Agrostis gigantea (Redtop)

Plant Info
Also known as: Black Bent Grass
Genus:Agrostis
Family:Poaceae (Grass)
Life cycle:perennial
Origin:Eurasia
Status:
  • Weedy
Habitat:part shade, sun; average to moist disturbed soils; waste areas, roadsides, fields, railroads, along shores
Fruiting season:July - September
Plant height:1 to 4 feet
Wetland Indicator Status:GP: FACW MW: FACW NCNE: FACW
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: Cluster type: panicle

[photo of flowering spikes] Loose panicles, taller than wide, 3 to 10 inches long, egg-shaped to pyramidal in outline, the main branches whorled at the stem nodes and spreading to ascending. Spikelets (flower clusters) are mostly appressed near the tips of the main branches, though at least some branches have spikelets all the way to the branch base. Spikelets are typically purplish red, somewhat flattened, lance-shaped tapering to a pointed tip, with a single floret and a short, straight, rough-textured stalk.

[photo of cluster branches] At the base of a spikelet is a pair of bracts (glumes) that are lance-shaped tapering to a pointed tip, 1-veined, 1.7 to 3.2 mm long, the upper and lower glumes about equal. Surrounding the floret is a pair of bracts (lemma and palea), both oblong-elliptic, papery thin, translucent whitish and usually obscurely veined, the lemmas smaller than the glumes, 1.5 to 2.2 mm long, the palea about half as long as the lemma. Lemmas are usually awnless, occasionally with an awn up to 1.5 mm long.

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

[photo of stem leaves] Leaves are alternate, 2 to 8 inches long, up to 1/3 inch (2 to 8mm) wide, flat, hairless and mostly smooth on both surfaces.

[photo of sheath, ligule and node] Sheaths are mostly smooth, the edges overlapping near the tip or sometimes more widely separated, forming a long “V” at the front. The ligule (membrane where the leaf joins the sheath) is longer than wide, the top edge rounded to straight across and slightly jagged, the upper ligules up to ¼ inch (7 mm) long, the lower ligules shorter. Nodes are smooth and typically reddish. Stems are smooth, multiple from the base forming loose clumps, unbranched, sometimes prostrate at the base and sharply angled up from the lower node (genticulate), erect to ascending above, may root at the lower nodes, and typically forms colonies from long rhizomes.

Fruit: Fruit type: seed without plume

[photo of maturing spikelets] Florets drop away when mature, leaving the glumes behind persisting on the stalk. The grain is elliptic, 1 to 1.5 mm long.

Notes:

Redtop was introduced as an agricultural crop, escaped cultivation, and is now a common weed of roadsides, farm fields and other disturbed soils. While the distribution map indicates it is absent from several counties in Minnesota, it is likely another under-reported weed and is present statewide. The typically purplish-red panicle with ascending to spreading branches, spikelets with a single floret, the long ligule, and colonies formed by rhizomes (underground stems) are a pretty distinctive combination. In shadier habitats it is less strongly red in color and can be mistaken for the related Creeping Bent Grass (Agrostis stolonifera), which is a smaller plant, has panicle branches that are only spreading during anthesis and become ascending to appressed after pollination, has narrower leaves, and its colonies are formed by stolons (above ground runners), not rhizomes.

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More photos

Photos by K. Chayka taken in Ramsey County. Photos courtesy Peter M. Dziuk taken in Anoka, Kittson and Marshall counties.

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