Stiltgrass (Microstegium vimineum)

Synonyms: Eulalia viminea, Andropogon vimineus, Pollinia viminea, Microstegium vimineum var. imberbe

Common Names: Japanese stiltgrass, Nepalese browntop, Asian stiltgrass, Japanese grass, packing grass, Mary's grass

Description:

Japanese stiltgrass (Microstegium vimineum) is an annual grass from Asia that was brought to the United States around 1919, reportedly as packing material for porcelain [1][3]. It now grows across much of the eastern United States, forming dense, sprawling mats in shady, moist places where few other grasses grow [1][3]. MISIN tracks it because those mats crowd out native forest wildflowers, grasses, and tree seedlings and spread quickly along trails, streams, and roadsides [1].

Habit:

Japanese stiltgrass is a low, sprawling annual grass that grows 1 to 3 feet tall, often leaning over and rooting where its stems touch the ground on stilt-like prop roots [1]. Its leaves are short, narrow, and pale green, and the surest clue is a silvery stripe of fine hairs running lengthwise down the middle of each leaf, slightly off to one side [1]. The thin, branching stems and small leaves give it a delicate, almost miniature-bamboo look [1].

It germinates in spring, sends up slender, three-branched flower spikes in late summer, and then dies back in fall, leaving behind tan, matted stems that lie over the ground through winter [1]. The easiest time to spot it is mid-to-late summer, when it has filled in and the silver-striped leaves stand out; the tan winter mats are also a giveaway [1].

At-a-glance cues:

1. Sprawling pale-green annual grass, 1?3 feet tall, forming mats in shade [1]
2. A silvery stripe of hairs running down the middle of each leaf, slightly off-center [1]
3. Stilt-like prop roots where the stems touch the ground [1]
4. Tan, matted dead stems lying over the ground in fall and winter [1]

Leaves:

1. Alternate along the stem, short and narrow (lance-shaped), about 1?3 inches long [1]
2. Pale green and thin-textured [1]
3. A silvery stripe of fine hairs runs lengthwise down the midrib, slightly off-center ? the key identifying trait [1]
4. Surface mostly smooth, sometimes sparsely hairy [1]

Stems:

1. Soft and green (herbaceous), thin, and branching, not woody [1]
2. Sprawling and leaning, 1?3 feet long, often rooting at the joints where they touch the ground (stilt-like prop roots) [1]
3. Roots are shallow and fibrous, so the plant pulls up easily [1]
4. An annual ? the whole plant dies back each fall to tan, matted stems [1]

Flowers:

1. Tiny grass flowers carried on slender, usually three-branched spikes at the stem tips [1]
2. Appear in late summer, roughly August into September [1]
3. The plant also produces hidden self-pollinating flowers lower on the stem, so even small plants set seed [1]

Fruit and seeds:

1. Produces small grass seeds ? a single plant can make up to about 1,000 in a year [1]
2. Seeds ripen in fall, then drop near the parent or are carried away [1]
3. Spread on shoes, tires, mowers, and equipment, in mud, soil, and fill, by water, and by animals such as deer [1]
4. Seeds can stay viable in the soil for up to about five years [1]

Habitat:

Japanese stiltgrass grows in shaded to partly sunny, moist ground, which sets it apart from most weedy grasses that need full sun [1]. It is common on forest floors and edges, along streams and floodplains, on trails and roadsides, in ditches, and in shady lawns and gardens [1]. It tolerates low light and quickly fills in disturbed ground [1].

The grass is native to Asia [3]. It was introduced to the United States around 1919 and is now widespread across much of the eastern United States [1][3].

Reproduction:

Japanese stiltgrass spreads only by seed, and the seeds move easily on the things people and animals carry. They hitch a ride in the mud on shoes, tires, mowers, ATVs, and equipment, wash downstream in water, ride along in soil and fill, and are moved by deer and other animals [1]. So a new patch often starts along a trail, driveway, or stream bank and then fills in from there.

It is a prolific seeder ? a single plant can produce up to about 1,000 seeds, and even small plants set seed through hidden self-pollinating flowers [1]. The seeds can survive in the soil for up to about five years [1]. Because it is an annual that comes back only from seed, the key to control is to stop seed production: remove plants before they flower and set seed in late summer, and repeat for several years to use up the buried seeds [1].

Impact and Damage:

Japanese stiltgrass is not poisonous to people or pets, so the damage is to your yard, woodlot, and natural areas rather than your health. It forms dense, sprawling mats that smother and crowd out native wildflowers, ferns, grasses, and tree seedlings on the forest floor [1][3]. Once it takes over a shady spot, little else grows beneath it.

For natural areas, the effect is larger. Stiltgrass can blanket the ground layer of a woodland, reducing the variety of native plants and making it harder for forests to regrow [1][3]. Deer tend to avoid eating it while they browse the native plants around it, which gives stiltgrass an extra edge and helps it spread [1]. Heavy infestations can change conditions at the soil surface and lower the habitat value of the woods for wildlife [3].

Similar species:

The plant most often confused with Japanese stiltgrass is whitegrass (Leersia virginica), a native grass of the same shady, moist woods. Whitegrass has a similar look but lacks the silvery off-center stripe down the leaf, its leaves are often a bit narrower and rougher, and it is a perennial that comes back from its roots rather than only from seed.

Two other grasses can also be mistaken for it. Crabgrass (Digitaria species) is a sprawling annual grass too, but it grows in sun rather than shade, has wider leaves, and lacks the silver leaf stripe. Nimblewill (Muhlenbergia schreberi) is a low, wiry native grass of lawns and shade, but it is a perennial with very fine leaves and no silver midrib stripe.

Monitoring and rapid response:

Signs to watch for:

1. Sprawling pale-green grass mats in shady, moist spots, with a silver stripe down each leaf [1]
2. Stilt-like prop roots where the stems touch the ground [1]
3. Tan, matted dead stems lying over the ground in fall and winter [1]

What you can do:

1. Hand-pull it ? the roots are shallow, so it pulls up easily, especially when the soil is moist [1].
2. For larger patches, cut or string-trim the plants at ground level in late summer, just before they flower (around August), so they cannot regrow and set seed; cutting too early in the season only lets them grow back and reseed [1].
3. Always remove or cut plants before the seeds form in late August to stop new seed [1].
4. Bag and trash any plants that are flowering or seeding ? do not compost them.
5. Clean seeds and mud off your shoes, tires, mowers, and tools before leaving an infested area so you don't carry seed to new ground [1].
6. Repeat the work every year for several years, since the seeds last about five years in the soil [1]. Natural area crews should target small and new patches and edges first, pull or mow across a site before seed set, and limit soil disturbance.

When to call a professional:

1. Large, well-established infestations across a woodlot or property.
2. Sites where yearly pulling and cutting are not keeping it in check.
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Penn State Extension. Japanese Stiltgrass. extension.psu.edu
2. USDA NRCS Plants Database. Microstegium vimineum (Trin.) A. Camus. plants.usda.gov
3. U.S. Department of Agriculture, National Invasive Species Information Center. Japanese Stiltgrass (Microstegium vimineum). invasivespeciesinfo.gov
4. Michigan Department of Natural Resources. Invasive Species: Japanese Stiltgrass. michigan.gov/invasives

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