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White Snakeroot (Ageratina altissima)

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Summary

A poisonous perennial herb native to eastern North America. One of the most common members of the Eupatorieae tribe of the aster family. In much of its range, among the last wildflowers to bloom in fall.

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Native

This tentative map is based on the FHWA's ERA. This data lacks information on Canada, but also overestimates native ranges, especially around the edges, as this post explains. We have not yet reviewed or fixed this map.

USDA Plants Profile for Ageratina altissima

Illinois Wildflowers Page for Ageratina altissima

Description & Identification

Typically 1½-3' tall, often taller in moister conditions and more fertile soil and where competing for light. It usually has opposite leaves (rarely whorled), branching occasionally, typically forming a neat, symmetrical growth habit. Leaves sharply serrate. Flowers white. Flowerheads with 12-25 florets, in clusters often with some space between individual flowerheads.

Similar Plants

Blue Mistflower (Conoclinium coelestinum)
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Late Boneset (Eupatorium serotinum)
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Habitat

Deciduous woodlands, especially open woods and woodland borders. Found in conditions ranging from moist conditions is sun to slightly dry areas in light shade, but usually in well-drained soils. Adaptable to a broad range of conditions.

Common and often abundant in partly-shaded disturbed habitats, including small, fragmented sections of woods along roadways and railroads, and wooded areas in urban, suburban, and industrial areas. Often grows underneath hedges and shrubbery in suburbs. Many of these populations are escapes from cultivation.

Life Cycle

Ageratina altissima is an herbaceous perennial. It can spread vegetatively by short rhizomes, but often grows with a clumping growth habit and is more likely to spread by seed, with each clump growing more dense and vigorous each year.

First year plants usually flower and set seed, but the plant usually grows larger and produces much more seed in its second and later years.

The above-ground parts are sensitive to cold and frost, and usually die down completely after a moderately hard frost. As this plant is late to flower and set seed, and often grows in partly-shaded areas that stay somewhat cooler, its pollination and seed production can be sensitive to weather conditions in fall. An early hard frost can halt its reproduction for the year, and prolonged cold temperatures can hinder its pollinators.

It can sometimes colonize areas with north-facing exposures due to buildings or steep slopes, where it initially thrives during the brightest months, but ends up with insufficient light to successfully flower and set seed late in the season.

Faunal Associations

This species is consistently eaten by leaf miners and aphids.

The flowers attract a large variety of pollinators, including bees, butterflies, moths, and flies.

The foliage is toxic to mammals and as such it is usually avoided by most mammals. Cows can occasionally be pushed to browse on this plant in overgrazed pastures, which can be dangerous as the toxin can be passed through the cow's milk.

Uses

Cultivated as an ornamental plant, although it can sometimes become aggressive in garden conditions, outcompeting other plants. A dark purple leafed cultivar, "Chocolate snakeroot" is most commonly cultivated; often, local populations established from this cultivar can show considerably more purple on the leaves than truly wild populations.

Used in ecological restoration projects, as it is easy to get established, competes favorably against invasive plants, thrives in disturbed habitats, resists browsing by deer, and has high value to insects and birds.

This plant may have had some historical medical uses, but all parts of the plant are poisonous.

White snakeroot has abundant relatives. There are 11 different Ageratina species native to North America, and one introduced one, but none of these are as anywhere near as common or widespread, so in most regions, Ageratina altissima will be the only plant of its genus.

In the broader Eupatorieae tribe, there are many other genera that occur together with this species, including Conoclinium, Eupatorium, Eutrochium, and many others.

Of these, the most visually similar plants are Conoclinium coelestinum (Blue mistflower), which is shorter and has pink-blue-lavender flowers and more rounded leaf tips, and Eupatorium serotinum (Late boneset), which is taller and has lightly longer, more slender leaves. Both prefer sunnier, wetter areas.

Of the closely related plants, white snakeroot is usually the most shade tolerant and particularly the most tolerant of slightly dry shade, but compared to most of these other species, is less tolerant of poorly-drained soils.

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