Kentucky bluegrass is actually a native grass of Europe but has become one of the most popular cool-season grasses for North American lawns. Thanks to selective breeding, more than 100 varieties of Kentucky bluegrass were developed in the latter half of the 20th century, many of which are more shade and drought tolerant than the original cultivar. Kentucky bluegrass reproduces two ways, through vegetative propagation and gametophytic apomixis.
Kentucky bluegrass (Poa pratensis) is a member of the angiosperm family of plants, a group known for its ability to produce flowers or fruits containing seeds. Unlike many angiosperms however, Kentucky bluegrass reproduces asexually in a process called gametophytic apomixis. It also reproduces vegetatively by means of rhizomes that are roots that grow near the soil surface and bear new plants at nodes and by stolons that are tillers that have grown along the soil surface and sprouted new crowns at nodes.
Alfred Gordon Etter pioneered the study of the Kentucky bluegrass plant in the Annals of the Missouri Botanical Garden. According to Etter, the bluegrass seedling begins life in spring as a rolled bundle of four or five leaves above a few roots. The base of the bundle, or nubbin, grows into a crown from which more bundles rise next to the first. As the crown expands it forms nodes or thick during early summer. The space between nodes, called the internode, increases as the plant grows. New plants grow at nodes close enough to the surface, creating a nursery of baby plants around the parent.
By fall, the rhizomes have formed a colony of new plants. Tillers rise from leaf axils and droop to the ground. As growth nodes of tillers touch the ground, new roots grow and form new nubbins that become crowns for new plants. The tillers anchor into the ground to form stolons that mirror the underground network of rhizomes. By late fall, rhizomes and stolons create the dense mat of grass plants that make Kentucky bluegrass a successful turf grass.
In late fall, the plant flowers and by the next spring, seed panicles appear, giving the plant its blue-green color. In gametophytic apomixes, seeds form without undergoing meiosis, the cell division that relies on fertilization; seed embryos form directly from eggs. Sven Asker, a researcher at the Institute of Genetics at the University of Lund, Sweden, suggests that apomictic reproduction may be the result of adaptation and recessive genes. Bluegrasses and some other apomictic propagators appear to come closer to reproducing sexually as mutations are manipulated by hybridizers.
Meiosis or apomixis--the result is the same--is when a seed contains a plant embryo. Seeds sown in fall germinate in warm soil and radicles reach down to form roots as plumules grow up to form leaf bundles. The seeds produced by gametophytic apomixes, however, will produce Kentucky bluegrass plants identical to the parent, just like those produced vegetatively by rhizomes and stolons.