By Jennifer Janasie - Boreal Community Media - August 22, 2025

As summer begins to wane and we note seasonal changes in the fireweed blooms (native perennial flowers frequently found along forest edges), there is still plenty of tansy (an invasive plant) blooming all around us as well. Instead of leaving invasive plant species intact around your yard, which are likely to out-compete native plants, why not remove them and put them to good use as natural dyes? There is no need to feel guilty about pulling up the whole tansy plant, for example, to make a natural dye bath, according to Theresa Hornstein, North House Folk School instructor and author of “Invasive Color: Using Invasive Species as Natural Dyes.” “In fact, you are doing a community service when you pull it!” she exclaims. Tansy, as well as some other invasive plants, can be used to dye yarn, cotton embroidery floss, and silk scarves, according to Hornstein.

Common tansy, a plant whose flowers and leaves produce a yellow to green dye, is on Minnesota’s "prohibited-control noxious weeds" list, according to the Minnesota Department of Agriculture’s website. Although tansy is already established in Cook County and throughout the state, efforts are underway to prevent the spread of this species because it tends to cover an area heavily -- choking out native plants and hampering reforestation projects, according to the site.

 Learn to explore the world of natural dyes in "Invasive Color," by Hornstein. 

Common buckthorn is another species that can be used in natural dye baths, but unlike tansy, it is not prevalent in Cook County, and experts are working to keep it that way. Nevertheless, its pervasiveness in other areas of the state is at the root of Hornstein’s natural dye story.

Hornstein’s personal story is one filled with a captivation for color, experimentation, and fiber arts. “I have always played with dyes and colors since childhood, doing those crazy kid things,” says Hornstein. “I was one of those kids who grew up mixing things together just to see what would happen, and when I got a little bit older, I started playing around with dyeing clothing with natural dyes. Working with the invasives, though, that started with me being a klutz!” she says.

While out snowshoeing to empty buckets on tapped maple trees about 30 years ago, she literally stumbled upon her first invasive species dye. After falling into a melted-out spot and getting stuck, she discovered while grasping at the branches of some nearby buckthorn that she had unintentionally dyed her hand-knit, white, wool mittens gold. “Even without a mordant (something you use to affix the dye to a fiber), the gold color would not wash out,” says Hornstein. “So I started playing!” she adds. She realized through experimentation that yellow, rust, and pink dye can be obtained from buckthorn leaves, that the berries produce a green dye, and that the plant generally offers a good variety of colors.

As a biologist by trade and an enthusiastic gardener, further investigating the subject of natural dyes came naturally to Hornstein. “I have always loved being outside, and plants, animals, and science in general,” she says. “And I also have always had an interest in art,” she adds. Though Hornstein was formerly trained and had several jobs connected to the field of science, she has continually woven art into her life in a number of ways. At the early age of five, not only was she playing with color, but she was also crocheting and knitting when her two grandmothers passed down their crafts. As a teenager, she started to explore fiber dyeing. “When I got a job teaching college in Duluth, I loved experiencing those aha moments I would witness in the eyes of my students. Having said that, to me, teaching was more of a process-oriented experience, and over time, I realized that I also had a product-focused mindset,” says Hornstein. “So, further exploring the crafts and art components I was interested in (such as knitting, crochet, felting, and dyeing) sort of completed me,” she says. And her two passions for science and art converged.

 Yarn dyed with invasives can produce subtle beauty.

Hornstein sees gardening as both a process-oriented and product-focused experience. Some of her gardens are quite organized, such as in nice raised beds, while others are “crazy with wildflowers,” she says. For example, “I will take an area full of tansy; for years now, I have been working on pulling spaces free of tansy, and then broadly casting wildflower seeds in the newly disturbed areas.” As a result, Hornstein has been slowly transforming sections once completely taken over by tansy into a self-described “chaos of color.”

In the realm of fiber arts, a variety of colors can be achieved in the natural dye process by altering factors like temperature, pH, and length of time in dye baths. For tansy specifically, by shifting the dye bath pH with vinegar, for example, products take on more of a yellow appearance; they transform to more of an olive-green color with the addition of iron. Hornstein uses an old, chipped, cast-iron dutch oven to leech iron into a dye bath, and is a big proponent of “using what you’ve got,” such as rusty nails, cider vinegar, or citric acid from canning. Within the required constraints, such as the use of mordants, there is so much flexibility, according to Hornstein. “I encourage people to experiment and see what they can come up with,” she says.

Adults tend to lose track of some of the sense of wonder and exploration present in childhood. “This is a way to bring that back,” says Hornstein. “A lot of folks when they first start to dye are kind of hesitant, and by the end they are like, oohh, I wonder what will happen if we add this or I wonder what will happen if we dip it in this one and this one?!” she says.

Retired from the formal classroom at Lake Superior College, Hornstein still teaches in informal classes and through her writing. “One of the first (non-college) classes I taught was at Yarn Harbor in Duluth, and that presented a very different type of teaching opportunity with students who were signing up as lifelong learners,“ she says. “I tend to run an open classroom, where I’m not afraid to go down rabbit holes and follow questions that get us a bit off topic, but still are related. I loved it at Yarn Harbor, and the students seemed to like it. So now I also teach at North House Folk School, Duluth Folk School, Spirit of the Lake Community Arts, and the Swedish Institute,” says Hornstein. She has also done demonstrations at events like the Viking Fest in Mankato. “Many of the invasives here were used in Northern Europe as dyes on a regular basis,” she says. It is an interesting process to learn about the archeological evidence of this, according to Hornstein.

 A display of yarn dyed by Hornstein with invasive species.

In Hornstein’s “Invasive Color” class at North House (held earlier this month and annually), students learn about why some of our invasive species were brought to this country and how to identify these species; then they explore how to use some as natural dyes. In her book of the same title, Hornstein encourages readers to experiment and to keep track of what they discover, such as on the note pages contained within the book.

Those interested in dyeing fibers with invasive species should harvest invasive plants responsibly, and in a way that helps to prevent their further spread. Proper plant identification and an awareness of plant toxicity are also critical. For example, the Minnesota Department of Agriculture recommends wearing gloves to handle tansy and removing tansy down to the roots, according to the agency’s website.

For more information about invasive species in Cook County, contact Environmental Specialist/County Agricultural Inspector Tia Parks, Cook County Invasives Team/Cook County Highway Department, at [email protected] or 218-387-3692.