Species: Poa pratensis ssp. pratensis

On any given parent plant the number of axillary buds is dependent on the number of leaves. Etter (1951) reports that in dry, shady localities, Kentucky bluegrass may develop as few as 7 to 9 leaves, 12 to 14 in a meadow, and as many as 18 in a moist pasture. Each axillary bud has the developmental possibility of early tiller formation or delayed rhizome development. <br><br>In rangeland terminology a "tiller" often refers to any aerial shoot. In the strict parlance of developmental morphology a "tiller" is an aerial shoot that develops in the axillary bud of live leaf tissue (Etter 1951, Dahl and Hyder 1977). These new shoots develop root systems of their own and are hence a method of vegetative reproduction. Because of their origin, tillers always develop in close proximity to the parent plant. Young tillers derive their nutrition from the parent plant until they have attained full growth (Dahl and Hyder 1977). Although tillers develop true root systems of their own, these systems are not extensive (Etter 1951) and mature tillers neither translocate nor receive carbohydrates to and from other shoots derived from the same parent plant. There is a high mortality of tillers during the season of formation (Etter 1951). <br><br>Tillering is favored by cool temperatures (Etter 1951, Darrow 1939) and short day lengths (Evans and Watkins 1939, Evans 1927) and reaches its maximum in spring and fall, but declines in midsummer. Tillering is induced by early removal of developing flowering culms while the floral initiates are still enclosed within the sheath (booth stage) (Dahl and Hyder 1977). Etter (1951) reports that tillering is correlated with short leaves and, in a separate context, that Poa pratensis is more likely to develop long leaves in the shade. Well-lit situations that foster an abundance of shorter leaves may therefore enhance tillering. Etter (1951) reports that tillering is encouraged by April mowing to prevent flowering, fall grazing, fall nitrogen fertilization, fall irrigation and removal of dead plant material and shading. <br><br>Both Poa pratensis and Poa compressa are rhizomatous perennial grasses. Some North American fields of Poa pratensis are known to be as old as 60 years (USDA 1948). Volland (1978) attributes the inter-seasonal longevity of bluegrass to the activity of the rhizomes. <br><br>Axillary buds that have not formed tillers can develop into rhizomes, lateral shoots that penetrate the enveloping leaf sheath and develop underground (Dahl and Hyder 1977, Etter 1951). Rhizomes can form on the surface of the soil (Evans and Ely 1935, Etter 1951), but soon turn downward. Rhizomes account for bluegrass's sod-forming capability and can extend the horizontal growth of the plant as much as 2 square meters in 2 years (Kannenberg and Wrede 1934). The mode of elongation is the same as for aboveground shoots. The length of the rhizome is a function of the degree of internode elongation. Under conditions of drought or on excessively drained soils, short internodes (and hence short rhizomes) are produced. Short sprout-like rhizomes appear to increase under adverse conditions such as high temperatures (Harrison 1934) fire injury, or too close grazing and mowing (Etter 1951). <br><br>Rhizome formation and growth occurs throughout the year except late winter and early spring. Initiation of new rhizomes from axillary buds that have remained dormant overwinter invariably occurs when the inflorescence begins to elongate (Etter 1951). Brown (1939) found that rhizome elongation peaks between 60 and 70 degrees F. Evans and Ely (1935) report a midsummer peak of rhizome formation in Ohio. Summer- formed rhizomes can remain dormant until the following spring or develop into aerial shoots anytime during the growing season. <br><br>Rhizomes constitute a major sink for storage of carbohydrates in Poa pratensis. Brown (1943) reports that late autumn is the most favorable period for carbohydrate storage in Kentucky bluegrass. Carbohydrates are also synthesized more rapidly than they are used in early spring. During late spring-early summer there is a loss of carbohydrates from roots. Plants that remain uncut or are watered continue to lose carbohydrates from their roots. Plants that are mowed semimonthly or go unwatered do not experience such a loss (Brown 1943). <br><br>The major consequence of a high ratio of rhizomes to tillers is the formation of a dense sod. As the sod becomes increasingly tighter, fewer aerial shoots are formed (Volland 1978). In northern mixed prairie region this "sod-bound" condition can develop to such an extent that "bluegrass slicks" are formed, in which the thatch is so thick and the sod so tight that the bluegrass eventually dies (Steuter pers. comm., Kruse pers. comm.). In southern mixed prairie Weaver and Rowland (1952) observed an absence of Kentucky bluegrass in heavily mulched prairie, but in the Pacific Northwest Volland (pers. comm.) reports that although thinning occurs, bluegrass pastures appear to reach an equilibrium condition and do not choke themselves out. <br><br>Because the number of buds available for rhizome formation is dependent on the number previously used to form tillers, it seems logical that rhizome development would be favored by conditions the converse of those favoring tillers: allowing flowering to occur, protection from grazing and mowing, mulch build-up and shading. <br><br>Conditions that contribute to the death of rhizomes include overfertilization with nitrogen (Etter 1951), repeated close mowing (McKell et al. 1969) especially at temperatures in excess of 100 degrees F (Harrison 1934), overgrazing during drought at temperatures in excess of 100 degrees F (Wilkins 1935) and summer irrigation (Brown 1943) which accelerates decomposition of old roots and causes the plant to draw down the carbohydrate reserves at unnatural rates (McKell et al. 1969). <br><br>Evans and Ely (1935) studied rhizome development in both Poa pratensis and Poa compressa and provide the following contrasts between the two species: Rhizomes of Poa compressa originate almost entirely below the surface of the soil whereas those of Poa pratensis often originate above ground and turn downward. Peak rhizome formation in Poa pratensis was observed in midsummer, in Poa compressa in late summer and early fall. Nearly all the spring-formed rhizomes of Poa compressa had formed aerial shoots by August, whereas only half of early-formed Poa pratensis rhizomes did so (Evans and Ely 1935). <br><br>Both rhizomes and tillers are morphological shoots. Both form true roots. Roots of Poa pratensis begin to grow in March in the northeast (Stuckey 1941) at air and soil temperatures below 45 degrees F (Sprague 1933). Maximum root elongation is in April (Stuckey 1941). Unlike rhizomes, roots cease formation and elongation in midsummer (Stuckey 1941, Sprague 1933) at temperatures above 80 degrees F (Brown 1939). Darrow (1939) found that roots incubated at 15 to 25 degrees C were whiter, more succulent and twice as long as those incubated at 35 degree C, perhaps helping to explain the apparently greater competitive advantage and persistence of Poa in cooler climates. <br><br>Kentucky bluegrass is well known for its ability to withstand and apparently thrive on successive defoliations. This ability contributes to its usefulness as a forage plant and lawn grass, but detracts from the usefulness of defoliation as a method of control. The biological basis of this characteristic lies in the development of the leaves. As the first leaf arises from the crown the blade elongates followed in sequence by the sheath, and the internode (Etter 1951, Dahl and Hyder 1977). In some grasses the internodes elongate considerably, carrying the shoot apex ever upward on the plant, above the level of previously formed leaves. But in Kentucky bluegrass the greatest growth occurs in the blade and sheaths, each successive blade growing upward apace with the elongation of the previously formed sheath. Internode elongation is minimal, leaving the shoot apex nearer the ground, below the level of the previously formed leaves. As long as defoliation is above the level of this apex, growth will continue on that particular shoot. This growth habit allows for more rapid recovery than on long-shoot plants whose recovery is based solely on tiller formation in response to the loss of apical dominance at the time of defoliation (Dahl and Hyder 1977). <br><br>McKell et al. (1969) report that defoliation of Poa pratensis at the time of full leaf expansion does not affect the leaf emergence rates of individual shoots, but the total number of leaves on the plant is reduced. Such periodic defoliation can actually have a positive effect on the plant because evapotranspiration is reduced with a concomitant reduction in the amount of carbohydrates withdrawn from storage in the rhizomes (Brown 1943). <br><br>Poa pratensis is a cool-season grass, greening up in early spring and coming into bloom in early summer (Nieland & Curtis 1956, Stuckey 1941). Evans and Watkins (1939) relate the more upright growth form and greater rhizome development of both Poa pratensis and Poa compressa in the early part of the season to the longer day lengths of early summer. In one Rhode Island study (Stuckey 1941) blossom primordia formed in May, Poa pratensis was in full bloom in June, with Poa compressa lagging somewhat behind in flower development, but bearing seed at the same time. In greenhouse studies in Wisconsin, Sprague and Graber (1938) observed increasing water use from spring into early summer by both unclipped plants and plants clipped at weekly intervals. As would be expected, unclipped plants used more water until they were clipped on June 13. Water use resumed to the pre-clipping level within two weeks. In a study of container-grown plants in Michigan designed to assess the competitiveness of groundcovers for apple orchards, Partridge (1941) found the highest water use by both Poa pratensis and Poa compressa from June 23 to July 18, but that Poa pratensis plants used an average of 2.6 inches more water than those of Poa compressa over the growing season. <br><br>Etter (1951) reports 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 (Etter 1951). Floral initiation is induced by a period of vernalization involving both an inductive developmental stage and a photoperiodic requirement (White pers. comm.) before the inflorescence is initiated (Dahl and Hyder 1977). Vernalization 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 (Dahl and Hyder 1977, White pers. comm.). This requirement applies even though apical dominance is removed by removing the flowering culm. <br><br>Poa pratensis is generally considered an apomictic species, with reports of the proportion of "aberrants" (sexually reproducing individuals) in given populations varying from 0 to 100%, but usually less than 20% (Smith et al. 1946, Akerburg 1939, Brittingham 1943, Brown 1941). <br><br>In 4-year studies of the Newport cultivar of Kentucky bluegrass, Evans and Canode (1971) report in excess of 200 seeds per panicle in the first year. Maximum seed production of around 900 kg/ha was attained in the second year because of a great increase in the number of panicles per m2. By the fourth year seed production leveled off around 4000 panicles/m2 and around 100 seeds per panicle. Despite the high seed production, production of new plants from seeds in an established prairie is thought to be virtually nonexistent (Steuter pers. comm.). The only available data on the numbers of Poa seeds in soils are those of Van Altena and Minderhoud (1972) who report a maximum of 560 seeds/m2 for Poa pratensis in soil samples from Netherlands pastures. The results of the Duval buried seed experiment indicate that Poa pratensis can germinate from depths as great as 42 inches, with over half the shallow seeds and over three-quarters of the deep seeds germinating within the first four years after burial (Toole 1946). <br><br>Poa pratensis is a fall germinating species. In laboratory studies of the germination requirements of Poa pratensis, Sprague (1933) found that freshly harvested seed required a cold treatment at 5 or 15 degrees C for 10-14 days for germination. Six months after harvest, such a period of cooling was not required. Alternating temperature was more effective in breaking initial dormancy than constant cool temperatures. The response of Poa compressa was more variable, but chilling the moistened seed at 10 degrees C for 10 days before subjection to alternating temperatures increased germination (Sprague 1933). In a study of species common to reservoir shores, Hoffman et al. (1980) found that Poa pratensis was among the species that germinated 10% or more in autumn and germinated better in water than on dry or moist filter paper. <br><br>Under unusual circumstances Poa pratensis has been reported as a viviparous plant (Beetle 1980). However, production of shoots directly in the spikelet of the inflorescence is not an effective means of reproduction in the wild (Aiken and Darbyshire 1984).