Purple Milkweed (Asclepias purpurascens). Photo courtesy of Kevin Doyle.
Each year, the Rare Plant Monitoring Program picks plants deserving special attention for the following year. In 2025, we are featuring Wisconsin's five species of rare milkweed. Monitors are not required to survey these plants, but we hope they might consider it. By highlighting these plants, we hope to get a fuller picture of their statuses in the state. If you have been unsure about choosing a survey in the past, these plants are a great place to start. Our 2025 species will provide opportunities throughout the state, so consider requesting a survey near you for the coming year.
Overview
Milkweeds are herbaceous perennials named for their distinctive milky sap, which is toxic to herbivores and provides defense for the plant. Wisconsin has 13 native milkweeds plus one non-native species.
Wisconsin's milkweeds are mostly limited to fire-dependent habitats like prairies, savannas, barrens and oak woodlands where the canopy is either entirely absent or, at most, only partially closed.
Green milkweed
(Asclepias hirtella)
One of three milkweeds in Wisconsin with alternate leaves (the others are woolly milkweed and butterfly milkweed) and the only one that is not hairy. This milkweed is often found in wetlands, whereas woolly milkweed and butterfly milkweed are limited to dry sites. The best time for identification is June through August.
Dwarf milkweed
(Asclepias ovalifolia)
This is most likely to be confused with common milkweed, but less robust, both shorter and with narrower leaves. The fragile stature of dwarf milkweed can be hard to describe but is easy to identify in the field. The flowers of dwarf milkweed are purer white, whereas those of common milkweed tend to have at least a hint of pink. Found in barrens and sand prairies, including the disturbed edges of these sites, such as roadsides. The best time for identification is throughout June.
Woolly milkweed
(Asclepias lanuginosa)
A very small, hairy plant with alternate leaves and white coronas. It is limited to dry prairies and barrens. It is most likely to be confused with butterfly milkweed, but it is much smaller and has milky sap, whereas butterfly milkweed does not.
Found in dry, dry-mesic and sand prairies, as well as barrens. Surrounding vegetation is often sparse or short-statured, and exposed sand or gravel may be present. The best time for identification is late May through late June.
Prairie milkweed
(Asclepias sullivantii)
Prairie milkweed is distinctive for its sessile leaves, meaning they have no leaf stalks. Only clasping milkweed and Mead's milkweed have this same feature, but both of those species have leaves with wavy margins and are found in dry or even sandy habitats, while prairie milkweed is found in wet or mesic prairies. The best time for identification is early June through early July.
Purple milkweed
(Asclepias purpurascens)
A tall milkweed found in a variety of habitats from wet-mesic prairies to oak woodlands. Most likely to be confused with common milkweed. Distinctive for its terminal and hemispherical umbels of deep purple flowers (vs. usually axillary and spherical umbels of light pink flowers in common milkweed). Purple milkweed also has lines of hairs running down the stem, whereas common milkweed stems are evenly hairy throughout. Finally, the hairs on the midvein on the upper side of the leaf of purple milkweed are hooked, while those of common milkweed lay flat and are somewhat wavy. This requires a hand lens to see. The best time for identification is late June through late July.
Ecological Importance
We all know that milkweeds are the larval host for the monarch butterfly, but they also provide nectar for a variety of other butterflies, along with bees, wasps and flies. Even some hummingbirds use milkweed nectar. And it's not just their nectar that makes milkweeds important members of their natural communities. Despite their toxic sap, some insects utilize milkweed stems, leaves and roots. In fact, one researcher found 457 species of insects on common milkweed plants alone! Vertebrates use them, too. Birds can use fibers from the stems and fruits for nest material. Milkweeds have been and continue to be used by humans for food, fiber and medicine.
Conservation
Throughout the Upper Midwest, approximately 20-40% of milkweeds are considered vulnerable to critically imperiled. Compared to surrounding states, Wisconsin has the highest ratio of rare milkweeds relative to the overall number of native milkweeds (38%).
Historically, the biggest threat to our rare milkweeds has been habitat loss from European settlement. Although land conversion continues to be a threat, new threats have also emerged. Today, fire suppression, invasive species spread, habitat fragmentation and pollinator decline all contribute to downward trends in milkweed. Unfortunately, measuring trends of our rare milkweeds has not been easy. While we collect a lot of data on rare milkweeds, we are often unable to revisit enough populations of a given species to assess statewide trends.
