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​Eurasian Milfoil in Cottage Lake:

On September 26, 2007, staff members from the King County Lake Stewardship Program found Eurasian water milfoil (Myriophyllum spicatum) in Cottage Lake. Surveyors estimated that individual plants and small patches covered a combined total of less than two acres scattered in shallow areas around the lake. Floating fragments were present as well. Milfoil was not present at the time of the last formal weed survey (July, 1995), and although milfoil has been seen in recent years in the lake, it was not identified as Eurasian milfoil until September, 2007.

If left uncontrolled, the Eurasian milfoil will likely grow into dense stands throughout much of the lake, extending from the shallow water at the shoreline to a depth of about 10 feet. Dense milfoil can:

  1. Make swimming dangerous
  2. Foul fishing gear and electric motors
  3. Prevent light and oxygen from penetrating to deeper water
  4. Reduce dissolved oxygen by increasing water temperature
  5. Add nutrients and further decrease dissolved oxygen during decomposition, and  
  6. Displace native plants and reduce biodiversity

Update (9/28​/17):

The treatment was complated on August 7th and the results look good. It took about 4 weeks before the chemical levels were low enough to remove the restriction on irrigation. We'll continue to survey the lake, and are currently planing on a follow-up treatment next summer.

Update (8/06​/17):

The milfoil treatment is scheduled for Monday, August 7th (tomorrow) - the contractor will be applying a selective herbicide (2,4-D) in the areas where milfoil was found during the survey. The herbicide will only kill milfoil, so please don't be surprised when you still see plants in the lake (the native plants provide habitat for fish, insects, etc.). We expanded the treatment area slightly based on the latest survey (milfoil was found all around the lake with the exception... of a section of the east side where the water is presumably too deep for milfoil to grow). The herbicide is targeted (to get the highest concentration near the milfoil), but it is a liquid so will spread. Please don't forget to turn off your irrigation systems if you use lake water (2,4-D will not harm grass, but could kill other ornamentals). There is a minimum 7 day restriction on irrigation using lake water and we will be testing weekly to determine when the water is safe to use again.

Since the contractor will already be out on the lake, it was also cost effective to have him treat water lilies again - the treatment last year was pretty successful, so there are just a few areas where we have large masses of lilies. Again, we are only targeting the invasive fragrant water lily (large, white blooms), and trying to avoid killing the native lilies or other native plants (e.g. spatterdock - yellow blooms, leaves stand out of the water)​

More Details on Restrictions:

The treatment with liquid 2,4-D will target areas with milfoil in the lake, but the herbicide will spread and there is a minimum 7 day restriction on irrigation using lake water. The herbicide will not impact grass, but could kill ornamental or other garden plants. We will be testing the water weekly to monitor the level of herbicide and hope the levels will drop quickly, but, based on reports from other lakes, I would plan for 2-3 weeks without lake water irrigation. I will update this email list as well as the website when we have results from the tests (starting Tuesday, Aug 15th).

Next year, we are trying to move the treatment earlier in the summer with the hope we could treat at the end of June (when the weather is generally cooler and not as dry). Thanks again for your willingness to deal with the restrictions and help fund the treatment.

Please note that Ecology is also recommending a 24 hour swimming restriction and you will see that recommendation on the postings around the lake. My understanding is that this recommendation stems from the other chemicals in the treatment solution which will quickly dilute, not the herbicide itself.

Update (7/07/17):

The community met in May and has decided to pursue a chemical (herbicide) solution using 2, 4-D. Treatment is scheduled for early August and notifications will be mailed out soon.


Update (7/21/16):

Hand pulling didn't meet residents expectations and the year-over-year expansion of the milfoil has continued. In addtion, we had a warm winter and much of the milfoil didn't die back (so we have even more this summer). Based on recommendations from specialists and looking at the solutions other lakes have tried, we may want to look into a chemical (herbicide) solution such as 2, 4-D. Some links that might be useful: 

Update (7/31/15):

We hired a larger dive team to see if we could cover more area since the milfoil seemed to be much worse this year with the warm weather. Sterling Marine had a team out on the lake for two days was out on the lake for a second day with a dive team pulling milfoil. This was the last day of milfoil treatment for 2015 (due to budget constraints).


Update (6/27/2015):

Friends of Cottage Lake selected Cameron Tilley as the contractor for milfoil control this year and he started this week at the north end. A fundraising letter has also gone out to all lakeside residents. If you want additional copies, please let me know. Cameron can also remove additional non-invasive weeds from around your dock or lakeshore - if you are interested in hiring him for some additional work, I can connect you. Please read this document​ on weed removal before moving forward (it spells out what you can and cannot do, and acts as a permit for basic hand pulling).

Initial 2015 Update (6/9/2015):

Friends of Cottage Lake will be hiring divers again this year to control the milfoil. Please look for a letter in the mail soon as we need more donations to fund the effort.

If you are interested in more information on milfoil, check out this story from King 5:


2014 Update (8/25/14):

Matt Harrington and his scuba crew will be back on the lake one last time today to do one more sweep and hit the spots where they found the most milfoil. Thanks very much for all the feedback you've sent - so far, I've been very impressed with Matt and amazed at how much milfoil they have pulled.

Matt has suggested that we can really help reduce the amount of milfoil that spreads each year by scooping out any fragments we see while out on the lake. Here's a link to a picture of a milfoil fragment - - the plants break very easily and these fragments sink down to the bottom and start new plants.

Matt and I tried out some small aquarium nets from Amazon that work really well for scooping up fragments - here's a link - these are only $6.99 and I'd be happy to do a bulk order if anyone is interested.

I'm also trying to see if we can get a workshop on aquatic weeds for the neighborhood - there's a workshop down in Maple Valley on Wednesday this week in case anyone is interested.

Current plans:

In 2014, Friends of Cottage Lake signed a contract with Alki Diving Services, LLC to hand-pull milfoil in the lake. The cost of hand-pulling was similar to the cost of chemical control, but without the environmental, recreational, or irregration impact.

Alki Diving Services will be in the lake the week of August 11th and I'll report on the results of this experiment once they have completed the work. If things go well, I'd like to purpose continuing this treatment for several years in an attempt to get the milfoil problem under control.

What's been done:

For three years (2008-2010), the milfoil was treated each summer with “Triclopyr”, a weed killer that targets broadleaf plants and has been extensively tested for usage in lakes. The treatment was paid for using grant funding and was completely managed by King County. The final report from the treatment can be found here:CottageMilfoil_FinalReport.pdf