The Seattle Parks Department said Monday that the spikes found in Green Lake over the past two weeks were part of a milfoil-control package...

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The Seattle Parks Department said Monday that the spikes found in Green Lake over the past two weeks were part of a milfoil-control package purchased from chemical giant DuPont in the mid-1980s.

And the Seattle Police Department said Monday it called off its investigation into the source of the spikes, now that the Parks Department has claimed responsibility.

The plastic sheeting the city used to suppress milfoil growth came packaged with stainless-steel spikes that were bent into curves at the top.

The city had no say in it, said Kathy Whitman, the city’s aquatic director for the past 15 years. It was a pilot program to try to control milfoil, she said.

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Eurasian milfoil found in Green Lake is an invasive aquatic plant that appeared in the lake in the early 1980s and has clogged swimming areas, snagged boats and collected along the lake edges.

Parks officials speculate that the straight, sharp spikes found in Green Lake over the past two weeks likely were originally curved at the top, and that the curves corroded and broke off.

“No doubt, that’s the case,” Whitman said. “They were put down a long time ago.”

In June 1984 the city placed 120,000 square feet of black plastic on the bottom of Green Lake near the small-craft center and along the southeast shoreline, according to a story that ran in The Seattle Times. And that’s where the spikes turned up this month.

The purpose of the plastic was to keep sunlight from the milfoil to block its growth.

After the city tried the DuPont plastic and spikes, it used another product, held down with sandbags, and yet another, held down with rubber plastic discs. “In each of these cases, we were buying a whole solution, a whole package,” Whitman said.

Over the weekend, the city paid for a team of three divers to scour Green Lake and Lake Washington for metal spikes, and more were found. That followed a dive in Green Lake on Friday when volunteers from the Virginia Mason Medical Center in Seattle pulled up 41 spikes. Whitman said the divers in Lake Washington found spikes there, too, but they were in good condition with the curved tops intact.

What Whitman can’t explain is why a major Green Lake cleanup in 2005 found no spikes. But she no longer thinks the spikes were sabotage. “At least most of [the mystery] has been solved,” she said.

Dewey Potter, spokeswoman for the Seattle Parks Department, said the department is trying to collect all the available milfoil records.

Whitman, who is trying to piece together a history of milfoil treatment, said it’s been at least 15 years since the city did any work with milfoil, other than hiring divers to pull out the weeds. She said that a few years ago fish that eat milfoil were added to Green Lake. She said the city never used any chemical treatments for the milfoil.

Potter said the Parks Department didn’t really frighten users of Green Lake because the spikes were found in areas that were off-limits to swimmers, and no beaches were closed.

“We’re glad to find an explanation,” she said. “I’m sorry it was us.”

Susan Gilmore: 206-464-2054 or

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