Uh oh, that’s not milfoil – Green Lake has a new aquatic weed to deal with

Egeria stem being held by someone with a person in a canoe in the background
Egeria plant found during the Green Lake survey in May 2021. Photo by Ben Peterson.

If you’ve swam or paddled in Seattle’s Green Lake, you’ve probably seen and felt the masses of feathery Eurasian watermilfoil in the water. It’s not fun to swim through, and its growth has been a challenge for lake users for many years, along with the various kinds of algae that give the lake its color and name.

Now the milfoil and algae have company – a newly discovered infestation of another aquatic noxious weed called Egeria, or Brazilian Elodea. Readers of this blog might remember the discovery of the same plant in Lake Rasmussen in Duvall last fall by a young nature enthusiast named Maggie. Green Lake’s Egeria discovery was also made last fall by Friends of Green Lake (FOGL) member, Rob Zisette, who also happens to be an aquatic scientist (lucky for Green Lake)!

In November 2020, Rob noticed pieces of Egeria washed up on the shore at the northeast end of the lake during FOGL’s fall milfoil cleanup. This was the first time anyone had seen the plant at Green Lake, even though volunteers check the lake every year. King County Noxious Weed Specialist Karen Peterson visited the lake the next day and confirmed that it was indeed Egeria densa, the Class B Noxious Weed, and not a similar native plant called common waterweed or Elodea canadensis. Rob and Karen alerted city, county, and state officials. Together they formed a plan to survey for the plant in the spring, with the hope that the fragments Rob found were not part of a larger population. Egeria is very difficult to get rid of once it becomes established in a water body, which is why we try to catch infestations early.

map of Green Lake showing the location of the first Egeria found in 2020
In the fall of 2020, Egeria was first found in the northeast corner of Green Lake.

Egeria is a leafy underwater plant that grows so densely that it can fill the water of slow-moving rivers and lakes, outcompeting the native aquatic plants. Released accidentally into lakes in Washington, the species is native to parts of South America.

Previously, Egeria was a commonly used aquarium plant in North America but it was banned from sale in Washington years ago. It has been found in fish tanks and classroom science kits in recent years and occasionally ends up in local waterways. Currently designated as a Class B Noxious Weed in Washington, Egeria control is required in some counties in the state. In King County, control is required where it hasn’t yet become widely established and removal is still feasible.

Four people on a dock get ready to survey for Egeria in Green Lake
The Green Lake Egeria survey team gets ready to embark. [L to R] Rob Zisette, Friends of Green Lake; Ben Peterson, King County Noxious Weed Program; Lizbeth Seebacher, Washington Department of Ecology; and Chris Baines, Seattle Parks and Recreation.

Recently, a team of local experts (above) set out to survey the lake. The group waited until late May to make sure the Egeria plants were up and visible. Because the lake is relatively shallow and clear, the team was expecting Egeria plants to grow almost anywhere in the lake, so they made sure to cover as much water as possible.

A canoe and a small motorboat each with two people on a lake
The May 2021 Egeria survey of Green Lake was done in two boats, one a canoe and the other a small motorboat, each with two people. Photo by Allison Bachner.

They had two boats – a canoe and a small motorboat loaned by the Green Lake Boathouse. The motorboat had sonar and an underwater camera (so they could see the plants in the water). The sonar boat mapped the Egeria that was visible by camera and sonar. At the same time, the canoe paddlers dropped a rake every 50 feet or so to get a sample of the plants in the water.

The results were mixed. First, the bad news. There is a lot more Egeria in the lake than anyone knew, far too much for divers to remove by hand-pulling, which is a highly effective approach for early infestations. It has clearly been there for a while and escaped notice until it washed up on the shore. Second, the not-so-bad news. Most of the Egeria seems to be in the middle of the lake farther from the shore, probably due to being crowded out by the more aggressive milfoil in the shallower areas. Egeria spreads by plant fragmentation, but that doesn’t seem to be happening very much at this point. Compared to milfoil, the Egeria also doesn’t appear to be topping out of the water as much, making it less of a nuisance to boaters and swimmers.

Map of Green Lake showing colored lines where Egeria was found
Map of the Green Lake survey for Egeria densa done on May 20, 2021.

Given how much Egeria there is in the lake, a plan of action is still under development. A second look in August will show just how much of the water column Egeria is taking up and give the team a better idea of the level of recreational and environmental impacts. They also need to consider how to mitigate the potential algae bloom that could result from controlling large amounts of aquatic plants at one time. Stay tuned for more information later this year or in 2022 as the plot thickens, along with the Egeria most likely!

Read more about Egeria in King County and Washington:

Duvall and King County plan rapid response to new aquatic weed found upstream of the Snoqualmie River

King County information on Egeria (Brazilian elodea) identification and control

Washington State Noxious Weed Control Board information on Brazilian elodea

Brazilian Elodea (Egeria densa) removal on the Chehalis River