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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Poa pratensis L.
Kentucky bluegrass (Poaceae)


Current level of impact
Known locations in RMNP: Widespread throughout RMNP. Kentucky bluegrass is found mostly in areas where it was planted such as old golf course in Moraine Park, old ski area, and housing areas on both side of Park.
Assessment: Several widespread and dense populations. When added together, all populations would cover an estimated area greater than 50 hectares. Has potential to inhibit secondary succession, and modify native communities.

Distribution
Origin: Introduced from Europe, possibly native of North America.
Geographic distribution: Widespread throughout North America.
Ecological distribution: Mesic to moist sites, in gardens, pastures, marshes, along roadways and waterways. In greasewood, grass-sagebrush, pinyon pine, oak-maple, aspen, ponderosa pine, lodgepole pine, aspen-spruce fir, and meadow communities. Adapted to a wide range of environmental conditions, and is somewhat flood tolerant.
Soils: Adapted to most soils types, common on fine textured soils. Tolerates pH range from 4.5-8.5, and does best at a pH of 6.5. Has somewhat high nutrient requirements (requires at least 6 ppm P and 1500 ppm N).

Reproduction
Perennial, cool season grass, reproduces by seeds and rhizomes. Plants can spread rapidly by rhizomes.
Seed production: Produces 100-200 seeds/panicle with as high as 4000 panicles/square meter.
Seed longevity: Seeds remain viable for only 2 years.
Germination: Generally germinates in fall. Cold, moist pre-chilling (5-15 C for 10-14 days) is necessary to produce germination in young seeds. Alternating temperatures can also be used to induce germination.

Competition
Increases rapidly in over-grazed pastures. In natural areas, Kentucky bluegrass can compete with native species, and can reduce species diversity. Kentucky bluegrass is believed to compete directly with cool season native grasses.
Level of impact: Considered a serious weed problem in native tallgrass prairie.
Response to shade: Moderately shade tolerant, does best in open areas.

Control
Seeds remain viable for only about two years, but plants can spread rapidly through vegetative means. The best option for natural areas may be to promote growth of native warm season grasses which compete with Kentucky bluegrass. Control of Kentucky bluegrass in RMNP may not be a realistic or feasible option because it is well established and widespread.
Mechanical: Mowing is an ineffective method of control, and may only increase tiller production and rhizome elongation. Mowing in mid-summer may also be detrimental to native grasses.
Biological: Research into biological control agents for bluegrass has been limited because of its wide use for lawns.
Chemical: Because Kentucky bluegrass grows intermixed with other species, chemical control is an unlikely option for natural areas. Spot treatments are difficult due to the plant's growth form, and may also expose native plants to herbicides. Roundup is the only herbicide specifically licensed for controlling Kentucky bluegrass. Roundup applied at 0.38 kg/ha provided complete control of 2 month old Kentucky bluegrass. 98 % of Kentucky bluegrass was controlled using late spring applications of Roundup at 1.12 kg/ha. Arsenal (imazapyr) and Oust (sulfometuron methyl) have also been used to control Kentucky bluegrass.
Other: Fire is the most commonly used method to control bluegrass in some areas. Most studies show that Kentucky bluegrass is reduced with late spring burns. However, prescribed burning strategies will vary according to the mix of warm and cool season grasses in the target area, the specific site characteristics, and the frequency and timing of treatments.

Notes
Extensively used for pasture, recreational turf, and erosion control through most of northern U.S. As a forage it is highly palatable to deer, and nutritious to all classes of livestock and elk. Although it can withstand continued heavy grazing better than almost any other grass, it is not drought tolerant and is therefore, satisfactory for range seeding only in mountain regions.

References

Anderson, K.L., E.F. Smith, and C.E. Owensby. 1970. Burning bluestem range. Journal 
    of Range Management 23:81 -92.

Bingham, S.W., J. Segura, and C.L. Of ay. 1980. Susceptibility of several grasses 
    to glyphosate. Weed Science 28(S):579-585.

Curtis, J.T. and M.L. Partch. 1948. Effect of fire on the competition between 
    bluegrass and certain prairie plants. American Midland Naturalist. 39:437-
    443.

Sather, N. 1988. Element stewardship abstract for Poa pratensis- Kentucky blue-
    grass and Poa compressa- Canada bluegrass. The Nature Conservancy, Minneapolis.

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