Butterbur (Petasites hybridus)

Synonyms: Petasites officinalis, Petasites vulgaris, Tussilago hybrida, Tussilago petasites, Petasites officinalis var. hybridus

Common Names: Common butterbur, bog rhubarb, pestilence wort, butterfly dock, butterdock, blatterdock

Description:

Butterbur (Petasites hybridus) is a large-leaved perennial from Europe and Asia that was brought to North America as a medicinal and garden plant [1][7]. It escapes into wet, shaded ground, where its oversized leaves and creeping underground stems form dense patches that crowd out native plants [3]. MISIN tracks it because those patches spread quickly along streams and wetlands and are hard to remove once they take hold [3][4].

Habit:

Butterbur is easiest to find in two seasons. In early spring ? often February through April, before any leaves appear ? it sends up thick, leafless stalks topped with dense, cone-shaped clusters of small pink-to-purple flowers [3]. By summer the flowers are gone and the plant is unmistakable for its size: round, heart- or kidney-shaped leaves up to 3 feet across, about the width of a trash-can lid, on stalks 3 to 4 feet tall [3][4]. The leaves are smooth and green on top but pale and woolly underneath [3]. In a damp, shady spot, a patch often forms a solid carpet of these giant leaves.

At-a-glance cues:

1. Round, heart- or kidney-shaped leaves up to 3 feet across, with toothed or scalloped edges [3][2]
2. Leaf undersides are pale and covered in soft, woolly hairs [3][2]
3. Pink-to-purple flower clusters on thick, leafless stalks in early spring, before the leaves [3]
4. Grows in dense patches in wet, shaded ground ? stream banks, ditches, and wetland edges [4]

Leaves:

1. Arrangement: all leaves rise from the base of the plant; the flowering stalk has no leaves above its base [2]
2. Shape: round to heart- or kidney-shaped, with a deep notch where the stalk attaches [2][3]
3. Margin: toothed to scalloped along the edge [2][3]
4. Surface: smooth and green above; pale, woolly, and hairy beneath [3][2]
5. Venation: 2 to 5 pairs of veins branching out from the base of the blade [2]
6. Size: very large ? up to about 3 feet (1 m) across [3][2]

Stems:

1. Habit: herbaceous perennial ? no woody stems [2]
2. Flowering stalk: thick and upright, leafless above the base, round in cross-section [2]
3. Bracts: small, leaf-like scales spaced along the flower stalk, often purple-tinged [2]
4. Surface: covered in tangled, woolly hairs [2]
5. Spreads underground by thick, branching rhizomes ? creeping, root-like stems [3][4]

Flowers:

1. Color: pink to purple [3]
2. Shape: small disk flowers with no petal-like rays, packed into rounded heads [2]
3. Arrangement: many heads crowded into a dense, cone- or spike-shaped cluster on one stalk [3][2]
4. Bloom time: early spring, roughly February through April, before the leaves emerge [3]
5. Sex: male and female flowers grow on separate plants (the species is dioecious); in much of North America only one sex is usually present [3]

Fruit and seeds:

1. Fruit: a tiny, dry, one-seeded fruit (achene) up to about 1/8 inch (3 mm) long [2][3]
2. Color: pinkish to purplish [3]
3. Each seed is topped with a tuft of fine white hairs [3][2]
4. Dispersal: the hair tufts catch the wind and carry seeds away from the parent plant [2]
5. Note: many North American patches set little or no seed and spread mainly by root fragments instead [3][4]

Habitat:

Butterbur is native to Europe and northern Asia [1]. It was introduced to North America as a garden and medicinal plant and is now found in scattered wet sites across the northeastern United States and the Great Lakes region [7][3]. It thrives in moist, shaded ground: stream banks, lake and pond shores, wetland edges, forested floodplains, wet meadows, and roadside ditches [3][4]. It tolerates deep shade and favors rich, damp soils [3]. Once it reaches a streamside, flowing water carries broken root pieces downstream to start new patches [4][6].

Reproduction:

Butterbur spreads in a few main ways. The most common is by its thick underground stems (rhizomes): a patch creeps a little wider each year, and even a small piece of root left in the soil ? or moved in fill dirt or dumped yard waste ? can start a brand-new patch [3][4]. Along water, root fragments break off and float downstream to sprout on a fresh bank [4][6]. Where male and female plants grow together it also makes seed, and each seed carries a tuft of white hairs that drifts on the wind [3][2]. In much of North America only one sex is usually present, so most spreading happens through the roots rather than by seed [3].

For control timing: the plant flowers in very early spring before the leaves open, so the big leaves do most of the work feeding the roots through the growing season. Removing those leaves again and again, from spring through fall over several years, slowly starves the root system [4].

Impact and Damage:

Every part of butterbur contains natural toxins called pyrrolizidine alkaloids, which can damage the liver if eaten [5][6]. Documented human cases from butterbur products that still contained these toxins include hepatitis and, in a few cases, liver failure [5]. Because of this, do not eat wild butterbur or feed it to pets or livestock, and keep it out of vegetable gardens [5].

In a yard, butterbur's giant leaves cast such heavy shade that little else grows beneath them, so a patch can take over a damp corner and smother ornamentals and ground covers [3].

In natural areas, dense butterbur stands crowd out native wetland and streamside plants and form near-solid mats that lower plant variety [3]. Where it lines a stream, its toxins leach into the water: one study measured these alkaloids at up to 90 nanograms per liter in small streams and up to 230 nanograms per liter in seepage water reaching groundwater wells [6]. As native streamside plants are pushed out, bare soil can be left between the stems, which may worsen bank erosion [3].

Similar species:

Several large or early-flowering plants are mistaken for butterbur.

Giant butterbur (Petasites japonicus) is the closest match ? another non-native butterbur with the same huge, rounded leaves and early-spring flowers. Tell them apart by size and color: giant butterbur's leaves can reach about 5 feet across and its flowers are whitish to yellowish, while common butterbur's leaves top out near 3 feet and its flowers are pink to purple [3].

Coltsfoot (Tussilago farfara) is another non-native that flowers on leafless stalks in early spring, but it is much smaller and its blooms are bright yellow and dandelion-like, not pink. Its leaves appear only after the flowers fade and are a fraction of butterbur's size [8].

Arctic sweet coltsfoot (Petasites frigidus) is a native relative. Its leaves are triangular or deeply lobed rather than the broad, rounded kidney shape of butterbur, which is the easiest way to separate the two [3].

Marsh marigold (Caltha palustris) is a native wetland plant that shares butterbur's wet, shady habitat but is far smaller, with bright yellow flowers and leaves only about 4 inches wide [3].

Monitoring and rapid response:

Signs to watch for:

1. In early spring, thick leafless stalks topped with pink-to-purple flower clusters in a wet, shady spot [3]
2. By summer, a spreading patch of oversized round leaves up to 3 feet across [3][4]
3. New patches showing up downstream of a known infestation, or near dumped soil or yard waste [4]

What you can do:

1. For a small patch, dig out the whole plant ? including the underground stems ? when the soil is moist; every root piece left behind can resprout, so remove as much as you can [4]
2. If you cannot dig it all out, cut or pull every leaf as it appears, repeating every 2 to 3 weeks from spring through fall for several years to starve the roots [4]
3. Smother a patch by covering it with a tarp or heavy mulch through the growing season to block all light [4]
4. Bag and trash every root and leaf ? do not compost it, and do not move soil off the site, since fragments start new patches [4][3]
5. Natural area crews can map and treat small outlying patches first, then work toward the core, planning repeated visits across a site over several years [4]

When to call a professional:

1. Large or long-established patches with deep, dense root mats that keep resprouting [4]
2. Infestations along streams, shorelines, or wetlands, where digging can worsen erosion and water rules may apply [3][4]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Plants of the World Online (POWO), Royal Botanic Gardens, Kew. Petasites hybridus (L.) G.Gaertn., B.Mey. & Scherb. powo.science.kew.org
2. Native Plant Trust, Go Botany. Petasites hybridus (butterbur sweet-coltsfoot). gobotany.nativeplanttrust.org
3. Minnesota Department of Natural Resources. Common butterbur (Petasites hybridus) and giant butterbur (Petasites japonicus). dnr.state.mn.us
4. Keweenaw Invasive Species Management Area (KISMA), Michigan Technological University. Butterbur. mtu.edu
5. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. Butterbur. National Institute of Diabetes and Digestive and Kidney Diseases / NCBI Bookshelf. ncbi.nlm.nih.gov
6. Kisielius V, Hama JR, Skrbic N, Hansen HCB, Strobel BW, Rasmussen LH (2020). "The invasive butterbur contaminates stream and seepage water in groundwater wells with toxic pyrrolizidine alkaloids." Scientific Reports 10:19784. semanticscholar.org
7. USDA NRCS PLANTS Database. Petasites hybridus (L.) G.Gaertn., B.Mey. & Scherb. plants.usda.gov
8. Native Plant Trust, Go Botany. Tussilago farfara (coltsfoot). gobotany.nativeplanttrust.org

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