Poa annua (Annual Bluegrass)

Plant Info
Also known as:
Genus:Poa
Family:Poaceae (Grass)
Life cycle:annual
Origin:Eurasia
Status:
  • Weedy
Habitat:part shade, sun; disturbed soils; lawns, gardens, waste places, trail edges, roadsides, shores
Fruiting season:June - October
Plant height:2 to 8 inches
Wetland Indicator Status:GP: FACU MW: FACU NCNE: FACU
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

Pick an image for a larger view. See the glossary for icon descriptions.

Detailed Information

Flower: Flower shape: indistinct Cluster type: panicle

[photo of panicle] Open panicle to 3 inches long, egg-shaped to pyramidal in outline. Spikelets (flower clusters) are short-stalked, light green sometimes purple-tinged, flattened, oblong to egg-shaped, 3 to 6 mm (to ¼ inch) long with 2 to 6 florets; the uppermost 1 or 2 florets may be sterile.

[close-up of branch] At the base of a spikelet is a pair of bracts (glumes) that are both lance to egg-shaped, hairless, awnless, light green with transparent whitish edging, the lower glume 1-veined and 1.5 to 3 mm long, the upper glume 3-veined and somewhat larger than the lower glume. Surrounding a floret is a pair of bracts (lemma and palea), the lemma 2.5 to 4 mm long, as long as or slightly longer than the upper glume, egg-shaped with transparent whitish edging, 5-veined, the lateral veins not reaching the tip of the lemma, long white hairs on the veins, hairless between them; the palea is shorter than the lemma, 2-veined with white hairs along the veins.

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of sheath, ligule, node and leaf tip] Leaves are alternate and basal, thin, flat or folded, floppy, up to 4 inches long, 1 to 4 mm wide, hairless, boat-shaped at the tip. The sheath is hairless and the edges are fused for about 1/3 their length (closed sheath). The ligule (membrane where the leaf blade joins the sheath) is up to 3 mm long and lacks a fringe of hairs. Nodes are smooth and elongated. Stems are multiple from the base, branched from near the base, erect to prostrate, sometimes rooting at a lower node. Plants form leafy clumps.

Fruit: Fruit type: seed without plume

[photo of glumes, lemma and palea] Individual florets drop away when mature, leaving the glumes behind persisting on the stalk. The grain (seed) is golden to brown, about 1 mm long.

Notes:

Annual Bluegrass is a cool-season grass introduced from Eurasia and is one of the most common weeds world-wide. It is a bane of turf grass lawns, gardens, and golf courses everywhere. As one lawncare guide put it, it sticks out from the usual Kentucky Bluegrass (Poa pratensis) and since it's an annual growing throughout the season, dying stems can leave unsightly brown patches, all of which ruin the aesthetic of a manicured lawn. Boo hoo. It is likely under-reported in the state.

Annual Bluegrass is recognized by its clump-forming growth, short stature, typically bright green foliage, florets that lack long, crinkly hairs around the base, lemmas with usually 5 veins that have long hairs at least on the lower half (hairless between the veins), and paleas with 2 hairy veins.

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More photos

Photos by K. Chayka and Peter M. Dziuk taken in the yard.

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