Anthoxanthum odoratum (Sweet Vernal Grass)

Plant Info
Also known as:
Genus:Anthoxanthum
Family:Poaceae (Grass)
Life cycle:perennial
Origin:Europe
Status:
  • Weedy
Habitat:part shade, sun; moist to dry disturbed soil; roadsides, fields, pastures, woodland edges, waste places, shores
Fruiting season:June - July
Plant height:10 to 24 inches
Wetland Indicator Status:none
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: indistinct Cluster type: panicle Cluster type: spike

[photo of flowering panicle] Spike-like panicle at the tip of the stem, 1 to 5½ inches long, the short main branches mostly erect but usually ascending to somewhat spreading at flowering time. Spikelets (flower clusters) are single at branchlet tips, 6 to 10 mm (to 3/8 inch) long, slightly flattened, narrowly lance-shaped with a pointed tip, each with a single fertile floret flanked by a pair of sterile florets and mostly hidden by them.

[close-up of immature spikelets] At the base of a spikelet is a pair of bracts (glumes) that are thin, translucent around the edge, broadly egg-shaped with a pointed tip, keeled, often sparsely hairy, the lower glume 1-veined, 3 to 4 mm long, the upper glume 3-veined, to 10 mm long, as long as the spikelet, and completely wrapped around the florets until maturity. Surrounding a floret is a pair of bracts (lemma and palea). Sterile lemmas are 2.8 to 3.6 mm long, shorter than the lower glume, densely covered in long brown hairs, the lower lemma with a straight awn 2 to 4 mm long arising from about the middle of the back, the upper lemma awn 6 to 9 mm long, arising from near the base of the lemma and initially straight and barely exceeding the upper glume but elongating with maturity and becoming bent below the middle; sterile paleas are absent or obscure. Fertile lemmas are 1 to 2.5 mm long, shorter than the sterile lemmas, smooth, shiny, awnless, obscurely 5-veined, wrapping around the floret and mostly hiding the palea.

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

[photo of sheath, ligule, node and stem leaf] Leaves are mostly basal, flat, hairless to sparsely hairy on both surfaces, often fringed with a few long white hairs near the base. Lower and vegetative leaves are up to 12 inches long, to 10 mm (3/8 inch) wide, the 1 to 3 stem leaves shorter and narrower. Sheaths are hairless to sparsely hairy, often with a few long white hairs at the tip, and usually have a pair of small lobes (auricles) at the sheath apex. The ligule (membrane where the leaf joins the sheath) is 2 to 7 mm long and lacks a fringe of hairs. Nodes are smooth. Stems are smooth, mostly erect, unbranched or few branched, and multiple from the base forming loose to dense clumps.

Fruit: Fruit type: seed without plume

[photo of mature florets and grain] Fertile florets turn reddish brown and drop away with the attached sterile florets when mature, leaving the glumes behind persisting on the stalk. Grains (seeds) are oval-elliptic, about as long as the fertile lemma.

Notes:

Sweet Vernal Grass is a cool season grass introduced to North America from Europe and is more common on the east and west coasts than in the midwest. There are about 30 records of it in Wisconsin but it's only been reported twice in Minnesota, the first time in Ramsey County around 1900, and the second when we encountered it in 2015 in Pine County near Sandstone. As with many weeds, it is likely under-reported in the state. It tolerates a wide range of conditions but is most often found with other weeds in disturbed soils, such as roadsides, trail edges, weedy shores, and waste areas. Crushed leaves are said to smell like fresh cut hay, with a hint of vanilla.

Sweet Vernal Grass should not be confused with other grasses. It is clump-forming and has a compact, spike-like panicle with long, slender spikelets that have a pair of long-hairy, awned sterile florets flanking a single, smaller, smooth, shiny, fertile floret. It can be quite attractive when flowering, the panicle branches spreading out some with the long, feathery styles and stamens with long, pale anthers poking out of the spikelet tip. The longer lemma awn is barely noticeable at this time. As they mature, the spikelets become more appressed and the glumes open up and flatten out, revealing the florets inside, and the longer awn becoming bent at or just below the middle.

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More photos

Photos by Peter M. Dziuk taken in Pine County and in Michigan. Anthoxanthum odoratum sheath hairs by Stefan.lefnaer, via Wikimedia Commons, used under CC BY-SA 4.0

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