Interim profile, incomplete information
Taxonomic name: Poa pratensis L.
Synonyms: Poa alpigena, Poa angustifolia L.
Common names: bird grass, blue grass, capim-do-campo (Portuguese), common meadow grass, English grass, grama de prados (Spanish), green grass, June grass, Kentucky bluegrass, narrow-leaf meadow grass, northern meadow grass, pâturin des prés (French), poa común (Spanish), schmalblättriges Rispengras (German), smooth meadow grass, smooth-stalked meadow grass, spear grass, spreading bluegrass, Wiesenrispengras (German), zacate poa (Spanish)
Organism type: grass
Kentucky bluegrass (Poa pratensis) is a rhizomatous grass that is regarded as a crop in some systems, but as a weed in others. It is highly valued as a pasture and turf grass, particularly in golf courses. However, it is considered an invasive weed in natural grassland ecosystems where it outcompetes native species and reduces biodiversity and alters nitrogen cycling and ecosystem function.
Poa pratensis is a perennial, cool season mat-forming grass. The wide, creeping rootstock produces underground runners and leafy shoots. Runners creep under the surface of the ground and produce upright leafy stems from their ends. The stems are from 15cm to 1m in height, and eventually develop into flower-bearing stems (Clarke & Malte, 1913). Seedhead panicles have an open-pyramid shape and produce numerous small seeds (Bush, 2002). Each branch of the panicle carries several spikelets which are bluish green to purple. Each spikelet has four to five flowers enclosed within two glumes of equal size (Clarke & Malte, 1913). Leaves are smooth, soft and measure 1/8 to ¼ inch wide and 6 to 12 inches long with keeled tips (Bush, 2002). Kentucky bluegrass can be distinguished from other grasses by its narrow leaf blade which is V-shaped in cross section and boat-shaped tip.
agricultural areas, range/grasslands, ruderal/disturbed, scrub/shrublands
Kentucky bluegrass can grow in many habitat types including meadows, roadsides, forest edges, dry hills, marshes and along seashores (Clarke & Malte, 1913).
Sod forming grasses such as Poa pratensis are known to have negative effects on native prairie species in the northern Great Plains of the United States (Larson & Larson, 2010 and references therein). It outcompetes native plant species and reduces biodiversity in invaded areas (North Dakota Department of Agriculture, 2005). For example the vital rates and population growth rate of a native grass, Anemone patens was greatly reduced when growing among P. pratensis and other invasive grasses (Williams & Crone, 2006).
Litter produced by Kentucky bluegrass forms thick mats of “slicks” on the soil surface (Hendrickson & Lund, 2010). Litter from Kentucky bluegrass is known to inhibit seedling establishment of other plant species (Bosy & Reader, 1995).
Kentucky bluegrass has been found to disrupt ecosystem function by altering nitrogen cycling and carbon storage, lowering plant diversity and shifting seasonal forage production. These impacts affect livestock production, wildlife habitat and ecosystem services (Hendrickson & Lund, 2010 and references therein).
Four subspecies have been identified: Poa pratensis alpigena, P. pratensis colpodea, P. pratensis irrigata and P. pratensis pratensis (ITIS 2008). There are also numerous cultivars.
Native Range: Europe, northern Africa, Siberia and North America, Australia, southern parts of South America (Clarke & Malte, 1913).
Introduced Range: Antarctica, Canada, Falkland Islands, French Southern Territories, Mexico, New Zealand, Saint Helena, South Africa, South Georgia and South Sandwich Islands, United States.
Introduction pathways to new locations
Seed contaminant: Seed contaminant.
Local dispersal methods
Water currents: Seeds were collected from irrigation water in Washington, USA and were found to have high germination rates (Kelley & Bruns, 1975).
Controlled areas should be closely monitored because seeds of Kentucky bluegrass can remain viable for two years and the plant can continue to spread by tillers and rhizomes.
Physical: Mowing and raking are generally ineffective against Kentucky bluegrass, and can actually stimulate rhizome and tiller production. Therefore mowing is not recommended (Hendrickson & Lund, 2010; North Dakota Deparment of Agriculture, 2005).
Burning: Burning of Kentucky bluegrass can have variable results. Some studies have found Kentucky bluegrass to be negatively affected by fire (Hendrickson & Lund, 2010 and references therein). Timing of the prescribed burn, moisture and site conditions are important factors that may influence the response of the plant to burning. It is also important to consider native species composition of the area and what species will replace Kentucky bluegrass. Burning usually needs to be repeated annually for several years (North Dakota Department of Agriculture, 2005)
Chemical: Glycophosphate is effective in reducing infestations of Poa pratensis (Wisconsin Department on Natural Resources, 2004; North Dakota Department of Agriculture, 2005). Imazapyr and sulfometuron methyl can also provide control. However chemical control may not be the preferred method of control in some regions due to the effect on native species (North Dakota Department of Agriculture, 2005).
In field experiments performed in the UK, Clay et al. (2006) found Poa pratensis to be moderately resistant to resistant to all graminicide herbicides tested. It was only moderately susceptible to susceptible to the broad-acting Glycophosphate in spring and autumn. Resistance was greater with autumn-applied herbicides than with spring applications. Established plants were more resistant than young plants.
Integrated Management: A study carried out in North Dakota, USA found that burning followed by herbicide (Imazapic) was more effective at reducing Kentucky bluegrass than burning or herbicide alone. Burning removed the heavy thatch layer associated with Kentucky blue grass which may have improved herbicide effectiveness. However this study also found that while burn/herbicide treatment decreased Kentucky bluegrass it caused an increase in smooth brome (Bromus inermis), a similar invasive grass species (Hendrickson & Lund, 2010), a patterns also found in a study by Williams and Crone (2006). Similarly mowing/raking which decreased smooth brome caused an increase in Kentucky bluegrass. Community composition was also important in determining the effectiveness of a control technique (Hendrickson & Lund, 2010). This study highlights the importance of considering other species, community composition and length of time since treatments were applied (Hendrickson & Lund, 2010).
Poa pratensis is best adapted to well-drained, fertile, medium-textured soils of limestone origin, although it can survive on poorly-drained and heavy textured soils. It prefers soils of pH 6.0 to 7.5. Optimum temperatures for growth are between 15.5 to 32°C and it grows best in humid areas. It prefers high sunlight areas but can do well in light shade if moisture and nutrients are sufficient. It is usually dormant during dry or hot weather but can survive severe droughts.
Kentucky bluegrass is a shallow-rooted, perennial grass that reproduces both vegetatively and via seed. Once established plants expand mainly through the formation and growth of rhizomes and tillers, which are responsible for the sod-forming capability of the plant (North Dakota Department of Agriculture, 2005).
Each panicle (flower) is capable of producing between 100 to 200 seeds that can remain viable for up to two years. Seeds germinate in autumn after a chilling period. Seedlings establish forming a short tuft, rhizomes and tillers (North Dakota Department of Agriculture, 2005).
Poa pratensis becomes dormant in summer, but regains green colour in autumn. Growth starts in early spring with tiller buds developing into stems or rhizomes (Bush, 2002). It grows early in the season when most other species are still dormant, and can thus spread very quickly (Wisconsin Department of Natural Resources, 2004).
Compiled by: IUCN/SSC Invasive Species Specialist Group (ISSG) with support from the EU-funded South Atlantic Invasive Species project, coordinated by the Royal Society for the Protection of Birds (RSPB)
Updates with support from the Overseas Territories Environmental Programme (OTEP) project XOT603, a joint project with the Cayman Islands Government - Department of Environment
Last Modified: Monday, 23 August 2010