Tansy Flowers: Invasive, Poisonous Plants

Plant taxonomy categorizes common tansy as Tanacetum vulgare.

Common tansy is an herbaceous perennial. It is also sometimes classified as an herb plant.

Do not confuse it with "tansy ragwort" (Senecio jacobea), a different plant altogether.

Characteristics of Tansy

The flowers are golden and appear in flat-topped clusters. The tansy flower's shape is frequently described as "button-like." Common tansy often grows to a height of about 3 feet with a similar spread.

The flowers bloom July-August. The foliage is feathery and fragrant. Common tansy is an invasive plant (see below), so growing it is not recommended. However, if this flower already grows on your property, you need to be able to identify it (see picture), because common tansy is a poisonous plant. Should a child, say, ingest sufficient amounts, medical treatment would be necessary (stomach pumping).​

Where It Grows, Sun and Soil Requirements

Indigenous to Europe and Asia, common tansy grows as a perennial in planting zones 3-9.

Tansy flowers tolerate average, somewhat dry soils (but not wet soils) and will grow in either full sun or partial shade.

Care: Cutting Back, Deadheading

Tansy flowers grow like a weed along roadsides in many areas of North America, so if you are curious enough to desire a look at the plant, some of you may easily be able to do so. If this invasive grows in your own landscape, at least deadhead the flowers to keep them from going to seed.

By late summer, you might wish to cut tansy to the ground, as the appearance of its fern-like foliage may start to suffer from the heat. If you cut it back early enough, a new batch of foliage will emerge in autumn (in warm climates, re-blooming may actually result).

Tansy Poisonous, Despite Medicinal, Pest-Control Uses

Tansy has been used in companion planting for centuries.

The leaves of these fragrant plants are said to repel flies and ants, for instance (although their aroma may repel only certain types of ants). To use the plants for ant control, dry the flowers and leaves and sprinkle them to form barriers.

Despite being poisonous (to humans, to livestock, to dogs, and to cats, if eaten in sufficient quantities) if ingested raw, this herb, according to Botanical Online, traditionally has had culinary and medicinal uses, too. The same source notes, however, that contact with common tansy puts one at risk for dermatitis.

On Its Invasive Nature 

Tansy flowers have naturalized (along roadsides, for example) in parts of North America, where the plant is widely considered to be invasive.

The publication of this article should not be considered an endorsement for growing tansy flowers; rather, the information here is provided for research purposes and for those who already have the plants growing in their yards.

Tansy Weed Control

If tansy flowers are already growing on your property and you are curious about what organic weed control measures you can take to eradicate this invasive, We do not have good news for you. The plant is difficult to eradicate without chemicals.

It spreads not only by reseeding but also via underground rhizomes. One way to check the spread of the rhizomes is by using bamboo barriers. Some people mow tansy down to weaken it and keep it from producing seed, but this approach will not eradicate the plants.

Meaning of the Name, "Tansy" and Some History

While tansy plant has now fallen out of favor in some quarters due to its poisonous nature and invasive tendencies, it was nonetheless once a much-admired herb.

Tansy plant's common name derives from the Greek athanatos, meaning immortality (either because it is long-lived or because tansy was used for embalming going back to ancient times). In Greek mythology, Zeus was said to have made Ganymede immortal by giving the latter tansy on Mount Olympus.

It was an important medicinal and culinary herb in Europe for centuries.

"In medieval times, tansy was used for a variety of ailments," says Stephen Byrnes, in Tansy: an Herb with a Rich History. "Its most well-known use was for expelling intestinal worms, particularly in children. Children infected by these parasites would have a cup of tansy tea in the morning, and another at night."

Given their pedigree in the European tradition, it is not surprising that tansy flowers were soon brought to the New World by the American colonists and granted a position of prominence there in the garden. "Use of common tansy led the governor of Massachusetts to list common tansy as a necessary plant for colonial herb gardens in the 1600s," according to Montana State University, which source notes that the herb, unfortunately, escaped cultivation and became a pest.

That escape has culminated in tansy's being listed by watchdog groups as one of the worst invasive plants in North America. It is quite a comedown from those Mount Olympus days, isn't it?