The Friends of the Wild Flower Garden, Inc.

Plants of the Eloise Butler Wildflower Garden

The oldest public wildflower garden in the United States


Common Name
Sky Blue Aster (Azure Aster)


Scientific Name
Symphyotrichum oolentangiense (Riddell) G.L. Nesom


Plant Family
Aster (Asteraceae)

Garden Location


Prime Season



Sky Blue Aster is a native, erect, perennial aster of prairies and open woods, growing 2 to 3 feet high on stems that may or may not have hair and are green during the growth season.

Leaves are more toward the bottom of the stem, usually entire, thick and stiff; lower leaves often on an winged stalk. Basal leaves have a heart shaped base and a long petiole; upper leaves stalkless, lance shape, with a rounded base, but not clasping. All have a rough surface. Within the floral array, the leaves become linear, then shading to bracts near the flower heads, all stalkless. There is considerable variation in leaf shape.

The floral array is a branched cluster (a panicle), usually of many flowers, with stiff ascending branches that are leafy and with appressed bracts on the long flower stalks.

Flowers have two types of florets - an outer group of 13 to 20+ ray florets which have light blue to pinkish rays, 1/2 to 1-1/3 inches across the open head. These are pistillate and fertile. These surround the tubular central disc florets which have yellow corollas and number 20 to 25+. These are bisexual and fertile. The disc florets have 5 triangular lobes on the open throats and turn color toward reddish-purple at pollen maturity - typical of asters of this genus. Disc florets open from the edge of the central disc first then toward the center. The anthers of the disc floret stamens are yellow when the floret opens, and the five stamens tightly surround the style which has a branched appendage at its tip. Styles are exserted from the corolla when the flower opens. Flower stalk bracts are green, linear and usually appressed to the stalk. These grade into 4 to 6 series of phyllaries around the outside of the flowerhead. These too, are usually appressed but the outer series may be slightly spreading or even curled. They are generally whitish but have a short, dark green, diamond shape tip and are without hair.

Seed: Fertile flowers produce a 4 to 5 nerved dry purplish cypsela, 1.8 to 2 mm long, with white to creamy rose pappus attached for wind dispersion. Seeds germinate best in cool soil but no pre-treatment such as cold stratification, is necessary.

Varieties: Some references list two varieties for the species. Today, based on the work of Nesom, varieties of this species are not recognized by most authorities including the University of Minnesota Herbarium and Flora of North America.


Habitat: Sky Blue Aster grows best in mesic to dry sandy to loamy soils in full to partial sun. It is found in prairies, bluffs, dunes, open wood savannas. Excess water is detrimental. It grows from a short rhizomatous root system or from woody caudices, which form clumps, not spreading colonies.

Names: That strange species name, oolentangiense, refers to the Oolentangy River in Ohio where Riddell, who named the plant, found the species in 1835. Its former scientific name was Aster oolentangiensis Riddell. Another older name, but the most descriptive, is Aster azureus, which species name means 'sky-blue'. However, all the new world asters, formerly in the genus Aster, have now been reclassified, most into the genus Symphyotrichum. That genus name is from the Greek symphysis, for 'junction', and 'trichos', for hair and, while obscure, it was first applied by Christian Gottfried Daniel Nees von Esenbeck in the 1800s in describing the type aster for the genus.

The author names for the plant classification are as follows: The first to publish was ‘Riddell,' which refers to John Leonard Riddell, (1807-1865), American chemistry professor, botanist, writer, and medical doctor. He was born in Massachusetts, but most of his botanical observations were in the central states from Ohio southward. His important work was a “Synopsis of the Flora of the Western States” defined at the time as the territory from the Allegheny mountains to the Platte River in Missouri Territory. It was in great part a compilation. He later published Catalogus Florae Ludovicianae. He served as Adjunct Professor of Chemistry and Lecturer on Botany in the Cincinnati Medical College where he also received his M.D. and he later became chair of the Chemistry Dept. in the Medical College of Louisiana (which then became the University of Louisiana), which position he held until his death. It was there he invented the binocular microscope. The plant genus Riddellia was named for him. His descriptions for this species were updated by ‘G. L. Nesom’ who is Guy L. Nesom (b. 1945) American botanist who has published papers on the nomenclature of asters.

For comparison, a similar blue aster flower grows in the upland Garden -- the Smooth Blue Aster, Symphyotrichum laeve; it has different leaves that are stalkless or clasp the stem and the flowerhead phyllaries are more green with the tips more an elongated diamond to linear. See comparison photo below.

See bottom of page for notes on the Garden's planting history, distribution in Minnesota and North America, lore and other references.

Skyblue Aster

Above: Leaves vary considerably. Within the floral array they become linear and stalkless and are appressed bracts near the flower heads.

Below: 1st photo - on the stem below the floral array leaves have tapered bases and winged stalks; 2nd photo - the lower basal leaves have heart-shaped bases and long stalks. 3rd photo - The underside of the leaf has a rough surface from short stiff whitish hairs.

Sky Blue Aster upper leaf Sky Blue Aster basal leaf leaf underside

Below: 1st photo - Center disc florets mature to a deep reddish color. 2nd photo - Phyllaries of the flower head have a dark green diamond shape tip. 3rd photo - COMPARISON: These are the phyllaries of the Smooth Blue Aster, S. laeve, which also grows in the Garden.

Sky Blue Aster Sky Blue Aster Bracts Smooth Aster phyllaries

Below: 1st photo - The stem of the flower cluster has a number of linear appressed bracts which graduate in size up the flower stalk until they become the phyllaries of the flower head. 2nd photo - A typical branch of a flower cluster. Note the older flower heads have the disc florets turning a reddish-purple color while the younger flower at far right still has many disc florets that have not opened.

Bracts flower cluster
Flower detail
Sky Blue Aster panicle


Notes: Sky Blue Aster is indigenous to the Garden area. Eloise Butler first catalogued it on Sept. 7, 1907. Then in 1917 she brought in large numbers of plants: 63 on Oct. 9, 51 on Oct. 13, and 51 on Oct. 17, all sourced from within Glenwood Park (which partially surrounded the Garden); five different plantings in 1918, four in '19 and six plantings in '20 and then every year thereafter through 1932. In 1917 she said she was “continuing blueing east hillside." The odd count of plants put in indicates they may have been seedlings obtained from the Park Board Nursery which was located across Glenwood Ave. from the Garden at Glenwood Lake. Martha Crone first reported planting this species in 1934, then again on Sept. 16, 1937 and again on Sept. 27, 1938 and seeds in 1944. Sky Blue Aster was listed on Martha Crone's 1951 inventory of plants in the Garden at that time. Both Eloise and Martha used the older name Aster azureus.

Gardener Cary George reported planting Sky Blue Aster in 1995. It is native to a wide band of counties in Minnesota stretching diagonally from the SE Corner of the State toward the NW, but not including the NW corner. In North America its range is from the central plains east as far as Ohio and New York, and southward to the gulf coast but excluding Pennsylvania, New Jersey, Maryland, the Virginias, Kentucky and South Carolina. Asters are difficult to study. There are twenty-four species just of Symphyotrichum listed by the DNR and the U of M Herbarium as being found in Minnesota, some with several subspecies.

Eloise Butler wrote about the asters in the Garden in her 1915 report to the Board of Park Commissioners. Of this species she said: "Aster azureus still burgeons on the hillsides (October 5). It is a sine qua non not only on account of its late blossoms, but because of their profusion and bright, pure color." Much the same text was incorporated into an essay that was sent to The Gray Memorial Botanical Chapter, (Division D ) of the Agassiz Association for publication in the Chapter's circular. Text here.

References and site links

References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.

graphicIdentification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.