Plant Profile: Prairie Ironweed

Prairie ironweed, Vernonia fasciculata Michaux

Widely considered a pollinator magnet, prairie ironweed, also called common, smooth, or fascicled ironweed, attracts many bees and butterflies late in the season. The plant flowers from July into September with intensely purple flowers gathered into heads, typical of plants in the aster or sunflower family.  

Each small flower, called a floret, has narrow, flaring lobes and an emergent style with a curved, two-parted stigma. Florets have five stamens that are fused into a tube that surrounds the style. (See Flower Parts for Plant ID for a review of flower parts.)

Left: Ironweed florets are grouped into heads at the ends of stems. Right: This single head is composed of many florets. The stigma, style, and fused stamens of one floret are labeled.

An involucre composed of a series of bracts, called phyllaries, enclose the base of each head. Each is green to purple, flat, bluntly tipped, with white hairs around the edges. Below the phyllaries is a short, often hairy stalk, called a peduncle, that attaches the head to the stem. 

At the base of each head is an involucre (in-vo-LOO-cur), a series of bracts called phyllaries. The phyllaries are green to reddish purple with usually blunt tips. White hairs ring the edges of the phyllaries.

Leaves are alternate and lance-shaped, the largest ones up to 6 inches long and 1+ inches wide. (Some sources report the leaves are less than 1/2 inch wide.) Leaf margins have short, spine-like teeth. The leaves are either sessile (lacking leaf stalks, aka petioles) or with short petioles. Athough the leaves may have a stray hair or two, they are essentially hairless. 

Left: Prairie ironweed typically grows 3 to 6 feet tall. Stems and leaves are smooth, although some may have stray hairs. Leaves are alternate, usually sessile, and toothed.

The lower surfaces of the leaves are minutely pitted. According to some references, the pits may appear shiny from embedded resin glands, also called glandular trichomes. These glands have many possible functions in plants, including protection from insects and diseases (1). Some sources also report that the upper surface, not the lower one, is glandular, although the lower surface is consistently pitted and is a reliable characteristic to help identify this species. Glands were not detectable on the upper surface of this leaf.

The lower surface of the leaf has tiny pits. Some references also mention black spots on the underside of the leaves, but it's unclear if the spots are part of the plant or caused by a rust fungus. The large white line is the prominent midrib, part of the plant's vascular system. 

Like the leaves, the stems are mostly hairless. They are green to red-green, round in cross section and typically 3-6 feet tall when fully grown. Depending on growing conditions, some stems may be shorter or taller. 

After flowering, the heads release small, brown, tufted seeds carried by wind. Several sources report that sparrows, goldfinches, and other songbirds eat the seeds in fall and winter. The stems remain standing through winter and may provide refuge for various stem-boring insects. Earlier in the season, the larvae of several moths and butterflies may feed on the leaves, and some insects will bore into the stem or roots for food or shelter (2). Mammals avoid the plant because it tastes bitter.

Left: Seed heads begin developing in late September or early October. Seeds are 2-3 millimeters long with a tuft of hairs that carry them on the wind.

In addition to reproducing by seeds, prairie ironweed also spreads vegetatively by rhizomes, stems that grow horizontally under the soil. This can produce a colony of genetically identical plants. Although it can spread in this way to form dense patches, prairie ironweed is not considered aggressive.

Prairie ironweed thrives in moist to wet soils and full sun. Typically it is found in meadows, shorelands, wetlands, ditches, and low prairies, although it can tolerate drier conditions. Its range generally covers central North America, but it is more common in the Upper Midwest and into Canada. The range map from the USDA Plants Database appears to be copyrighted, but the link should lead directly to the species. Scroll down at the site to view the map.

The Minnesota Biodiversity Atlas (3) reports two species in Minnesota: prairie ironweed and Baldwin's ironweed (V. baldwinii). Prairie ironweed is by far more common. Only two records of Baldwin's ironweed are included in the atlas: one from Crow Wing County in 1890 and another in Goodhue County in 2000. 

Many species of ironweed are available in nurseries, so other kinds of Vernonia may be found in gardens or even in natural areas if their seeds have spread there. Ironweeds are known to hybridize readily (4), so some plants may have intermediate characteristics that make them hard to identify to species.  

The origin of the name "ironweed" is uncertain. Some think it comes from the plant's tough stems, but others think it describes the rusty-brown color of the seed heads. The genus Vernonia honors William Vernon, an English naturalist who explored the East Coast in 1698. The species fasciculata refers to the tight fascicles, or bundles, of flowers in the heads. 

Cited References

1. Glandular trichomes: micro-organs with model status? Robert Schuurink and Alain Tissier. New Phytologist, Vol. 225, Issue 6. March 2020. 

2. Ironweeds. Missouri Department of Conservation. Accessed 10/26/26.

3. Minnesota Biodiversity Atlas. Bell Museum, University of Minnesota. Accessed 10/26/26. 

4. Distribution and hybridization of Vernonia in Missouri. Cora Shoop Steyermark. Botanical Gazette, Vol. 100, No. 3. March 1939. 

Other References

Minnesota Wildflowers

Illinois Wildflowers

The Buckeye Botanist

Michigan Flora

Flora of Wisconsin

South Dakota State University

The Friends of the Wildflower Garden, Inc. 

Plants for Bee Specialists

Jerusalem artichoke, Helianthus tuberosus, is one of several sunflower species favored by the sunflower mining bee, a specialist pollinator.

The sunflower mining bee, Andrena helianthi, has discriminating tastes.

This native bee is a specialist, gathering pollen primarily from plants in the aster family, Asteraceae (formerly Compositae, also called composites). To be more specific, it favors pollen from plants in the genus Helianthus, the sunflowers, to feed its larvae. (1).

In pollinator terminology, the sunflower mining bee is oligolectic, meaning “few chosen.” It’s far from alone in having narrow food preferences. According to the recent Minnesota Statewide Bee Survey (2), about 30% of the nearly 360 bee species confirmed in the survey are oligolectic.

Benefits and Drawbacks of Oligolecty

Given that high number, there must be advantages to oligolecty. One possibility is that the bees co-evolved with a few host plants that offer more digestible pollen (3). Entomologists at the University of Wisconsin found that the larvae of the blueberry mason bee, Osmia ribifloris, thrived when fed preferred host pollen that also included the microbes naturally found in that pollen. In contrast, larvae fed microbe-free pollen from the preferred host plant were much less fit, and larvae fed pollen from non-host plants had intermediate fitness (4).

Plants benefit from the relationship, too. Species visited by oligolectic bees have dedicated pollinators that transfer pollen among only a few kinds of plants, which makes successful pollination more likely. The plant loses less pollen to insects that carry it to a wider variety of plants, most of which can’t use it.

The potential disadvantage for both oligolectic bees and their plant hosts is that if either one becomes rare, its partner could become rare, too. A spiraling decline of both bees and plants happens when, for example, a plant population is displaced by an invasive species or fails to thrive in a warmer or wetter environment. As the plant becomes less abundant, so does the oligolectic bee that depends on it. In turn, as the oligolectic bee becomes less abundant, so can the plant that depends on it for pollination. If fewer seeds are produced, the population may decline further, with consequent effects on the oligolectic bee, and so on. It’s a vicious circle that can be difficult to interrupt.

The Habitat Solution

Difficult but not impossible. The answer is to provide habitat, including preferred host plants, for the sunflower mining bee and other oligolectic species. Fortunately, there are several resources to learn which plants or groups of plants benefit which bee species.

Entomologist and ecologist Jarrod Fowler compiled a list of bee specialists documented in the Central U.S., a region that includes Minnesota, Iowa, Wisconsin, North Dakota, and South Dakota (3). He also tabulated their preferred plant(s) and found that species in the aster or sunflower family, Asteraceae, and the bean or pea family, Fabaceae, were visited most frequently by specialist bees in this region.

In addition, he noted the top 25 genera that support oligolectic bees. The genera found in this region include Helianthus (sunflowers), Heterotheca (false goldenasters), Solidago (goldenrods), and Symphyotrichum (asters).

Although Helianthus is a favorite of sunflower mining bee, the insect has also been collected from the flowers of (l to r) cup plant (Silphium perfoliatum), New England aster (Symphyotrichum novae-angliae) and goldenrod (Solidago) species, here showy goldenrod (S. speciosa).(1)

The Minnesota Department of Natural Resources’ Minnesota Bee Species List is another useful resource (5). The list of around 460 bees includes both those that collect pollen and those that are parasites on other bees’ nests. For those that collect pollen, the table provides the species’ lecty (range of pollen preference, either oligo- or poly-) and its nesting habitat, if either is known.

The species list in the Minnesota Statewide Bee Survey (2) includes not only the bees’ names and lecty, if the latter is known, but also the ecological province(s) where each species was found. The report includes distribution maps of the bee species as well as their conservation status, or S-rank, which can range from S1 (critically imperiled) to S5 (secure).

The 2024 Featured Plant series from the Board of Water and Soil Resources (6) highlights several plants that support specialist bees or other insects. A Featured Plant is posted online at the beginning of each month.

If you observe and photograph bees visiting plants, consider submitting your records to iNaturalist. Several bee-related projects are hosted on this online platform, including Minnesota Native Bees. To find other projects, go to the iNaturalist website, choose Projects from the Community drop-down menu, and type “bees” into the search box.

Who knows, maybe the sunflowers you watch this summer and fall will host sunflower mining bees. Although the bees were uncommon to rare in the state’s bee survey, you could be the lucky one who spots this specialist pollinator.

References

1)  Andrena helianthi, Robertson 1891. Discover Life. Website accessed July 3, 2024.

2)   Minnesota Statewide Bee Survey 2014-2023. Minnesota Department of Natural Resources.

3)   Pollen Specialist Bees of the Central United States. Jarrod Fowler, 2020.

4)   Dharampal, P.S., Hetherington, M.C., and Steffan, S.A. 2020. Microbes make the meal: oligolectic bees require microbes within their host pollen to thrive. Ecological Entomology 45: 1418-1427. DOI: 10.1111/een.12926. https://par.nsf.gov/servlets/purl/10253224

5)    Minnesota Bee Species List. Minnesota Department of Natural Resources, August 2023.

6)     Board of Water Resources Featured Plant series, 2024. (Red-berried elder is the July featured plant; plants featured in earlier months are in the Featured Plant archive.)