Species: Poa 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.