(Editor's Note: This article was originally published on October 19, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
On the tallgrass prairie of northern Illinois, the blooming of the prairie grasses in August signals the transition of the seasons, even if it is not quite meteorological autumn. As the giant silphiums that dominate the prairie in summer begin to fade, the grasses reach their ultimate height, until only the last, tallest flower heads stand above the waves of Big Bluestem and Indiangrass. But even in the grassland there are flowers blooming until the first frosts: gold, white and purple.
While summer is the height of the season for the golden composite flowers that dominate the prairie landscape, several varieties linger on until fall, when the goldenrods take over.
Sweet Black-Eyed Susan (Rudbeckia subtomentosa)
This species is a close cousin to the more common Rudbeckia hirta, and blooms later in the season. It differs slightly by having multiple flower heads on branching stems and a taller habit of growth.
Downy Sunflower (Helianthus mollis)
One of the shortest members of the sunflower family on the prairie, only about two feet in height. Its flower heads are about three inches across. The leaves are wide and covered with a soft fuzz that give it an overall gray appearance, which accounts for its also being called the Ashy Sunflower. The species name "mollis" means "soft."
Prairie Sunflower (Helianthus paucifloris)
A sunflower of medium height, growing to three or four feet, its flower heads are borne singly on their stems and are about four inches wide. It is one of the latest-blooming sunflowers and remains in flower for up to two months. It prefers a dry location. Several other sunflowers are also sometimes called "prairie sunflower."
Evening Primrose (Oenothera biennis)
As its name indicates, this is a biennial plant. It tends to invade disturbed or open areas but dies out as more vigorous perennial prairie species take over. Even though it is a native to the area, some prairie managers regard it as a weed. Nonetheless, the bright yellow flowers are attractive and make a nice display, growing over eight feet tall and covered at the top with blooms over a long period in summer. This plant is widely used by herbalists.
These gold-flowering forbs are widespread on the prairie in fall. There are several species of goldenrod, so they can be in bloom from July until frost, but the most prominant species bloom in the same season as the prairie grasses. Goldenrods, like so many prairie flowers, are members of the aster family, and the individual blooms, while quite tiny, are composites with both ray and disk flowers. All the goldenrods are extremely attractive to bees and other insects. Many people mistakenly attribute their attacks of hayfever to this flower, when the real culprit is generally ragweed. While some people consider these plants to be weedy, it is hard to imagine the prairie in fall without goldrod in bloom.
Early Goldenrod (Solidago juncea)
This species is one of the shortest goldenrods, and, true to its name, one of the earliest to bloom, its flowers often appearing in July. The flower stalks usually arch to the sides rather than standing upright.
Tall Goldenrod (Solidago altissima)
As its name suggests, this is the tallest goldenrod, with some specimens growing over six feet, although some individuals might be much shorter. This is the most common goldenrod of the tallgrass region.
Rigid Goldenrod (Solidago rigidum)
This species is also known as Stiff Goldenrod. The blooms are in flat clusters at the top of the stems. It is notable for the size of the individual flower heads, which are at at least twice the size of the common variety. This makes it a very attractive plant in the early fall prairie or prairie garden.
Old Field Goldenrod (Solidago nemoralis)
One of the latest species of goldenrod to bloom is another of the smallest, an inconspicuous plant for a goldenrod, and often considered a weed. This one is known by several different names, including Gray Goldenrod and Field Goldenrod. Each unbranched flower spike curves to one side.
The latest large group of forbs to bloom on the tallgrass prairie are the asters. There are both white and purple varieties of aster, but white-flowering plants of all types are common on the prairie in fall.
This photo shows white Hairy Asters growing with blue Drummond's Asters and purple New England Asters.
Hairy Aster (Aster pilosus)
A very common aster, often considered weedy. It greatly resembles the fleabanes, which are also members of the aster family, but the aster has fewer, larger ray flowers. The plant grows to about four feet in height, with numerous small daisy-like flower heads. The weight of the flowers often causes the plants to fall over, as in these photos.
Heath Aster (Aster ericoides)
Brushy plants, about three feet tall, with a branching habit. The flower heads are very tiny and very abundant, as if to make up for their size. It is one of the last asters to bloom, and one of the last flowers to bloom on the prairie.
Tall Boneset (Eupatorium altissima)
There are several similar species of boneset on the prairie. Tall Boneset grows to four or five feet in height, and it is covered at the top with clusters of small, white composite flower heads with tubular disk flowers. Its appearance in massed plantings is striking.
Cream Gentian (Gentiana flavia)
One of the few fall-flowering forbs on the prairie not a member of the aster family. Gentians comes in several colors, and while the cream species is sometimes considered yellow, the color of these specimens is off-white. They are a low-growing plant, with flower clusters on stalks less than two feet tall. Flowering is in early fall. The petals do not open fully and may remain entirely closed. The gentians in fall play the same role in the prairie garden as the Pasque Flowers do in spring.
Illinois Bundleflower (Desmanthus illinoensis)
An unusual member of the bean family, one of the few members of this family to flower later in the year. The leaves are compound and fine-cut, and it bears small greenish-white flowers that form convoluted pods, shown in the photo below.
Flowering Spurge (Euphorbia corollata)
A member of the spurge family, it may be in flower at any time from summer into fall. I found it blooming in August. It is a low-growing plant with numerous flower heads, less than a half inch wide, on wide-branching stems.
Queen Anne's Lace (Daucus carota)
Wildflower purists will not be happy with me for listing this plant, as it is generally considered a weed. But it is a very familiar part of many prairie landscapes, and some people may want to include it in a prairie garden. The plant can grow over six feet in height, and it bears flat, white flower heads from late summer into early fall. The bottom photos show it growing with Common Chicory (Cichorium intybus), sometimes known as Blue Cornflower, another common field weed. I think they make an attractive combination, despite their outcast status.
There are as many varieties of asters in shades of purple and blue as there are white-flowered species. While the asters are most common on the prairie landscape later in the fall, a number of other plants have blooms in this color range from August through October.
Some taxonomists place the asters in the genus Symphyotrichum, rather than Aster.
Smooth Blue Aster (Aster laevis)
The flower heads of this September-bloming aster are lavender blue, about an inch wide — larger than the white Hairy Aster, but not in such abundant clusters. It is named for the smoothness of its waxy leaves. One of the more attractive and desirable asters for the prairie garden.
New England Aster (Aster novae-angliae)
One of the most striking asters of the prairie, its flower heads are a deeper purple and larger than the Smooth Blue Aster, about an inch and a half wide. It prefers a moist soil and will seed itself readily.
Drummond's Aster (Aster drummondii)
The purple form of A sagittifolius, this variety prefers the shade of the woods, but I have found it growing in the open prairie as well. It is a bushy, fairly tall plant for an aster, profusely covered with small, pale, lavender-blue flower heads, no more than half an inch wide.
Ironweed (Vernonia fasciculata)
Another member of the aster family, Ironweed is often seen in the late summer tallgrass prairie. The plants grow to about four feet and the flower heads are a striking dark purple, clustered at the top.
Rough Blazing Star (Liatris aspera)
There are several different species of Liatris that commonly grow on the prairie, all members of the aster family. The flower heads of this variety alternate along the stem, instead of forming a solid spike. It blooms in late summer.
Obedient Plant (Physostegia virginiana)
A member of the mint family, this plant takes its name from the way the pink-lavender flowers can be turned on the stem and remain where they are placed. The flowers grow on spikes, with the bloom proceeding from the bottom to the top of the stem. The plant grows to about four feet and blooms in July and August.
Nodding Wild Onion (Allium cernuum)
A wild onion that is every bit as attractive as any cultivated allium, with pale pink flower clusters that nod at the end of their stems. It is a small plant, not much more than a foot high, and easy to overlook when not in flower.
Bottle Gentian (Gentiana andrewsii)
One of the very last plants to bloom on the fall prairie in October, after the Cream Gentian has faded. It is also known as Closed Gentian because the petals remain closed, like buds that never open. Only the bumblebee can force its way inside to pollinate these flowers. The color is usually an attractive indigo, borne on stems that may be over two feet tall. Some of the specimens in these photos have been trampled and are not standing upright. It prefers a moist location.
By the time frost comes to Northern Illinois in late October, even the last lingering asters and blue gentians have begun to fade, and the prairie is going dormant for the approaching winter, turning brown. Even in monochrome, however, the browns are surprisingly variable, from pale silvered tan of some seedheads to deep ruddy bronze. Throughout the winter, seedheads of many plants will remain standing to provide food for wildlife, and many fruits and pods are attractive and lend interest to the late season garden. A few of these are shown below.
White Wild Indigo (Baptisia leucantha) Pale Dogwood (Cornus amomum)
Pasture Rose (Rosa Carolina)
Common Milkweed(Asclepias syriaca)
This concludes my year on the prairie, although I will continue to walk the trails there until the snow makes them impassable. These photos were taken at the Russell R. Kirt prairie restoration project of the College of DuPage, and some of the reserves managed by the DuPage County Forest Preserve District. I want to give special thanks to Bob at Willowbrook, who knows what all the plants are and answered so many of my questions.