Poa infirma
Weak blue grass
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Family: Poaceae · Type: Grass · Not Native
Poa infirma (Weak blue grass) is a non-native California grass in the Poaceae family, recognized by visible axis and long terminal internode.
Weak blue grass is a naturalized annual grass found in southern San Joaquin Valley, southern Central Western California, and southwestern California in disturbed low ground, especially sandy places at elevations below 500 meters. Flowering from March to May, this grass produces yellow-green flowers in compact, ascending inflorescences 1 to 6 centimeters long. Growing in dense tufts to 15 centimeters tall with slender stems, it forms delicate clusters of thin stems. Its leaves are flat, 1 to 4 millimeters wide, with open sheaths and small ligules 1 to 5 millimeters long. The spikelets have densely hairy lemma veins and crowded branches, creating a distinctive compact grass form.
Identification and appearance
Key characters: 26 sympatric species; diagnostic visible axis and long terminal internode
Habitat and range
Habitat: Disturbed low ground, especially sandy places
Elevation: < 500 m
Outside California: scattered across temperate North America and worldwide, native to Mediterranean
Bloom period
Poa infirma blooms Mar-May.
How to tell it apart
vs Festuca: Poa has boat-shaped, keeled leaf tips; Festuca leaves are flat or rolled but never keeled like a ship's prow.; vs Melica: Poa has fine, narrow leaves with boat-shaped tips; Melica has wider, flat leaves without the distinctive keel.; vs Nassella: Poa leaves have boat-shaped tips you can feel; Nassella leaves roll inward and lack the keel.; vs Koeleria: Poa has cobwebby hairs at seed base and keeled leaf tips; Koeleria lacks both features.; vs Holcus: Poa has boat-shaped leaf tips; Holcus leaves are flat and soft-hairy without any keel.
Frequently asked questions
Is Poa infirma native to California?
No. Poa infirma is not native to California (recorded as not native).
When does Poa infirma bloom?
Poa infirma blooms Mar-May.
What plant family does Poa infirma belong to?
Poa infirma is in the Poaceae family, genus Poa.
What habitat does Poa infirma grow in?
Poa infirma grows in Disturbed low ground, especially sandy places.
Related species
Other California species in the same genus, family, or bioregion.
Sources: Jepson eFlora · CalPhotos
Last updated 2026-10-07
Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, Cal-IPC, and more.