August is for Defeating Stiltgrass

“Thrives in diverse environments and adapts readily to varied conditions. A prolific producer with multiple strategies to achieve success. Makes the most of opportunities.”

If invasive plants had their own LinkedIn accounts, this might be on the profile Japanese stiltgrass (Microstegium vimenium) and August is the month of its ascendency when it densely crowds roadsides, drainage ditches, utility rights of way, trail edges, stream banks, and just about any place where a little sunlight interrupts the shade, particularly where the ground has been disturbed.

Japanese stiltgrass was my “gateway invasive”—the species that introduced me to invasive plant ecology when I first became a Virginia Master Naturalist ten years ago because it had already dramatically altered the northern Blue Ridge woodlands where I’d grown up. In the invasive plant geek parlor game of “If you were a time-traveling superhero, what invasive plant species would you stop?” I
don’t waffle over my answer. In the mountains and piedmont, August is the season of stiltgrass dread while the fall and winter holidays bring the despair of the inescapable posthumous hummocks carpeting the forest floor. (It was dried-out dead stiltgrass that first invaded North America, used as packing material and loaded with seeds.)

But don’t let me bring you down, read on!

Stiltgrass is an annual grass from Asia that reproduces primarily through seed production. It’s one of the most destructive invasive plants in North America and nearly ubiquitous in eastern deciduous forests where, by late summer, it forms stiltgrass seas that turn to vast masses of dried “thatch” in winter.

No invasive plant species proves that “timing is everything” quite like stiltgrass. Manage it properly in late summer prior to flowering–every year it grows–and you can curb its spread, even eliminate incipient incursions. But misguidedly mow, weed whack, or pull it after it’s gone to seed and you’ve just been an unwitting accomplice in an ecological crime. Do the same too early, in June or July, and stiltgrass will respond with vigorous new growth.

How does stiltgrass stand out within the ranks of invasive plants that by definition dominate the environments they invade?

 Each plant can produce as many as 1,000 seeds.
 Those seeds can remain viable in the ground for up to five years, possibly longer (germination rates do drop off dramatically after two or three years).
 Stiltgrass spreads in sun or shade, dry or moist conditions.
 But it particularly loves wetlands and riparian areas (adjacent to streams, creeks, and rivers) and the flowing water carries stiltgrass seeds downstream to new outposts.

Stiltgrass also gets some help from us, animals, and the elements:

 Seeds hitchhike on the bottoms of our hiking boots and the tires of vehicles and machinery.
 White-tailed deer–particularly where populations are too high– heavily browse native plants, giving stiltgrass an advantage, while seeds get a ride on hooves and hides.
 Off-leash (and off-trail) dogs also can transport seeds on their fur.
 Stormwater runoff transports and deposits stiltgrass seeds.
 Land management and landscaping practices in natural areas that disturb soils and cut back native species woody and herbaceous vegetation set the stage for stiltgrass colonization.

Identifying stiltgrass is easy, but there are native grasses that might become cases of mistaken identity. Here are the basics:

 Stiltgrass leaves are 2 to 4 inches long and about half an inch wide,
well-spaced alternately on the stem, with smooth edges
 Leaves sport a distinctive silvery, off-center mid-rib
 At maturity, stiltgrass can reach a height of three feet or more
 Stems often branch in multiples from the base
 Seed heads extend from delicate stalks Alabama Natural Resources provides this excellent Field Guide to Identification of Japanese
Stiltgrass (with comparisons to look-a-like species).

So now that you’ve just realized you have stiltgrass on your property, what should you do about it? Here is the one nice thing I have to say about stiltgrass: it has a weak root system that pulls up easily. If you have just small patches of stiltgrass, hand pulling is the way to go. Hand pulling also allows you to carefully avoid collateral damage to native plant species, including grasses like Virginia white-grass and deer tongue grass. If you have larger expanses of stiltgrass and you’re confident that no native plants are hiding out, you can use a stringed weed trimmer to cut the stiltgrass as close to the ground as possible. I still use the (freshly sharpened) sickles and weed whips my family used to open trails and
other clearings at home in the ‘70s and ‘80s . . . . trails and clearings that were colonized by stiltgrass by the new century!

Whether manual or mechanical, you want to tackle stiltgrass soon, and since it does not uniformly go to seed at the same time in all habitats, inspect it closely to make sure seeds have not yet formed. For more detailed information on managing Japanese stiltgrass, check out this fact sheet from Blue Ridge PRISM.

Laura Greenleaf is a Certified Virginia Master Naturalist and a founding steering committee member of the James River Park System Invasive Plant Task Force. She lives near the Pony Pasture section of the JRPS.