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Species Abstracts of Highly Disruptive Exotic Plants

at Scotts Bluff National Monument

Poa pratensis

Park: Scotts Bluff National Monument (SCBL)
Species: Poa pratensis L. syn. P. agassizensis Boivin & Love
Common Name: Kentucky bluegrass, Junegrass
Urgency Ranking: Medium
Overall Ranking: 5
Significance of Impact: 55
A. Current impact: 23
B. Ability to be a pest: 32
Feasibility of Control: 22

Taxonomic Description:
Kentucky bluegrass is a strongly rhizomatous, mat-forming, perennial grass. Plants can range from 10 cm to 1 m in height. Culms are slender and wiry, slightly flattened, and erect to decumbent at the base. Leaf blades are flat to folded (1-37 cm long and 1-6 mm wide) and glabrous with a double mid-rib (resembling railroad tracks) and a prow-shaped leaf tip. Ligules are membranous and truncate (0.7-2.1 mm long). Sheaths are rounded to somewhat keeled, partially closed, and glabrous. The inflorescence is an open to slightly contracted, long-exerted panicle (3-13 cm long and 3-8 cm wide) and pyramidal in shape when open. All panicle branches are flexuous, and the lower branches occur in a whorl. Spikelets are three- to six-flowered (3-6 mm long and wide) and laterally compressed. Glumes are subequal in length (2-3.5 mm long), acute, keeled, and scabrous on the keels. Lemmas are keeled (2.5-4 mm long) and acute or obtuse. Cobweb-like hairs occur at the base of the lemmas. Over 100 Kentucky bluegrass cultivars have been developed, resulting in a highly variable species.

Canada bluegrass, a similar species found in the park, is distinguishable from Kentucky bluegrass vegetatively by its obviously flattened culm, compressed keel leaf sheaths, and more blue-green appearance. Canada bluegrass has a narrower inflorescence with the lower branches in groups of one to four (two is the most common) and fewer cobweb-like hairs at the lemma base. In all other respects, Canada bluegrass closely resembles Kentucky bluegrass.

Kentucky bluegrass is native to Europe, Eurasia, and possibly North America. Some scientists theorize that Kentucky bluegrass may have migrated across the Bering Land Bridge 12,000 to 20,000 years ago. It was probably introduced to the east coast by early colonial settlers sometime after 1600 and spread westward by settlers. This species is now considered to be naturalized. Kentucky bluegrass is found throughout Canada, in each of the 50 states of the United States, and in Mexico.

Kentucky bluegrass is adapted to a wide range of habitat conditions but favors moist habitats and can withstand temporary flooding. It is found in meadows, open woodlands, prairies, and disturbed sites. It is commonly planted as a lawn grass and a pasture grass in some areas. Kentucky bluegrass is well adapted to most soil types, although it is most common on fine-textured soils. It grows best at a pH of about 6.5, but will tolerate a pH range of 4.5-8.5. Kentucky bluegrass has a relatively high nutrient requirement, growing best on sites containing at least 6 ppm P and 1500 ppm N. It can tolerate a temperature range of 43 to -44°C, with an optimal growth occurring between 16 and 32°C. Precipitation ranges of 500 to 1250 mm produce optimal growth. Kentucky bluegrass may become dormant during extended periods of inadequate soil moisture. It is somewhat shade tolerant, but produces more leaves in full sunlight.

Generally, Kentucky bluegrass germinates in the fall. Studies have shown that it is necessary to expose fresh seed to a moist chilling of 5 to 15°C for 10 to 14 days before germination occurs. However, other studies indicate that the seed is no longer dormant 6 months after harvest. Alternating temperatures may be more efficient than continuous temperatures in breaking dormancy. Vivipary has been reported for Kentucky bluegrass, but it is extremely uncommon. Seedlings rapidly establish and form a short tuft. Plants rapidly spread vegetatively by rhizomes and tillers forming a sod. Rhizomes represent important carbohydrate sinks for Kentucky bluegrass.

Rhizome buds on established Kentucky bluegrass are most commonly initiated when air temperatures reach 15 to 23°C, corresponding with the summer and fall when the inflorescence begins to elongate. Most rhizomes develop just below the soil surface. Those developing at and slightly above the soil surface turn downwards. Rhizome elongation is dependent on environmental conditions. Short rhizomes are produced under low moisture conditions, resulting in numerous, closely spaced plants. Under adequate moisture conditions, internodes will be longer, resulting in more widely scattered new shoots. One plant can spread to occupy and area 2 m2 in 2 years.

Tillering is induced by photoperiod and temperature and modified by soil moisture, nutrients, and defoliation. Production of tillers of Kentucky bluegrass is enhanced by relatively cool temperatures and short photoperiods. Thus, most tillering takes place in the spring and fall. Some tillers produce inflorescences, while others remain vegetative. Vegetative tillers eventually form true roots, allowing them to be independent of the parent plant. Root development of Kentucky bluegrass generally begins in March, with maximum root elongation occurring in April and May. Elongation continues until early summer or when mean air temperature reaches 27°C.

Kentucky bluegrass flowers between April and August. Each panicle of Kentucky bluegrass produces between 100 and 200 seeds. One study showed that it could produce as many as 4000 panicles/mm2 and as much as 900 kg/ha of seed. Few studies report on the seed bank characteristics. A study in the Netherlands showed a maximum seed bank of 560 seeds/mm2. Other research determined that in more than one-half of the seeds in the shallow portion of the soil and over three-quarters of the deeply buried seeds would germinate within the first 4 years.

Kentucky bluegrass is primarily used as a lawn grass and as a forage species. It can not only tolerate successive, intensive defoliation, but it may flourish under these conditions. This ability to survive intensive, repeated defoliation is related to how the tillers elongate. First, the leaf blade elongates and is quickly followed by elongation of the sheath and then the internode. The shorter and later internode elongation allows the meristematic tissue to be retained low and near the soil surface while the leaves and sheaths grow well above this level. Thus, defoliation by mowing or grazing removes the leaves and not the meristem. The tiller may be defoliated, but it will continue to grow and produce foliage. Timing of defoliation does not appear to affect the emergence of other tillers, although the number of leaves may be reduced. Some studies indicate that defoliation increases tiller numbers.

An intermediate number of populations of Kentucky bluegrass occur at Scotts Bluff National Monument (SCBL) combining to cover an area of 5 to 10 hectares. These plants are found in mid-successional sites that have been disturbed between 11 and 50 years ago. Kentucky bluegrass is found scattered throughout the park. It has the potential to invade and modify existing native communities and poses an endangerment to the prairie vegetation. It has a minor negative visual impact on the vegetation of SCBL.

A limited number of control options exist for Kentucky bluegrass. Control methods for this species have been primarily chemical and cultural. An important consideration in controlling these species is that the seeds have the potential to remain viable in the seed bank for only about 2 years, however, plants readily spread by tillers and rhizomes. A further consideration is that there are many sources of propagules surrounding PIPE. An important consideration prior to using any control method is to determine if enough plants of desirable species are present to replace the eradicated bluegrass. If desired vegetation is scarce or absent control will be of little value.

Fire may either control or stimulate Kentucky bluegrass. The response is dependent on timing of the prescribed burn. Initial species composition must also be considered. It is desirable to switch the competitive advantage to the native warm-season species. Most studies have shown that Kentucky bluegrass is reduced by late spring burns. Studies of annual prescribed burns indicate that spring burning can result in as much as a 96% reduction in number of Kentucky bluegrass plants. A 6-year study showed a 36% reduction in Kentucky bluegrass reproductive culms. A 10-year study showed repeated mid-April prescribed burns reduced Kentucky bluegrass 95% in comparison to an unburned area. Late spring burns, conducted in early May, reduced Kentucky bluegrass by 96%. Late spring prescribed burns in another study resulted in a reduction of 84% of Kentucky bluegrass reproductive culms. An October prescribed burn did not reduce the reproductive culms of Kentucky bluegrass. Need for repeated annual burning is often necessary. A study on the effect of number of burns on Kentucky bluegrass showed annual reductions of 26, 13, and 28% in the total Kentucky bluegrass composition following fires in 3 successive years. It is apparent that environmental variables such as time of rainfall (cool-season versus warm-season) and temperatures also influence results.

Mowing will not control Kentucky bluegrass. Mowing may result in stimulating tillering and rhizome elongation. Tillage, other than clean tillage for several years, is not effective. Kentucky bluegrass increases under most grazing practices. However, intensive early spring grazing on primarily warm-season prairie will help reduce the competitive ability of Kentucky bluegrass. Few chemical control options exist for Kentucky bluegrass. Many herbicides are not specific to Kentucky bluegrass or may not be specifically licensed for this particular use. It is important to read and follow all herbicide label directions. Most herbicide studies have evaluated Roundup (glyphosate) and AAtrex (atrazine). Of these two, only Roundup (glyphosate) is currently labeled for use on Kentucky bluegrass, and Roundup (glyphosate) gives excellent control or Kentucky bluegrass. Arsenal (imazapyr) and Oust (sulfometuron methyl) have been shown to control Kentucky bluegrass.

Research into biological control has received little attention because of the extensive use of Kentucky bluegrass in lawns. Kentucky bluegrass, has numerous pests that may hold some potential as biological control agents. As research is not being conducted in this area, only the names of these pests and their effects will be listed. The larvae of May beetles (Phyllophaga spp.) and green June beetles (Cotinis nitida L.) feed on the roots and rhizomes and may kill the plants if these "white grub" larva are abundant. Sod webworms (Crambus spp.) feed on the leaves but rarely kill the plants. Adult bluegrass billbugs (Stenophorus parvulus Gyllenhal) feed on the leaves and stems, but rarely cause extreme damage. The black grass bug (Labops hesperius Uhler) is a piercing and sucking insect that will damage but not kill Kentucky bluegrass. Other insects that affect this species are a leafminer (Phytomyza nigra Meigen), meadow plant bugs (Leptopterna dolobrata L. and Amblytylus nasutus Kirsch.) that feed on the florets and prevent seed formation, wireworms (Dimonius), cutworms (Agrotis spp.), gelechids (Gelechia spp.), mites (Pediculopsis spp.), grass thrips (Anaphothrips obscurus Mueller), and grass mealybugs (Pseudococcus spp.).

Kentucky bluegrass is affected by a number of diseases such as leaf spot (Bipolaris sorokiniana), melting out [Drechslera poae (Baudys) Shoemaker], summer patch (Magnaporthe poae), necrotic ring spot (Leptoshaeria korrae), and dollar spot (Lanzia and Moellerdiscus spp.). In addition to these diseases, pathogens such as stem rust (Puccinia graminis Pers.) attack Kentucky bluegrass. Whitetop or silvertop (Fusarium poae Pk.) is a fungus that will attack the florets in association with a mite (Pediculopsis graminum Reut.) and reduce seed production. Another species of fungus [Fusarium roseum (Lk.) Snyd. & Hans] will attack Kentucky bluegrass. All of these diseases and fungi are enhanced by excessive thatch levels.


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