Bulbous bluegrass (Poa bulbosa)

Common Names: Bulbous meadow-grass

Description:

Bulbous bluegrass (Poa bulbosa) is a small, cool-season grass native to Europe, Asia, and North Africa, brought to North America in the early 1900s as a forage grass [1][2]. It has spread on its own across much of the western United States and beyond, mostly on roadsides, abandoned fields, and other disturbed ground [2]. MISIN tracks it because it spreads in an unusual way ? its flower heads turn into little leafy bulblets that sprout new plants ? and it can take over degraded, dry ground as native grasses decline [2].

Habit:

Bulbous bluegrass is a tufted grass, usually about 6 to 24 inches tall, with mostly basal, narrow leaves that have the boat-shaped tip typical of bluegrasses [2][3]. Two features make it easy to confirm. First, the base of each stem is swollen into a small, onion-like bulb ? pull up a plant and you can feel it [2][3]. Second, and most telling, its seed head is not full of normal flowers and seeds but of tiny leafy bulblets ("bulbils"), each with a dark purple base, that look like miniature grass shoots [2][3]. It is a cool-season grass: it greens up in fall or early spring, flowers in late spring, then dries up and goes dormant through the summer heat even if there is moisture [2]. Look for it in spring, when it is green and the bulblet heads are showing, often standing out along roadsides [2].

At-a-glance cues:

1. Small tufted grass, about 6 to 24 inches tall, with narrow boat-tipped leaves [2][3]
2. Stem base swollen into a small onion-like bulb [2][3]
3. Seed head made of tiny leafy bulblets with dark purple bases, instead of normal flowers and seeds [2][3]
4. Green in spring, then brown and dormant through summer [2]

Leaves:

1. Arrangement: mostly basal or on the lower stem [2]
2. Blade: narrow (about 1 to 3 mm wide), flat to folded, hairless, with a boat-shaped tip [2][3]
3. Length: short, up to about 4 inches [2]

Stems:

1. Habit: erect to spreading, tufted, hairless, growing in clumps; about 6 to 24 inches tall [2][3]
2. Base: swollen into a small, onion-like bulb ? the plant's namesake feature [2][3]
3. Life cycle: a short-lived cool-season perennial that goes dormant in summer [2]

Flowers:

1. Seed head: a branching cluster (panicle), egg-shaped, compact to open, a few inches long [2][3]
2. Bulblets: in most North American plants the florets are replaced by leafy bulblets ("bulbils") with dark purple bases, looking like tiny grass shoots [2][3]
3. Bloom time: late spring [2]
4. True flowering and seed set are mainly seen in its native range; North American plants reproduce mostly by the bulblets [2]

Fruit and seeds:

1. In North America the plant rarely makes normal seed; instead the seed head produces leafy bulblets that drop and sprout [2]
2. The swollen stem bases also act as bulbs that resprout each year [2]
3. Bulblets and bulbs spread in moved soil and gravel, on equipment, by water and wind, and when animals cache them; roadsides are a major spread corridor [2]

Habitat:

Bulbous bluegrass is native to Europe, temperate Asia, and North Africa [1][2]. It was introduced to North America as a forage grass in the early 1900s and has since spread widely on its own, especially across the western United States and into southern Canada [2]. It favors open, disturbed, and dry ground ? roadsides, trails, abandoned fields, overgrazed rangeland, and gravelly or low-fertility soils ? and it tolerates salty, shallow, and poor soils in semiarid country [2][3]. It is much less of a problem in intact, undisturbed natural areas [2].

Reproduction:

Bulbous bluegrass spreads in an unusual way. In North America it mostly skips making seed and instead turns its flower heads into tiny leafy bulblets, which fall to the ground and sprout into new plants; the swollen bulb at each stem base also resprouts year after year [2]. These bulblets and bulbs travel easily in moved soil and gravel, on mowing and grading equipment, by wind and water, and when small animals cache them ? which is why it spreads first and fastest along roadsides [2].

It is a cool-season grass: bulblets sprout in fall or early spring, plants green up and produce new bulblets by late spring, then dry out and go dormant for the summer [2]. For control timing, target plants in spring before the new bulblets ripen and drop, and remove the bulbs from the soil since the plant regrows from them [2][3].

Impact and Damage:

Bulbous bluegrass is mainly a problem on open, disturbed, and degraded land rather than a hazard to people or pets. It is an aggressive colonizer of roadsides, abandoned fields, and overgrazed rangeland, where it can build up and increase as native bunchgrasses decline [2]. Because it dies back early and leaves bare ground through summer, a heavy stand can shift the makeup of a grassland and reduce the cover that native grasses would otherwise provide [2]. It is, however, generally not considered especially invasive in healthy, undisturbed natural areas [2]. Its bulblets are eaten by wildlife such as deer, elk, and game birds, and it provides some early-spring forage for livestock before it cures [2][3].

Similar species:

Bulbous bluegrass resembles other bluegrasses when it is not in flower.

Kentucky bluegrass (Poa pratensis) is the most likely mix-up ? a common lawn and pasture grass that shares the narrow, boat-tipped bluegrass leaves. Tell them apart by the base and the seed head: bulbous bluegrass has a swollen, onion-like bulb at each stem base and a seed head full of leafy purple-based bulblets, while Kentucky bluegrass has slender (not bulbous) stem bases, spreading underground runners, and a normal seed head with true flowers and seeds [2][3].

Monitoring and rapid response:

Signs to watch for:

1. Small tufted grass with onion-like swollen stem bases and seed heads made of tiny purple-based bulblets, on roadsides, trails, or disturbed and gravelly ground [2][3]
2. Patches that are green in spring and brown and dormant by early summer [2]
3. New plants showing up along road corridors or where soil or gravel has been moved [2]

What you can do:

1. For a small patch, dig out the plants in spring before the bulblets ripen, and lift the bulbs from the soil since the plant resprouts from them [2][3]
2. Bag and trash the bulblet seed heads and bulbs rather than composting them, so they can't sprout elsewhere [2]
3. Avoid mowing once the bulblets have formed ? mowing scatters them ? and clean soil, gravel, and plant bits off mowers, tools, and vehicles before leaving an infested area [2]
4. Keep desirable grasses and ground cover healthy, since bulbous bluegrass mainly takes hold on bare, disturbed, or overgrazed ground [2][3]
5. Land managers can prioritize new, outlying patches along roads, time removal for spring before bulblet drop, and repeat over several years since buried bulbs persist [2]

When to call a professional:

1. Large or well-established stands on rangeland or right-of-way, where hand methods cannot keep up [2]
2. 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. Plants of the World Online (POWO), Royal Botanic Gardens, Kew. Poa bulbosa L. powo.science.kew.org
2. USDA Forest Service, Fire Effects Information System (FEIS). Poa bulbosa. fs.usda.gov
3. Utah State University Extension. Bulbous Bluegrass (Poa bulbosa), Range Plants of Utah. extension.usu.edu

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