Bulbous bluegrass (Poa bulbosa L.)
Introduced from Europe, bulbous bluegrass has become weedy in pastures, grain fields, roadsides, and disturbed areas throughout much of the western United States. It is a perennial grass that grows from distinctive basal bulbs, which allow it to survive drought, grazing, and adverse conditions.
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Plants form tightly clustered stems (culms) that typically grow ½ to 2 feet tall (8–32 inches). Growth is dense and tufted.
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Leaves are narrow (2–4 mm wide), flat or loosely rolled, and deep green in color. Leaf blades are parallel-sided, keeled, and V-shaped in cross section. Sheaths are slightly compressed, split with overlapping margins, and not sharply keeled. Membranous ligules are approximately 1/8 inch long.
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Flowering occurs from May through June. The panicle is open and pyramid-shaped, measuring 1.2–5 inches long. Lower branches occur in clusters of 3–5. Each spikelet contains 3–5 flowers borne on short stalks.
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A key identifying feature is that many flowers are modified into bulblets rather than viable seeds. These bulblets often have a dark purple base and detach easily, allowing vegetative spread. When seed is produced, the fruit is a caryopsis (a dry, single-seeded grain typical of grasses).
Quick ID Guide
Life Cycle:
• Perennial grass
• Grows from basal bulbs
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Height:
• ½–2 feet tall
• Dense, tufted growth
Leaves:
• Narrow (2–4 mm wide)
• Flat or loosely rolled
• Deep green
• Membranous ligule (~1/8 inch)
Flowering (May–June):
• Open, pyramid-shaped panicle
• Branches in clusters
• 3–5 flowers per spikelet
• Flowers often replaced by purple-based bulblets
Reproduction:
• Primarily by bulblets
• Seeds (caryopsis) when present
Management Options
Prevention and cultural control strategies should be emphasized. Maintain healthy, competitive perennial vegetation to reduce establishment. Avoid overgrazing, as disturbance favors bulbous bluegrass spread.
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There are currently no approved biological control agents available for bulbous bluegrass.
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Mechanical control options are limited. Hand pulling may be effective for very small infestations, but bulbs must be removed completely to prevent regrowth. Mowing is generally ineffective because plants are low-growing early in the season and reproduce through bulblets.
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Tillage may reduce populations in cultivated settings, but repeated cultivation is often required to exhaust bulb reserves.
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Larger infestations in pastures or non-crop areas can be managed with herbicides. Early spring treatments during active growth are generally most effective. As with all pesticide applications, read and follow the herbicide label carefully. Contact Sublette County Weed & Pest for assistance with product selection, rates, and timing.
