August is the time of the most active growth and flowering for a surprising number of our native plants. Since fewer insects are active in late summer, it’s easier to grow then without getting chewed. And since early-season plants are waning in vigor, late summer plants enjoy reduced competition for water, nutrients, and light. To be sure, late summer heat and drought can still pose a challenge, as can a premature fall that hits before growth is complete. But some of our most widespread and successful native plants have evolved adaptations that make this season congenial.
Perhaps the best example is the Island’s most iconic grass — little bluestem. This valuable native grass turns up in any open, reasonably dry habitat on the Vineyard, and in the most suitable places, little bluestem dominates. Indeed, that’s true not just on the Island but across the continent: a range map on a U.S. Department of Agriculture website shows that little bluestem occurs in prairies or dry grassland across 46 of the 48 contiguous states and eight of the ten Canadian provinces.
Perhaps the best place for Islanders to get acquainted with this grass is at the Land Bank’s Trade Wind Fields Preserve in Oak Bluffs, where it thrives. But once you learn to recognize it, you’ll find this grass nearly everywhere you look.
On the Vineyard, bluestem’s gray-green leaves begin growing in May; in August, its tall flowering stalks shoot up. The purplish color near the base of these stalks give the plant its not entirely apt name. When mature, bluestem stalks produce loose clusters of feathery seeds and then senesce into a pale brown straw that stands erect through most of the winter. It’s an attractive plant. Various selectively bred ornamental varieties may have more dramatic color, but I don’t think they’re any more attractive than our wild bluestem, which makes a fine addition to a yard or garden. It grows readily from seed and transplants well except in the fall, when transplants often can’t reestablish their root systems.
In addition to its late-season growth pattern, little bluestem has physical features that contribute to its adaptability. For one thing, its seeds disperse readily, borne by the wind on a canopy of fine fibers, like a tiny milkweed seed. For another, little bluestem has the good judgment to put more effort into growing roots than it puts into top growth. A mature clump of this grass, which can live for decades, may have roots eight feet deep, and this fibrous mass anchors the plant, traps water, and stores large reserves of energy as a buffer against drought or disturbance.
Little bluestem exemplifies what ecologists refer to as “keystone species” — plants or animals that are central features to an ecosystem and support many other kinds of wildlife. The caterpillars of many species of butterflies prefer bluestem as their food source. A host of other insects, including grasshoppers, leaf hoppers, and beetles, also eat it, but few of these species ever get numerous enough to damage bluestem plants. Spiders anchor their webs to the stiff, upright stems of this grass; in winter, song and field sparrows eat its seeds. Bluestem even makes an acceptable forage grass for livestock, producing tender growth after most early-season pasture grasses have gone dormant.
Perhaps best of all, little bluestem tends to grow in clumps, not as sod: rather than choking out other plants, the clumping habit leaves room for other plants to grow — asters, milkweed, goldenrod, other grasses. Along with these neighbors come the insects that associate with them. A healthy bluestem grassland is a diverse and productive habitat, supporting a wide variety of species both common and rare.
If bluestem has a vulnerability, it is a weakness that afflicts all grasses, which are short by nature: Growing three feet tall at the most, our bluestem is vulnerable to being shaded out by taller vegetation. Unless something happens that retards growth of shrub and tree seedlings, a bluestem meadow will eventually grow up into woodland. Often, fire is the mechanism that keeps bluestem grassland open. On the Vineyard, prescribed spring burns at grassland sites like Katama and Quansoo benefit this important grass. Fire readily burns off the dry, dead straw from the past season but leaves the deep roots of a bluestem plant unharmed. And by releasing nutrients and reducing competition from surrounding plants, a burn stimulates rapid growth and, later, vigorous seed-set in bluestem.
The more common a species is, the less we tend to notice it. Little bluestem, perhaps the most abundant native grass on Martha’s Vineyard, is something we take for granted. But it’s an attractive, well-adapted plant that performs a host of functions in Island ecology, and if it disappeared, the Island would be a very different place. Use the coming weeks to get to know the Vineyard’s keystone grass.