^{Above: View of a grassland remnant with yellow prairie grass (Sorghastrum nutans) and vervain thoroughwort (Eupatorium pilosum). Credit: Drew Chaney}

By David Bellangue, Plant & Systems Research Technician & Conservation Nursery Manager
Piedmont Discovery Center

Imagine you’re exploring a Piedmont savanna remnant in early August. In the summer heat, the grassland comes alive with the fresh green growth of little bluestem (Schizachyrium scoparium), yellow prairie grass (Sorghastrum nutans), and other warm-season grasses. You spot the bare seed heads of Small’s ragwort (Packera anonyma) whose flowers lit up the grassland in rich yellow during the early summer. Ah, the starry rosinweed (Silphium asteriscus) is in full bloom, its cheery butter-yellow flowers buzzing with the activity of native bees that dash here and there. And as you look out across the remnant, you see so many white puffs spanning the sun-drenched ecosystem you wonder if you’ve discovered patches of clouds given vegetable form. These, too, are buzzing with insect visitors—sweat bees, bumble bees, wasps, flower flies, beetles, and more—such an abundance of pollinators that trying to track them all is dizzying. What you’re seeing, my friend, is the fascinating genus of thoroughworts (Eupatorium spp.).   

Thoroughworts are perennial herbs in the Aster family (Asteraceae) with an erect growth form ranging from 1.5–5 feet. Leaves are mostly opposite and entire, though some species have toothed edges. Flowers are borne in a level cluster on top of the plant, usually white, rarely pinkish (Weakley et al. 2020). The genus name Eupatorium comes from Mithridates VI Eupator (132–63 BCE), King of Pontus, a state that comprised much of modern-day Turkey and the Black Sea Coast (Bayton 2020). He purportedly discovered the medicinal uses of hemp-agrimony (Eupatorium cannabinum), the sole species native to Europe. The common name “thoroughwort” is said to come from the species name for boneset (Eupatorium perfoliatum). Perfoliatum references the plant’s perfoliate leaves, shaped such that the stem appears to go through the leaves. In the past, there was less of a distinction between “through” and “thorough,” which, in combination with the word “wort” from the Old English wyrt meaning plant or root, gets us to the present day common name (Missouri Department of Conservation 2024). Eupatorium perfoliuatum is also known to the Eastern Band of the Cherokee Nation as Gadudzuli (“growing on the mountain”) or  Gadusuehi (“rejoins”) (Banks et al. 2004).  

 There are around 40 Eupatorium species found throughout the Northern Hemisphere, with most species found in North America and Asia. The genus was once much larger (around 800 or so species), but genetic studies resulted in many plants being moved to other genera (Ageratina, Eutrochium, Conoclinium, Mikania, Fleischmannia, etc.) (Weakley & Southeastern Flora Team 2023). 20 species can be found in Virginia, most commonly encountered in the Piedmont and Coastal Plain regions. Thoroughworts grow in a range of open, sunny habitats such as forest edges, Piedmont woodlands and savannas, old growth remnant patch-prairies, roadsides especially colonial era roads and paths that preserve scraps of prehistoric ground surface, and in utility rights-of-way rights-of-way. Weedy eupatoriums, especially Eupatorium serotinum, are prolific in disturbed landscapes associated with old pastures, abandoned plowed fields, and other unnatural settings.

• Eupatorium album (white thoroughwort)
• Eupatorium altissimum (tall thoroughwort)
• Eupatorium capillifolium (dog-fennel)  [primarily Coastal Plain & s. and e. Piedmont]
• Eupatorium cordigerum (clasping round-leaf) [primarily Coastal Plain]
• Eupatorium godfreyanum (Godfrey’s thoroughwort)
• Eupatorium hyssopifolium (hyssop-leaf thoroughwort) 
• Eupatorium leucolepis (white-bracted thoroughwort)  [primarily Coastal Plain]
• Eupatorium linearifolium (narrow-leaf bushy thoroughwort)  [primarily Coastal Plain]
• Eupatorium maritimum (seaside thoroughwort)  [primarily Coastal Plain]
• Eupatorium mohrii (Mohr’s thoroughwort)  [primarily Coastal Plain]
• Eupatorium perfoliatum (boneset)
• Eupatorium pilosum (vervain thoroughwort)
• Eupatorium pubescens (hairy thoroughwort)
• Eupatorium rotundifolium (roundleaf thoroughwort) [primarily Coastal Plain & eastern Piedmont]
• Eupatorium saltuense (pasture thoroughwort)
• Eupatorium semiserratum (small-flower thoroughwort)  [primarily Coastal Plain]
• Eupatorium serotinum (late thoroughwort)
• Eupatorium sessilifolium (upland boneset)
• Eupatorium torreyanum (Torrey’s thoroughwort)
• Eupatorium vaseyi (Vasey’s thoroughwort)

As stated previously, thoroughworts depend on open, sunny habitat to grow and reproduce, and more conservative species are hypothesized to be indicators of unplowed grassland remnants in the Virginia Piedmont. Their nectar attracts a wide range of insects, including long and short-tongued bees, wasps, small butterflies, and beetles. Thoroughworts also serve as the host plant for several moths, such as the Clymene moth (Haploa clymene) and eupatorium borer moth (Carmenta bassiformis) (Hilty 2025).

Thoroughworts are also an important and interesting component of the total diversity of plants in the Piedmont region, as many of the species listed above are endemic to the Southeastern United States. In fact, some species (Eupatorium godfreyanum and torreyanum) are only abundant in the Virginia Piedmont and the Piedmont region of adjacent states, while E. saltuense is almost endemic to Virginia, though it can also be found in 4 bordering counties in North Carolina (Vascular Plants of North Carolina 2019). This alone warrants their increased use in restorations and landscaping throughout the Piedmont region, though they should be planted with other native grassland taxa like Solidago, Schizachyrium, Pycnanthemum, and Lespedeza to emulate grassland ecosystems for the greatest success and resilience. This underscores the importance of restoring grassland vegetation to the wider landscape of the Piedmont. It’s unacceptable that we are losing the incredible diversity of thoroughworts and other flora that contribute to the floristic uniqueness of the Piedmont region!    

Another interesting aspect of thoroughworts is their genetics. One of the main ways North American species arose was through hybridization! For example, Godfrey’s thoroughwort is thought to have evolved from a hybridization event between upland boneset and round-leaf thoroughwort (Cronquist 1985). This event resulted in Godfrey’s thoroughwort having more sets of chromosomes than the standard two sets that makes an organism diploid (this phenomenon is called polyploidy), meaning it is unable to perform meiosis and produce viable pollen grains. However, Godfrey’s thoroughwort has found a workaround: its unfertilized ovaries develop directly into viable seeds without fertilization (this is called apomixis in botany, similar to parthenogenesis in some animals). This means that the seeds of Godfrey’s thoroughwort are genetically identical clones of the mother rather than genetically diverse offspring (Siripun & Schilling 2006). What’s even more interesting is that, throughout most of their ranges, upland boneset and round-leaf thoroughwort are also polypoid plants that produce seeds via apomixis (sexually fertile populations for both species do exist, but only in limited areas of their range). Similar hybrid processes are thought to have created the pasture thoroughwort (hypothesized to be hybrid between white thoroughwort and tall thoroughwort) and Torrey’s thoroughwort (hypothesized to be a hybrid between hyssop-leaf thoroughwort and another species) (Weakley & Southeastern Flora Team 2023). Much more work has to be done to fully understand the taxonomy of this genetically complex genus.

If you’ve gotten this far in the article and are thinking, “Man, these thoroughworts are awesome! I wanna try growing some.” You’re in luck! Thoroughworts are among the easiest native plants to grow from seed. Vigorous plants growing in full-to-part sun produce abundant seed with high viability that is easy to gather and dry. Seeds that are cold stratified (exposed to cold, damp conditions simulating winter) for 30 to 60 days will germinate vigorously in warmer temperatures and grow quickly. Some can flower within their first growing season given ample water and frequent light fertilizations. Despite their general ease of cultivation, only boneset and hyssop-leaf thoroughwort are commonly available in the horticultural trade, a mind-boggling state of affairs taking into account the diversity of thoroughworts in our region. Given this trend, one top priority of the PDC Conservation Nursery is to grow large numbers of thoroughworts— not only for our restorations, but to increase the visibility of this important grassland genus and make it more widely available to the general public. However, it cannot be overstated that these plants should be planted together with other heliophytic (sun-loving) vegetation in a way that emulates grassland ecosystems rather than forcing them into traditional Eurocentric landscaping or in generic/novel “wildflower meadows.” We must respect where these plants came from, where they choose to grow, and celebrate the ecosystems they belong to. This is a great way to ensure that local genetics and plant and animal relationships are considered in conservation action. It also facilitates a shift from our consumer mindset paradigm from land use and extraction to land care and stewardship. 

References

Banks, W. H., Kemp, S., & Great Smoky Mountains Natural History Association. (2004). Plants of the Cherokee : medicinal, edible, and useful plants of the Eastern Cherokee Indians. Great Smoky Mountains Association.

Bayton, R. (2020). The gardener’s botanical : an encyclopedia of latin plant names. Princeton University Press.

Cronquist, A. (1985). Eupatorium godfreyanum (Asteraceae), a “New” Species from Eastern United States. Brittonia, 37(3), 237. https://doi.org/10.2307/2806066

Hilty, J. (2025). Tall Boneset (Eupatorium altissimum). Illinoiswildflowers.info. https://www.illinoiswildflowers.info/prairie/plantx/tl_bonesetx.htm

Kunsiri Chaw Siripun, & Schilling, E. E. (2006). Molecular confirmation of the hybrid origin of Eupatorium godfreyanum (Asteraceae). American Journal of Botany, 93(2), 319–325. https://doi.org/10.3732/ajb.93.2.319

Thoroughworts (Bonesets). (2024). Missouri Department of Conservation. https://mdc.mo.gov/discover-nature/field-guide/thoroughworts-bonesets 

Vascular Plants of North Carolina. (2019). Ncparks.gov. https://auth1.dpr.ncparks.gov/flora/plant_list.php 

Weakley, A.S., and Southeastern Flora Team. 2023. Flora of the southeastern United States: FloraQuest: Northern Tier. University of North Carolina Herbarium, North Carolina Botanical Garden, Chapel Hill, U.S.A. and High Country Apps, Bozeman, U.S.A

Weakley, A.S., J.C. Ludwig, J.F. Townsend, and G.P. Fleming. (2020). Flora of Virginia. With significant additions and updates. Bland Crowder, ed. Mobile app. Foundation of the Flora of Virginia Project Inc., Richmond, and High Country Apps, Bozeman, Montana.