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

Current Status:

Sept 18th, 2008

We are still seeing higher concentrations of the herbicide (~20 ppb) - this is still too high for safe irrigation.

August 26th, 2008:

King County measurements from Aug 20th show that the herbicide levels are still too high for the water to be used for irrigation.  Details:

August 20th, 2008:

King County measurements from Aug 13th show that the herbicide levels are still too high for the water to be used for irrigation.  Details:

Site A: 28 ppb

August 12th, 2008:

King County took two water samples on Wed, August 6th. The herbicide levels are still too high for the water to be used for irrigation.  Details:

Site A: just north of the lilies in the southern end of the lake, which was the middle of our largest treatment area, and
Site B: down the outlet stream near the Bassetti’s

Residual herbicide concentrations were:

Site A: 30 parts per billion (ppb)
Site B: 20ppb

The irrigation level is 1ppb, so we still have a ways to go - the next round of results will be available on Monday, August 18th.

A couple of people asked when we should see the milfoil dying back - here's the answer from Murph:

"As far as plants showing effects, it’s likely to take about 3-weeks before plants start falling from the water column, and signs of herbicide treatment are fairly subtle until at least 2-weeks after treatment. I did notice a bit of twisting in some of the plants when I was there Wednesday, and given relatively high residual concentrations, I expect the treatment will have desired effects…"

July 28th, 2008

Here's the status I got from Murph at King County after the treatment on July 28th:

"The treatment went well - 560lbs of gray pellets sprayed or hand-thrown into the water.

As far as monitoring, we’ll take the first set of samples tomorrow (Wed, Aug 06), and we should have results by Monday morning. We’ll sample again on Aug 13, and if necessary, again on Aug 21. I have a list of people to contact directly with monitoring results, and you can refer people to me if you’d like, or just pass along info about results I send to you.

My speculation/hope is that we’ll see concentrations close to but slightly above the irrigation limit on this test, and below on the next test, meaning people could water safely after I get 2nd test results on Monday Aug 18, but I really can’t say if this will be the case. However - people can water their lawns right away - there is no restriction for established grasses. For broad-leaf plants and food crops in particular, advise people to wait for the go ahead from me."

So, it sounds like the treatment went well and was completed on schedule. As soon as I have results from the water testing, I'll send them out. If anyone has been out on the lake and has feedback on the results of the treatment, please let me know.
 

Previous Updates:

	King County has found funding for treating the milfoil and the water lilies this year. They expect to get separate funding for second treatment next year.
	Beth Cullen and Michael Murphy will be surveying the lake in July to identify locations that milfoil is growing. The locations will get mapped using GPS technology.
	The weeds will be treated at the end of July with “Triclopyr”, a weed killer that targets broadleaf plants and has been extensively tested for usage in lakes. It has no restrictions for recreation.
	There is a restriction on water use for irrigation after the treatment. King County will measure the concentration of the chemical in the water after application and will let us know when the concentration is small enough so that it should not affect our gardens (likely two to three weeks). The full impact to garden plants is not known.
	King County expects to do only one herbicide treatment for the milfoil this year.

The proposed timeline is:

June 24: Notice about herbicide treatments sent to residents and businesses within ¼ mile of the lake (download the online version: page 1, page 2)

July 7-10: First lily treatment

July 16: Detailed milfoil survey (snorkeling)

July 29-30: Milfoil treatment, lily follow-up

Aug/Sept: Follow-up survey, milfoil pulling (snorkel and dive)
 

December 2007:

We had a community meeting on Dec. 13th, 2007 to discuss the milfoil problem in Cottage Lake. You can view the presentation and read the minutes to get up-to-date. We will talk about this issue again at the January 30th, 2008 General Meeting.

After the meeting, Michael Murphy put together a summary of the project proposal presented at the meeting.

Links:

Department of Ecology Page