SportsTurf

36 SportsTurf | July 2016 www.sportsturfonline.com FIELD SCIENCE time, those 30 million seedlings will be reduced to some lower number, maybe thousands or maybe hundreds. In addition, the surviving plants will very often tend to look very different from one another due to their genetic uniqueness. This is one explanation of how and why putting greens tend to segregate over time into a network of patches. THE MAINTENANCE FACTOR However, the most important force for determining the level of competition and subsequent survival among these bentgrass seedlings is most likely the type and level of maintenance given by the turfgrass manager. Are these greens initially mowed at 1/8 inch or at 1/10 of an inch? Are these greens regularly cultivated, fertilized and irrigated, or are they kept lean and hard? These different management parameters have an extremely important impact on the final composition of the resulting bentgrass population because the genetically unique individuals will have different abilities to respond to different levels of management. For example, some individuals will tolerate lower mowing heights better than others, some will respond more to increased levels of nitrogen fertilizer than others, some will tolerate traffic more than others, etc. And thus, the original seedling population, which initially started at 30 million plants, will be steadily winnowed down to a much-reduced number of plants depending on the level and type of management received. Additionally, once a particular genotype has been eliminated from the population, it cannot be resurrected because turfgrasses like creeping bentgrass do not typically flower and set seed underneath our mowing heights. Thus, if management practices are abruptly changed, after say 10 years, to a different level of intensity, then the genotypes best adapted to the new management practices might have already been eliminated from the existing population. And once these individuals are gone, they are gone, never to come back again. It is at this point when my students typically ask, "But what about overseeding?" Well, in some cases overseeding can add or replace lost genetic variation, but in the case of creeping bentgrass, it is surprising to me how little genetic variation can actually be added through overseeding (see Sweeney, P. and K. Danneberger, Introducing A New Creeping Bentgrass Cultivar Through Interseeding: Does It Work? It sounds like a good idea, but there are drawbacks. USGA Greens Section Record Sept-Oct 1998). Thus, the point of all this is for superintendents to know and value their seedling population and to appreciate the fact that their management techniques will greatly influence the resulting plant population for a long, long time to come. This same principle described above also holds true for home lawns and landscape turf. However, for lawn and landscape turf, the seed numbers game is initially more important during the act of initial establishment. This is because most lawn and landscape turfs are established as a mixture of different species, the most common of which in the Northeast is a mixture of Kentucky bluegrass, perennial ryegrass and fine fescue. The important point here is that these three species have very different sizes of seed and that the available information on the seed label provides only the percentage of each type of seed by weight. Thus, when calculating out the final composition of species in the final product (the lawn itself), the Pure Seed component that is listed on the seed label as percent by weight of each species, in combination with PLS, must be converted to percent by number. This is because each pure live seed, regardless of its weight, gives rise to an individual seedling plant, and thus, the numbers are often very, very different. As can be seen in photo on page 35, the number of Pure Live Seed per pound of each species is very different than the amount of Pure Seed that is listed on the seed label as a percent by weight. For example, as a percent by weight, Kentucky bluegrass only registers 10% of the above bag of seed. Moreover, when the percent germination is included into the calculation in order to determine PLS, Kentucky bluegrass is only 7.5% of the seed mix. However, when we take into account that Kentucky bluegrass has 2.2 million seeds per pound, then the final species composition of the resulting lawn is a whopping 43.1% Kentucky bluegrass. This example demonstrates the importance of knowing about the effect that seed numbers have on establishing various turfgrass species. David R. Huff, PhD, is Professor of Turfgrass Breeding and Genetics at Penn State. This article originally appeared in the Winter 2016 edition of Pennsylvania Turfgrass, the publication of the Pennsylvania Turfgrass Council. Segregating bentgrass putting green.

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