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 (07/29/22):

The milfoil treatment has been scheduled for Monday, August 1st, 2022.

If you irrigate from the lake, I recommend turning off your system for 3 days starting Monday to avoid any chance the herbicide damages your plants (it is safe for turf, but not ornamentals or broadleaf plants). Otherwise, there are no restrictions with the chemical we are using (and the recommendation for irrigation is just 1 day, but we are suggesting 3).

Update (05/31/22):

A community Meeting on lake treatment options was held on May 14th. You can view the presentation from Aquatechnex here. Aquatechnex will be back on the lake to do an invasive plant survey (focused on milfoil) the week of May 31st, 2022.

Based on the presentation and information provided, we are hoping to treat the milfoil this year and are updating our permit to add ProcellaCor (Florpyrauxifen-benzyl) as an option for treatment (faster acting and fewer restrictions than 2,4-D)

General Update (08/26/21):

We last treated the lake for Eurasian milfoil in June of 2018 – we saw some milfoil return in 2019 and more in 2020, but it has not yet grown back to pre-treatment levels.

We’ve been tracking new treatments for milfoil – one newer chemical, ProcellaCor, has been approved for use in Washington State and has been successfully used on several larger lakes. The biggest challenge with previous treatments (using 2,4-D) was the irrigation restrictions, but there was also some community concern about the herbicide. The new herbicide is used at a much lower concentration, applied directly to the target weeds, and has only a very short restriction on irrigation (less than one week). The cost is expected to be similar, but there will be additional costs to survey the lake before treatment.

Please report large milfoil infestations when you see them in the lake and provide feedback on future treatment plans. We will have to treat again to deal with this invasive weed before it gets back to the level of infestation we saw before treatments in 2017 and 2018. You can read about our problems with invasive weeds in the Integrated Aquatic Vegetation Management Plan.

Latest Update (06/22/18):

Lake treated June 14th – if you have seen any small milfoil plants this year, you should see them die back in the next 2 weeks. Remember this is a selective herbicide, so it will not kill other plants in the lake.

The first water sample results (taken June 20th) are back and we are still above the safe limited for irrigation (expected based on our experience last year). We’ll sample each week and post updates as they as the results come in, but expect it to be about 4 weeks before it is completely safe to irrigate with lake water again.

Update (06/19/18):

Woodland Resources treated the lake for milfoil on June 14th. Do not use the lake for irrigation of anything other than lawns until we post the all clear (the herbicide can kill gardens and ornamental plants).

Update (8/08/17):

In 2017, we agreed to try a herbicide treatment for the milfoil. We hired a contractor to treat the lake with 2,4-d in August and the lake was treated on Aug 7th.

The contractor applied 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)

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.

What’s been done in the past:

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