Canada bluegrass (Poa compressa)

Synonyms: Poa fertilis, Poa pallens, Poa polynoda, Poa compressa var. contracta

Common Names: Canadian bluegrass, flat-stemmed bluegrass, wiregrass

Description:

Canada bluegrass (Poa compressa) is a low, cool-season perennial grass from Europe and Asia that was introduced to North America, both as a contaminant in seed lots and on purpose for erosion control and forage [2][3]. It is now naturalized across much of the continent, mostly on poor, dry, disturbed ground [1][3]. MISIN tracks it because it spreads by seed and creeping underground stems and can take over thin, infertile soils where native plants struggle [3].

Habit:

Canada bluegrass is a wiry, blue-green grass that usually grows 10 to 24 inches tall [3]. Its most telling feature is the stem: it is strongly flattened and two-edged, not round like most grasses, which is easy to feel by rolling it between your fingers [3][4]. The narrow leaf blades are blue-green and hairless, and the plant creeps outward by underground stems to form a loose, open turf [3][4]. The flower heads are narrow, branched clusters that start out grayish-blue and turn tan as they dry [3].

It is easiest to spot in summer, when the flower heads appear ? seed heads are usually showing by August in the North [4]. The bluish color and flat, wiry stems stand out in dry, sunny spots with poor soil. Earlier in the season, look for low patches of thin blue-green leaves on roadsides, in pastures, and on compacted or worn ground [3].

At-a-glance cues:

1. Stems strongly flattened and two-edged, not round ? noticeable when rolled between the fingers [3][4]
2. Wiry, blue-green color throughout the plant [3]
3. Low grass, 10 to 24 inches tall, spreading by underground stems into a loose turf [3]
4. Narrow flower clusters that are grayish-blue, turning tan by late summer [3]

Leaves:

1. Arrangement: mostly low and clustered at the base, with a few blades up the stem [4]
2. Shape: narrow, flat to folded blades [3][4]
3. Surface: hairless and smooth [4]
4. Color: blue-green to gray-green [3]

Stems:

1. Habit: herbaceous (non-woody) perennial; solitary or in loose tufts, 10 to 24 inches (25 to 60 cm) tall [3]
2. Cross-section: strongly flattened and two-edged, not round ? the key field trait [3][4]
3. Color: blue-green and wiry [3]
4. Spread: creeps outward by long underground stems (rhizomes), forming a looser turf than Kentucky bluegrass [3]

Flowers:

1. Color: grayish-blue, turning tan as it dries [3]
2. Shape: many tiny flower clusters (spikelets) along the branches, with no showy petals ? typical of grasses [3]
3. Arrangement: a narrow, compressed branched head (panicle) at the top of the stem [3]
4. Bloom time: summer; seed heads are usually visible by August in the North [3][4]

Fruit and seeds:

1. Fruit type: a tiny dry grain (the grass seed), light tan to brown [3]
2. Output: very high ? roughly 2.5 million seeds per pound of cleaned seed [3]
3. Dispersal: the small seeds are carried by wind and water, on mowers and equipment, and as a contaminant in seed and hay [3]

Habitat:

Canada bluegrass is native to Europe, the Mediterranean region, and temperate Asia [2]. It was brought to North America and is now naturalized from Newfoundland to Alaska and south through most of the United States [1][3].

It is a grass of poor ground. It does best on infertile, dry, acidic, or compacted soils, and on sites that are too droughty or nutrient-poor for other grasses ? places where it can grow in nearly pure stands [3]. It needs full sun and does not tolerate shade [3]. Look for it on roadsides, worn pastures, vacant lots, old fields, reclaimed mine land, gravelly openings, and thin lawns [3]. On rich, fertile soil it is a weak competitor and tends to be crowded out [3].

Reproduction:

Canada bluegrass spreads two ways. It produces large amounts of seed ? about 2.5 million seeds per pound ? and these tiny seeds travel easily on the wind and water, on mowing and tillage equipment, and mixed into seed or hay [3]. It also creeps outward underground by rhizomes (horizontal underground stems), so a patch slowly widens on its own and can push in from neighboring ground [3].

It is a cool-season perennial: it greens up and grows in the cooler parts of the year, flowers in summer, and sets seed by late summer [3][4]. For control timing, the key is to cut or mow before the seed heads ripen in mid- to late summer, and to dig out the underground stems when removing a patch by hand [3].

Impact and Damage:

For homeowners, Canada bluegrass is a minor problem rather than a hazard. It is not toxic to people or pets. It mostly shows up as a wiry, blue-green grass in thin, dry, or worn parts of a lawn or driveway edge, where richer lawn grasses have trouble growing [3].

For natural area stewards, the concern is on poor, dry, open ground. Canada bluegrass is an early colonizer of disturbed sites and can form stands on dry prairies, rocky openings, barrens, and other low-fertility natural areas, where it takes up space and can reduce the cover of native plants [3]. Because it was widely planted for erosion control, reclamation, and forage, it spread far beyond where it was first introduced [3]. Its impact is limited on fertile soils and in shade, where it cannot compete, so healthy woods and rich sites are at low risk [3].

Similar species:

Several lawn and pasture grasses in the bluegrass group look alike. The surest check for Canada bluegrass is its strongly flattened, two-edged, wiry blue-green stem [3][4].

Kentucky bluegrass (Poa pratensis) is the most common look-alike and a close relative. It has round stems rather than flattened ones, grows taller and greener, and forms a much denser, fuller sod, while Canada bluegrass stays wiry and blue and grows on poorer soils [3][4].

Annual bluegrass (Poa annua) is often confused with it in lawns and disturbed ground. Annual bluegrass is a low, bright-green grass that usually lives a single season, lacks creeping underground stems, and flowers for much of the year, unlike the blue-green, rhizome-spreading, summer-flowering Canada bluegrass [4].

Rough bluegrass (Poa trivialis) is another bluegrass of similar size. It prefers shady, moist ground and creeps by above-ground runners, and its stems are round rather than the flattened, two-edged stems of Canada bluegrass [3][4].

Monitoring and rapid response:

Signs to watch for:

1. Low, wiry, blue-green grass on dry, poor, or compacted ground ? roadsides, worn pastures, gravelly openings, thin lawns [3]
2. Stems that feel flattened and two-edged when rolled between the fingers [3][4]
3. Narrow, grayish-blue flower heads in summer, turning tan by August [3][4]

What you can do:

1. Dig out small patches by hand, removing the underground stems so the plant cannot resprout ? easiest when the soil is moist [3]
2. Cut or mow before the seed heads ripen in mid- to late summer to stop seeds from forming and scattering [3]
3. Bag and dispose of seed heads rather than leaving them on site, since the tiny seeds spread easily [3]
4. On bare or worn ground, improving the soil and establishing dense, desirable plants helps, because Canada bluegrass competes poorly where richer plants can grow [3]
5. For natural areas, volunteer crews can target small outlying patches on dry prairies and openings first, then work across larger stands with repeated cutting and digging over several seasons [3]

When to call a professional:

1. Large or well-established stands on dry prairies, barrens, or reclaimed ground that keep returning after cutting and digging [3]
2. Sites where removing the grass could leave erosion-prone bare soil that needs a managed replanting plan [3]
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. USDA NRCS Plants Database. Poa compressa L. plants.usda.gov
2. Plants of the World Online (POWO), Royal Botanic Gardens, Kew. Poa compressa L. powo.science.kew.org
3. Esser LL (1994). Poa compressa. In: Fire Effects Information System (FEIS). U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. research.fs.usda.gov
4. NC State Extension Gardener Plant Toolbox. Poa compressa (Canada Bluegrass). plants.ces.ncsu.edu

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