Invasive species are a big ecological problem. But former Lake Superior College biology professor Theresa Hornstein has found they can serve a purpose.

She collected tansy from her own backyard and put it in a mesh bag in boiling water, similar to how one might steep tea. She adds plain Happy Feet sock yarn, lets it “steep” for at least 30 minutes, and it comes out yellow. If she adds other ingredients, like iron water, the color will change to green.

“I’ve always played with dyes, but mostly it’s been synthetics,” Hornstein said.

But about 30 years ago, when she moved to her rural Duluth home, she discovered something else she could use for dying.

It started with a mishap when she went out to check the taps on her maple trees.

“I hit one of those melted-out spots,” she remembered. “I hung up my snowshoes in this scrubby little tree/shrub.”

She described being stuck on her back like a turtle and curling up, trying to grab the tree to get leverage.

“And when I got done, my … white mittens were gold,” Hornstein said.

It turns out the tree was buckthorn. And it inspired years of experimenting with other invasives to see what colors they produced.

All of her knowledge working with tansy, buckthorn, Amur maple, woad, and more is now collected in a book: “Invasive Color.”

“I had written textbooks before but not – what do I want to say? – a fun book,” she said.

She covers what makes an invasive species, basic dye techniques, and then the specifics of what long-lasting colors certain plants will make.

“If you’re going to rip up the buckthorn or the tansy, find a use for it,” Hornstein said.

She also teaches classes. Her next at the Duluth Folk School is July 19. She’ll also teach two at North House Folk School in Grand Marais on Aug. 1 and 2.