Taxonomic and distribution databases

Every species page links directly to that species' record in each database below, resolved by identifier rather than by search, so the link always lands on the right taxon.

Works consulted

24 works underpin the identification, cultivation, toxicity and monarch guidance across the site. Individual guides list the subset each one draws on.

1. Woodson, R. E. (1954). The North American species of Asclepias L.. Annals of the Missouri Botanical Garden 41(1): 1–211. The monograph the modern taxonomy of the genus still rests on. View
2. Flora of North America Editorial Committee (eds.) (2026). Flora of North America North of Mexico. Oxford University Press, New York and Oxford. Accepted names, keys and descriptions for the North American flora. View
3. Royal Botanic Gardens, Kew (2026). Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Nomenclature and global distribution, keyed to IPNI name identifiers. View
4. Kartesz, J. T. (Biota of North America Program) (2026). North American Plant Atlas. BONAP, Chapel Hill, North Carolina. County-level distribution maps, the basis for the ranges generalized here. View
5. USDA, NRCS (2026). The PLANTS Database. National Plant Data Team, Greensboro, North Carolina. State-level distribution, wetland status and nomenclature. View
6. GBIF Secretariat (2026). GBIF Backbone Taxonomy. Global Biodiversity Information Facility. Occurrence records aggregated from herbaria and citizen science. View
7. Agrawal, A. A. (2017). Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution. Princeton University Press. The standard general account of the milkweed and monarch relationship. View
8. Pleasants, J. M. and Oberhauser, K. S. (2013). Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity 6(2): 135–144. Quantifies the loss of Midwestern milkweed to herbicide-tolerant cropping. View
9. Pleasants, J. M. (2017). Milkweed restoration in the Midwest for monarch butterfly recovery: estimates of milkweed stems needed. Insect Conservation and Diversity 10(1): 42–53. Source of the widely cited 1.6 billion stem restoration target. View
10. Satterfield, D. A., Maerz, J. C. and Altizer, S. (2015). Loss of migratory behavior increases infection risk for a butterfly host. Proceedings of the Royal Society B 282: 20141734. Year-round breeding on evergreen milkweed raises OE infection sharply. View
11. Altizer, S. M., Oberhauser, K. S. and Brower, L. P. (2000). Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies. Ecological Entomology 25(2): 125–139. Establishes the link between migration and low OE prevalence. View
12. Faldyn, M. J., Hunter, M. D. and Elderd, B. D. (2018). Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies. Ecology 99(5): 1031–1038. Cardenolide levels in tropical milkweed rise with temperature, harming larvae. View
13. Malcolm, S. B. (1994). Milkweeds, monarch butterflies and the ecological significance of cardenolides. Chemoecology 5/6: 101–117. How larvae sequester cardenolides and what that buys them. View
14. Wyatt, R. and Broyles, S. B. (1994). Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25: 423–441. The pollinium mechanism, self-incompatibility and very low fruit set. View
15. Kephart, S. R. (1983). The partitioning of pollinators among three species of Asclepias. Ecology 64(1): 120–133. Which insects actually move milkweed pollinia, and how they divide up. View
16. Luna, T. and Dumroese, R. K. (2013). Monarchs (Danaus plexippus) and milkweeds (Asclepias species): the current situation and methods for propagating milkweeds. Native Plants Journal 14(1): 5–15. Practical propagation protocols, including stratification requirements. View
17. Borders, B. and Lee-Mäder, E. (2014). Milkweeds: A Conservation Practitioner’s Guide. The Xerces Society for Invertebrate Conservation, Portland, Oregon. Seed production, establishment and habitat management at field scale. View
18. Burrows, G. E. and Tyrl, R. J. (2013). Toxic Plants of North America, 2nd edition. Wiley-Blackwell, Ames, Iowa. The reference work behind the livestock and pet toxicity guidance here. View
19. Zalucki, M. P. and Brower, L. P. (1992). Survival of first instar larvae of Danaus plexippus in relation to cardiac glycoside and latex content of Asclepias humistrata. Chemoecology 3: 81–93. Why latex, not toxicity, is the main hazard to a newly hatched caterpillar. View
20. Bhowmik, P. C. and Bandeen, J. D. (1976). The biology of Canadian weeds: Asclepias syriaca L.. Canadian Journal of Plant Science 56(3): 579–589. The standard account of common milkweed rhizome behavior. View
21. Ackerfield, J. (2015). Flora of Colorado. BRIT Press, Fort Worth, Texas. The regional flora used for the Colorado species accounts. View
22. The Xerces Society (2026). Milkweed Seed Finder. Xerces Society for Invertebrate Conservation. Directory of regional milkweed seed and plant vendors. View
23. Monarch Joint Venture (2026). Monarch conservation resources and monitoring protocols. Monarch Joint Venture, St Paul, Minnesota. Habitat guidance, monitoring methods and regional planting lists. View
24. Monarch Watch (2026). Monarch Waystation Program and tagging data. University of Kansas, Lawrence, Kansas. Waystation criteria and long-running migration tagging records. View

How the range data is built

Each species carries a list of US states in which it is recorded. Those lists are generalized from county-level data in BONAP and state-level data in USDA PLANTS, cross-checked against regional floras where one exists.

Generalising to the state is a deliberate trade-off. It answers the question most people arrive with, which is whether a species belongs where they live, and it keeps 72 species and 51 states tractable. It also loses real detail: a species listed for Texas may occur in four counties at the far western edge of it. Every species page links to the BONAP county map for exactly this reason.

State pages count a species as present if it appears in that state's list, whether native or naturalised. Naturalised species are labeled as such everywhere they appear.

What is measurement and what is judgement

Some things here are sourced facts. Others are editorial calls, and they are worth separating.

• Sourced: accepted names, authorship, state-level distribution, federal and state conservation listings, bloom months, mature size, hardiness zones, stratification requirements.
• Editorial judgement: the garden value, monarch value and nectar value ratings, the choice of "best species to plant" on each state page, and the relative emphasis in each species summary. These are informed opinions, not measurements, and reasonable botanists would score some of them differently.

Corrections

Range errors, outdated names and identification mistakes all matter. Regional botanists know their own floras better than any national compilation, and this guide gets better when they say so.

Reusing this material

The text and data are published under CC BY 4.0. Photographs carry their own licenses, shown on each image. Full details are on the license page.

Cite this page

This page is licensed CC BY 4.0. Quote it, adapt it and reuse it, including commercially, as long as you credit Milkweed USA and link back.

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