Kentucky Bluegrass was introduced as a forage plant but most of us know it better as a lawn grass, a niche at which it is admirably suited. It is a cool-season, sod-forming perennial grass which will go dormant in hot weather. When left to go to seed, the seed stems are 18 to 24 inches tall, but will be much less (4 to 6 inches ) if grazed or are periodically cut. Stems are somewhat round to weakly compressed and non-branching.
Leaves: The stem blades are dark green, narrow, 1/8 to 1/4 inch wide (to 5 mm) and 6 to 12 inches long, parallel-sided, with a keel (boat-shape) at the tips. The underside is usually smooth but the upper surface may have sparse rough hairs. Middle stem leaves are the longest and the flag leaf blade is from 1/2 to 4 inches long. (The flag leaf is the one immediately under the seed head.)
Sheaths and ligules: The leaf sheath is split at the top but closed for 1/4 to 1/2 the length at the base, dark green, without hair and distinctly veined. Ligules are 0.9 to 2 mm long, and quite variable - either smooth or rough, truncate to rounded, with fine marginal hair or without.
Inflorescence: The flowering panicle has an open pyramidal shape, narrow to broad, 2 to 15 cm long, with 30 to 100+ spikelets and 2 to 7 branches per node. The branches become spreading with the spikelets crowded in the upper half of the branch.
The spikelets are 3.5 to 6 mm long, the length 3.5 times the width. They have 2 to 5 florets each. The glumes are unequal in size, usually distinctly shorter than the adjacent lemmas, distinctly keeled with the keel having sparse to dense fine rough hairs. The lower glume is 1.5 to 4 mm long, 1 to 3 veined and narrowly lanceolate. The upper glume is 2 to 4.5 mm long, shorter than or equaling the lowest lemma. The lemmas are 2 to 4.3 mm long, distinctly keeled, the keels and marginal veins with long fine hairs. Below the lemma keel appears a web of crimped hairs. There are 3 anthers.
Seeds are very small, 1 - 4 mm long, ellipsoidal in shape - about 2,177,000 seeds per pound.
Habitat: Kentucky Bluegrass grows from long, creeping rhizomes. Tiller buds at the base develop into stems or new rhizomes. As a forage grass, it is very palatable to horses, cattle and sheep and also to elk and deer (not to forget Canada Geese). In the wrong environment it is invasive and when used for pasture, difficult to get rid of. Most of the subspecies require full sun to do well and adequate moisture.
Names: The genus Poa is the Greek word for grass. The species name, pratensis, means 'of the meadows' referring to the common habitat. The author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: Kentucky Bluegrass is one of 32 species in the Poa section Poa. Over 60 cultivars have been developed for commercial use. There are six accepted subspecies of P. pratensis: subsp. alpigena, Alpine Bluegrass ; subsp. colpodea (these two are considered the only native subspecies and are found in the arctic); subsp. irrigata; subsp. agassizensis; subsp. angustifolia; and subsp. pratensis. The latter 3 are are recognized in Minnesota. Most of the commercial cultivars are derived from subsp. pratensis and that is the subspecies usually found in Minnesota. The DNR does not track county level locations of the other two. Details of the differences can be seen in the diagram presented below and are detailed in reference #W6. Canada Bluegrass, P. compressa similar but has a lighter blue-green foliage, has shorter and and tapering leaf blades, a longer ligule and a flat stem such that it cannot be rolled between thumb and fingers and a panicle with rough branches. It also matures later.
Above: Tall panicle of Bluegrass in development stage and mature stage. The main side branches of the panicle holding the spikelet branches are erect against the main panicle rachis until such time as the spikelets are ready to mature, then they spread horizontally as shown here.
Below: Complete plant specimen of Kentucky Bluegrass, ©Anna Gardner, Iowa State University. Drawing of Kentucky Bluegrass courtesy SDA-NRCS PLANTS Database / Hitchcock, A.S. (rev. A. Chase). 1950. Manual of the grasses of the United States. USDA Miscellaneous Publication No. 200. Washington, DC.
Below: 1st photo - The leaf sheath is split at the top on the front, the ligule on this exampe with smooth rounded tip. 2nd photo - The back side of the sheath. 3rd photo - the sheath of the flag leaf at the base of the panicle with several panicle branches rising.
Below: Detail of the spikelet of Kentucky Bluegrass. Photo ©Anna Gardner, University of Iowa
Below: 1st photo - the under side of the lef blade showing the slight center ridgeline. 2nd photo - the root system.
Below: Florets in bloom.
Below: Comparison drawing of the four non-arctic subspecies of Poa pratensis. Drawing by Sandy Long, ©Utah State University.
Kentucky Bluegrass is a perennial introduced from Europe about 1700 and now found throughout the United States and Canada. In Minnesota it is present in most of the counties in the State. It is one of 18 species of Poa found in Minnesota, 10 of which are considered native grasses. Likewise, Canada Bluegrass is an introduction with similar coverage.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"