Poa pratensis
(smooth meadow-grass)

P. pratensis is a perennial, cool season mat-forming grass. It is native to Europe and Asia and has probably been introduced to the USA and Canada, where it is invasive in the northern prairie states, as well as in New Zealand, Indian Ocean islands and South Africa. The wide, creeping rootstock produces underground runners and leafy shoots. In the USA, where it is known as Kentucky bluegrass, it 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, but is considered an invasive weed in natural grassland ecosystems, where it outcompetes native species, reduces biodiversity and alters nitrogen cycling and ecosystem function. It is defined as ‘high risk’ for the Pacific region and invasive in Hawaii by PIER (2016).

P. pratensis is certainly native to Europe and Asia and probably some countries of northern Africa. It has been introduced to Mexico, South America, southern Africa, Atlantic islands, Antarctica, Australia, New Zealand and Pacific islands including Hawaii.

Its status in North America is not completely clear, with some authorities suggesting it is native across most of USA and Canada (Fernald, 1950; Canadensys, 2016) while others believe it is exotic (Hitchcock, 1950; ISSG, 2016). BONAP (2016) and USDA-NRCS (2016) take an intermediate view, suggesting that some forms of the plant may be native, at least in the northern states of the USA and Canada, while others (presumably those of commercial interest) have been introduced. Overall, it seems most probable that it is effectively introduced across North America. In any case it is most common in the north and much less common in southern states. Most introductions have been deliberate, for cultivation. In some cases it may continue to be restricted to cultivation, and not naturalised, but it has not been possible to make this distinction in the Table below.

P. pratensis is widespread throughout the UK and north-eastern USA in plant assemblages of pasture and arable land, roadsides, waste-ground, forest edges, dry hills, marshes, along seashores and walls. P. pratensis is used for hay, grazing and for park and sportsground turf (Clarke and Malte, 1913; Phillips, 1980).

P. pratensis has been reported to impact on, Triticum aestivum, Hordeum vulgare and Anemone patens.

Genetics

P. pratensis is very variable genetically, with chromosome numbers ranging from 2n=14 to 154, but the most common numbers are 2n = 28 or 56 (Missouri Botanic Garden, 2016). The genetic variability in P. pratensis has been studied by Raggi et al. (2015) where they found relatively little consistent variation between cultivated and wild types, but variation according to region of collection. The most distinct variation is shown by populations from China and Mongolia and it is suggested that this is a likely area of origin of the species.

Reproductive Biology

P. pratensis may hybridize but some forms produce seed apomictically, whereby the seed forms from cells contained within the ovary wall of the flowers (the progeny are identical to the parent plant) (Hubbard, 1959). Breeders have been able to make controlled crosses and in many cases the resulting crosses are still highly apomictic. Many early cultivars, including ‘Merion’, are simple selections of highly apomictic plants.

Physiology and Phenology

Seeds of P. pratensis germinate in autumn after a chilling period. In a study by Aamlid and Arntsen (1997), the optimum constant temperature for germination was 16°C but higher rates of germination were obtained under fluctuating temperatures. Light was beneficial at constant temperatures, but had no benefit, or was even inhibitory at alternating temperatures. Germination is delayed under dry soil conditions, more so than in related Poa species (Springer and Goldman, 2016). Seedlings establish forming a short tuft, rhizomes and tillers (North Dakota Department of Agriculture, 2005). It may take up to three years for plants to establish fully from seed but is then very long-lived.

Valuable detail on the phenology and ecology of P. pratensis are provided by Sather (2016). He records that the growth of each aerial shoot or underground rhizome is indeterminate until its shoot apex is triggered by environmental stimuli to initiate floral development. Each tiller or rhizome produces a single terminal aerial flowering stem. Floral initiation is induced by a period of vernalisation involving both an inductive developmental stage and a photoperiodic requirement before the inflorescence is initiated. Vernalisation is not transferred from one shoot to another, with the consequence that aerial shoots from either tillers or rhizomes formed in any given year (whether spring or fall) must overwinter before they will bloom. This requirement applies even though apical dominance is removed by removing the flowering culm. Flowering in the UK occurs from May to early July (Phillips, 1980). Each panicle (flower) is capable of producing between 100 and 200 seeds. Seeds can remain viable for up to two years.

Environmental Requirements

P. 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 to 32°C and it grows best in humid areas. It prefers high sunlight 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 (ISSG, 2016). Frost is tolerated but tolerance is reduced under increased nitrogen availability (Malyshev and Henry, 2012).

P. pratensis is affected by a very wide range of fungal and nematode enemies, but none are known to be very widespread or damaging. Of greatest economic significance perhaps are those which affect P. pratensis on golf courses and other amenities.

Among many insects P. pratensis is eaten by the caterpillars of the meadow brown (Maniola jurtina) and gatekeeper (Pyronia tithonus) butterflies. Adults of the common sun beetle (Amara aenea) feed on the developing seeds. The leafhopper Eupelix cuspidata and the grassbug Myrmus miriformis feed on young blades and developing seeds.

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Amenity
• Landscape improvement
• Wildlife habitat

P. pratensis is similar in appearance to a number of closely related species in the same genus. P. pratensis flowers from May to July, as opposed to P. annua, which is in flower for eight months of the year. P. pratensis has a fairly prominent mid-vein in the middle of the blade. The ligule of P. pratensis is extremely short and square-ended, in contrast to P. annua and P. trivialis, in which it is silvery and pointed. P. pratensis is a dark green compared to the apple green colour of P. annua and P. trivialis.P. angustifolia may also be confused with P. pratensis; it differs mainly in narrower, stiffer leaves, but in any case is regarded by some authorities as a subspecies of P. pratensis. P. compressa, often occurring with P. pratensis in Europe and in the USA, is distinct in having flattened stems.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/Mechanical Control

Mowing and raking are generally ineffective against P. pratensis and can actually stimulate rhizome and tiller production. Williams and Crone (2006) found that mowing and raking decreased Bromus inermis (smooth brome), a similar invasive grass, but increased P. pratensis. Common varieties of P. pratensis however, perform well when lightly grazed or mowed at 5 cm or higher, actually improving the stand. However, when intensively grazed, or mown below 5 cm, diseases can then develop which generally destroy the stand. The dwarf or compact turf varieties require mowing below 2 cm; so mowing is therefore not recommended for control unless there is disease already present in the stand below cutting level (North Dakota Department of Agriculture, 2005; Hendrickson and Lund, 2010). For eradication complete turf removal down to 12-25 cm is recommended.

Sather (2016) comments that the most widely used management procedure for controlling P. pratensis in natural areas is the use of fire. In Kansas and Nebraska, USA three successive years of annual spring burning are considered sufficient for conversion of rangeland from P. pratensis to dominance by native warm season grasses. In the north it appears that much longer periods of annual burning are required. In central North Dakota, refuge managers suggest that even longer periods may be required and that P. pratensis may never be eliminated but only held in control by fire. The 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 critical to consider native species composition of the area prior to burning. Burning usually needs to be repeated annually for several years (North Dakota Department of Agriculture, 2005). USDA Forestry Service (2015) makes detailed comments on the factors influencing the effectiveness of fire in control of P. pratensis.

Chemical Control

In the USA, glyphosate is effective in reducing infestations of P. pratensis (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 its effect on native species (North Dakota Department of Agriculture, 2005), as these chemicals will likely also affect the native species that are desired. Sather (2016) records that atrazine, DMPA (‘Zytron’), DCPA (‘Dacthal’) and trifluoralin at suitable doses can provide some selective suppression of P. pratensis in a pasture situation.

In field experiments performed in the UK, Clay et al. (2006) found P. pratensis to be moderately resistant to resistant to all graminicide herbicides tested. It was only moderately susceptible to the broad-acting glyphosate in spring and autumn. Resistance was greater with autumn-applied herbicides than with spring applications. Established plants were more resistant than young plants. Bayer (2016) recommend chlortoluron, isoproturon, diflufenican and flufenacet alone or in mixtures for control of Poa species including P. pratensis in winter wheat and barley

IPM

A study in North Dakota, USA, found that burning followed by the application of a herbicide (imazapic) was more effective at reducing P. pratensis than burning or using a herbicide alone. Burning removed the heavy thatch layer associated with P. pratensis, which may have improved herbicide effectiveness. However, this study also found that while burn/herbicide treatment decreased P. pratensis, it caused an increase in smooth brome (Bromus inermis) (Hendrickson and Lund, 2010).

24/06/16 Updated by:

Chris Parker, Consultant, Bristol, UK

22/06/15 Original text by:

Philip Roberts, CABI, UK