you think Glyphosate is the next DDT (see links section for some examples). I believe that testing,regulations, and the governing agencies (EPA, Ecology, etc) have come a very long way since the 1950s - I have read several studies on long term use of Glyphosate that make me believe that the risks in using it are outweighed by the benefits of eradicating the water lilies. Toxicity is one way to measure the impact of a chemical - this is a very complex subject, and I encourage people to read the research - the key thing to remember is that the concentration of the chemical in question is usually one of the key pieces in measurements of toxicity. Here's a quick tutorial from Katie Messick (King County):

".... here is the website address for the USDA's risk assessment for glyphosate:

This is a hefty document, but if you look at section 4 and appendix 9 it gives you the results of tests of the effects of specific glyphosate formulations on fish (other organisms are in other parts of the document). A quick tutorial: if it says "96 hours LC50 42.0ppm" for coho salmon fry, this means that within 96 hours at a concentration of 42 parts per million, 50% of the test population died. When you think about how many gallons of water are in Cottage Lake (around 300 million), you'd need to put 42x300 = 12,600 gallons of herbicide in the lake in order to approach a lethal dose. In fact, the maximum amount allowed for your infestation is 7.5 pints per acre. Worst case scenario, if you have 10 acres of lilies it would be 75 pints, or about 9 gallons, for the whole lake, which is .0007% of the LC50 dose. I've stuck all these numbers in to make sure that any of you who decide to read the risk assessment understand that the studies indicated were purposely done to measure lethal doses, not to indicate the actual effect on organisms when used properly."

Additional Important Points:

"1. When you are looking at toxicity data for Glyphosate (or other herbicides), be sure you are looking at data for the active ingredient alone (in this case, Glyphosate), and not specific formulations of herbicide (such as Roundup or Aquamaster). Exceptions of course are when you're trying to determine the toxicity of a specific formulation, in which case you should be sure that the data are referring to that formulation. Remember that most formulations of Glyphosate herbicides will have additives (surfactants) that are included in order to help deliver the active ingredient to the plant: they make it stick to the leaves or increase absorption rates, for example. Many of these surfactants are much more toxic than the active ingredient, and most are prohibited from use in an aquatic environment. Note, however, that some formulations (such as Aquamaster) do not contain any surfactants but will not work without them -- you can choose the surfactant that will work best for your needs (and there are a number that are specifically formulated for aquatic use).

2. Remember that many of the studies done were conducted to determine toxic levels of a given chemical. To use a more familiar example: one aspirin tablet will help cure a headache, but a whole bottle may kill you. The role of the contractor you hire to apply the herbicide is to ensure that they are delivering the proper dose to achieve the desired results but not damage the lake. I think the numbers I sent to Jonathan earlier (quoted above) do a pretty good job of demonstrating the difference.

3. Finally, this is not a hard science, with absolute numbers and clear answers. If you look for scientific papers or studies to support any given point of view, you will undoubtedly find them. Glyphosate has been in use for over 20 years and there is a large body of work studying its effects on various aspects of the environment. The majority of the scientific community holds that using this chemical in the prescribed manner is well within the accepted level of risk. You have every right to determine your own accepted level of risk. Remember that this also applies to all your other options: whether you control the lilies or let them remain, there are potential risks to the lake involved. The goal is to know what those risks are and make your decisions accordingly."



What can you do?

  •  Read more about the issue including the answers to the frequently asked questions on this page.
  •  Talk to your neighbors - we need everyone's help on this issue
  •  Make a donation
  •  Ask more questions -
    email me at:

Informational Links:

Water Lily Info (Ecology)

2006 Noxious Weed List

USDA Risk assessment for glyphosate

Glyphosate exposure study (terrestrial)

Herbicide issue has muddied the waters (Seattle PI article)

International Programme on Chemical Safety (Glyphosate)

North American Lake Management Society's position on herbicides

Wikipedia Entry on Glyphosate