Curtis Prairie

Common milkweed

Common milkweed

The first species of plant that we saw today has always intrigued me. Sometimes it catches my eye because of the unique shape of the flowers, or the way the flowers are packed together in a round burst of umbels. In the case of common milkweed (Asclepias syriaca) these umbels are shaped like popcorn balls almost the size of my fist.  The small individual flowers have five soft light white-pinkish petals that unfold and curve downward as they bloom, and five sturdy pointed nectar cups curving upwards. The fragrance and the color of the flowers attract a variety of insects to the nectar that is inside these cups. As with other flowering plants, the nectar is an enticement designed to attract pollinators to lift and spread the pollen from one plant to another.

However, the pollination system of milkweeds is not like most of the plants we typically see. The pollen is not in the form of free grains attached to an anther that is picked up by or brushed onto the insect. The pollen is contained in a waxy sac called a “pollinium” [plural: pollinia] with each sac having about a few hundred grains. These sacs are attached together by a filament that works like a shopping bag handle, designed to get caught on the leg of the would-be pollinator and carried over to the next plant. To hedge against the pollen being deposited in flowers of the same plant, these handles will actually hesitate a few minutes before rotating these sacs into the optimal position to be deposited in the cups of the next plant. This delay is an adaptive strategy to increase the chances that the nectaring insect is not depositing the pollen in the same plant.

During the few months the milkweed is blooming, it will produce between 300 and 500 flowers. Only a few will develop into pods—about one pod for every 100–150 flowers. But each of these pods will typically produce more than 200 seeds. So each individual shoot can produce around 800 seeds. You may have seen these seeds floating around at the end of the summer. They are tiny flat brown seeds attached to tufts of silky white filaments that act like little parachutes floating the seeds through the air. Like many other plants, this isn’t the milkweed’s only strategy for reproducing. Chances are when you see a milkweed, you will see a patch or a row of them. This is because what looks like individual plants are actually shoots of a clonal plant. Not far underneath the surface the shoots will send out horizontal growths, called rhizomes, from which stems or shoots will grow to three- to six-foot-tall plants—all the ones connected will actually be the same plant growing out of this single rhizome.

This is why you might see rows of these plants in a strip of land along the side of a building or in a cluster in an abandon field. As farmers and other property owners have gotten rid of these edges or patches of unclaimed soil, or paved or sprayed the milkweeds and other “weeds,” the monarch butterfly population that relies on milkweed plants to lay its eggs and as food for growing caterpillars is declining. So, there is a movement to plant and encourage space where the milkweeds will grow. Three years ago I had one small milkweed shoot pop up in a thin patch of flowers alongside my house. Now I have at least two clones on this narrow strip with about 10 to 15 hardy shoots—and each year the monarchs and other butterflies have found these plants.

Because milkweeds and other clonal plants can be aggressive and out compete other desirable plants, they can be a threat to gardens or native plant restorations. Another clonal native plant that we saw today is an effective competitor in most of the prairies, including the Arboretum prairies—Canadian goldenrod (Solidago canadensis). This goldenrod will grow up to six feet tall and will be topped with small bright yellow flowers along the terminal branches forming a pyramid of waving branches covered with small flowers. Like the milkweed, if you get close to the flowers you’ll get a front row seat to a frenzied show of activity from a wide variety of insect species. However, the Canadian goldenrod flowers are not blooming just yet—but by the end of the month you’ll see these yellow flowers throughout the prairie. There are at least seven types of goldenrod in our prairies, and many more throughout the world. We did see stiff goldenrod (Solidago rigida) and showy goldenrod (Solidago speciose) in the Native Plant Garden that wraps around the Visitor Center. Those are on a south-facing slope getting lots of sun and will also bloom soon—perhaps a bit earlier than the ones within the prairies.

Also along edges of the Native Plant Garden we saw the great mix of blooming colors summertime brings to the prairie: bright yellow prairie coreopsis (Coreopsis palmata), small purple flowers blooming upwards on stalks of hoary vervain (Verbena stricta), and interesting shades of two other milkweeds in bloom at this time: small pinkish-red flowers of swamp milkweed (Ascelpias incarnata), and bright orange flowers of butterfly milkweed (Asclepias tuberosa).

Walking along the prairie we did see several common weedy plants: St. John’s-wort (Hypericum perforatum) and daisy fleabane (Erigeron strigosus). These plants can be competitive in an open area, but are kept pretty well in check by the other established prairie plants. Leafy spurge (Ephoribia esula)is a weedy plant that seems fairly soft and delicate is a much more tenacious competitor. It has a milky sap that can cause blisters if it gets on your skin and is toxic to humans and other mammals if ingested. It will shoot out seeds from a very small seed pod in late summer and fall, but its main strategy in crowding out other plants by sending up shoots from its deep and vigorous rhizome growth.

As we walked along the paths in Curtis Prairie that lead south and west, we encountered pockets of wet areas. There we saw several stands of willow and areas where clumps of tussock sedge (Carex stricta) were growing. Also in these areas we saw the thick purplish-red stemmed plant Angelica (Angelica atropurpurea). It was nearing the end of its bloom time. Flowers are small greenish-yellow, but showy in round umbels (a similar pattern to the common milkweed).

Another interesting thing we noticed as we walked these paths was a seemingly healthy vole population in the prairie. At least six voles crossed in clear view scurrying along the paths in front of us during our hour-and-a-half walk. Voles are small rodents that grow to 3–5 inches. They have dark brown short hair and look a bit like mice, with less prominent ears and rounder or stouter bodies. They can reach adult size in about a month, and may have 5 to 10 litters per year. They build tunnels and burrow through the soil, feeding on all the bountiful roots and rhizomes abundant in the prairie. They will also eat a variety of nuts, fruits, and seeds, and will scavenge on dead animals.

Voles can girdle small trees and ground cover, much like a porcupine. This girdling can easily kill young plants and is not healthy for trees or other shrubs. However, like other burrowing insects and animals, they play their part in the overall health of the ecosystem by breaking up the soil and distributing nutrients.

Along the edges of the prairie we often see red-tailed hawks and owls. I was expecting one of their favorite foods—voles—to draw some out today. But we didn’t hear or see any today. We did hear field sparrows, a common yellow-throat, and a catbird, and saw barn and tree swallows and a downy woodpecker.

In the drier spots of the prairie we saw the final blooms of tall beard-tongue (Penstemon digitalis). Also in bloom were black-eyed Susans (Rudbeckia hirta), some fairly large stands of pale purple coneflowers (Echinacea pallida), and smaller patches of downy phlox (Phlox pilosa).

Joe-Pye-weed (Eutrochium maculatum), pale Indian plantain (Arnoglossum atriplicifolium), prairie rose (Rosa arkansana), elderberry (Sambucus canadensis) and sumac (Rhus glabra) were abundant along the edges and in the shaded oak savanna areas around Curtis Prairie.

This is the time of the year when each week there are many species coming into bloom in the prairies—so be sure to come out and enjoy the show!

—Paul Borowsky

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