Flower:

Open to compact branching cluster at the top of the stem, to 6 inches long, the branches spreading to ascending to erect at flowering time. Spikelets (flower clusters) are single at the tips of short branchlets, green to purple-tinged, with 2 to 7 florets, though florets are usually modified into elongated bulblets up to 20 mm (~¾ inch) long, sometimes with the lowest 1 or 2 florets in the spike normal and fertile.

At the base of a spikelet is a pair of bracts (glumes), both lance-elliptic with a pointed tip, keeled, usually smooth except rough along the keel, thin and whitish around edge, the lower glume 2 to 3 mm long and 1 to 3-veined, the upper glume about as long as or slightly longer than the lower glume and 3-veined. At the base of normal florets is a pair of bracts (lemma and palea), both much like upper glume, 3 to 4 mm long, sometimes hairy along the veins and keel. The thickened base of the floret (callus) sometimes has a few long, crinkled hairs, usually hidden by the glumes. Modified florets form elongated, leafy bracts around a bulblet, the bulblet's lemma and/or palea significantly enlarged.

Leaves and stems:

Leaves are mostly basal or on the lower stem, up to 4 inches long, 1 to 2.5 mm wide, hairless, folded or flat, and boat-shaped at the tip. Sheaths are hairless and the edges fused together (closed) on the lower fourth. The ligule (membrane where the leaf joins the sheath) is 2 to 4mm long, blunt to pointed at the tip and not fringed with hairs. Nodes are smooth.

Stems are erect or spreading from the base and rising at a lower node (geniculate), unbranched, hairless, multiple from the base forming clumps of mixed flowering and vegetative shoots. The base of the stem is thickened, forming a bulb like a scallion.

Fruit:

At maturity, bulblets are dark purplish at the base and green to purple on the elongated tip; florets and bulblets shed individually, leaving the glumes behind on the stalk. Grains (seeds) are seldom produced. The bulblets can take root as soon as they hit the ground.

Notes:

Bulbous Bluegrass was introduced from Europe as a forage grass to the Pacific Northwest, where it escaped and spread throughout the western US. It has only been recorded a few times in Minnesota but is very likely under-reported. We stumbled upon two of those previously undocumented populations ourselves, once at the State Fairgrounds in St. Paul, and once in the campgrounds at Old Mill State Park in Marshall County. From those two heavily trafficked sites it likely spread to many other locations but has probably gone unnoticed as just another weedy grass.

There are 2 subspecies of Poa bulbosa: subsp. bulbosa is not widespread, present in scattered locations from Illinois to Pennsylvania and does not produce bulblets; subsp. vivipara produces mostly bulblets and is the more common, widespread in the western third of the US with scattered populations eastward to the Atlantic coast. Once noticed, it is not likely to be confused with any other grass, as the bulblets are quite eye-catching. If they are absent, the bulbs at the base of the stem are diagnostic; the bulbs vaguely resemble some native Onion (Allium) species, which never have jointed stems or leaves with boat-shaped tips.