Seed: what’s in the numbers?

By David R. Huff, PhD

Seed serves as a remarkable means of plant propagation, particularly in terms of commercial application of plant material. It ships well, it stores well, and large-scale establishment can be achieved relatively quickly at low cost. In the turfgrass industry, it is most typical that cool-season species are propagated by seed, whereas warm-season turfgrass species are propagated vegetatively through either sprigs or sod. While there are exceptions to this general tendency (i.e., vegetatively propagated cool-season species and seeded warm-season species), for the most part, our industry uses seed to establish cool-season turfgrass species like bentgrass, perennial ryegrass, tall fescue, Kentucky bluegrass and fine fescues, while vegetative propagation is commonly used for warm-season species like bermudagrass, St. Augustine grass, zoysiagrass and seashore paspalum.

Much has been written about using seed as a means of turfgrass propagation, including the basic importance of various aspects like purchasing, applying and establishing seed; for example, being able to properly understand and use the important information contained on a seed label like the calculation of the percent Pure Live Seed (PLS) for comparing the true costs of different seed products (example, see

However in this article, I would like to present some of the different, though no less important, aspects of using seed to establish areas of turf. For example, over the years, I have found that most turf managers view their seed as an individual cultivar or variety, in that all the seed in a bag, of say “Penncross” creeping bentgrass, is genetically all the same. The truth is that each individual seed gives rise to a genetically unique individual plant. By genetically unique, I mean that some plants will be big and others small, some will be dark green and others light green, some will tiller more while other stay less dense, etc. This is because each seed in the bag was the result of a fusion between a sperm from the pollen-donator parent plant and an egg from the seed-bearing parent plant. And just as in animals, insects and human beings, each individual is genetically unique and different from all others because each was derived from a unique combination of sperm and egg that gives rise to their genetic uniqueness.

A numbers game

I often like to tell my students that there are more stars in the universe than all the grains of sand on all the beaches of planet Earth and that there are more genetically different individuals of Penncross creeping bentgrass than there are stars in the universe; but that statement seems to lose its impact unless you do the math, so let’s do the math.

There are approximately 700 trillion cubic meters of beaches on planet Earth, and using an average medium sand particle size that is equivalent to 8,000 sand grains per cubic centimeter, that equals 5 quintillion (5 x 1018; in this decimal notation, 1018 is a 1 followed by 18 zeros) grains of sand on all the beaches of planet Earth. Now, there are approximately 250 billion stars in our Milky Way galaxy, and there are approximately 200 billion galaxies in the known universe. That equals 50 sextillion (50 x 1021) stars in the universe (give or take).

These are very large numbers of things, too large really to even comprehend; however, they pale in comparison with the number of genetically unique individuals of sexually reproducing species, like creeping bentgrass. To estimate the number of genetically unique individuals of creeping bentgrass, we’ll apply a standard genetics formula that uses the number of different combinations of different forms of any given gene to the power of the total number of genes that an organism possesses. On average, this number is 1 x 1030,000, or 10 different combinations per gene to the power of 30,000 genes per organism. I don’t even know that there is a name for this number, but I do know that it is a very large number.

Finally, of the total amount of all this genetic variation within the creeping bentgrass species, approximately 2/3 of the genetic variation have been found to reside within cultivars, while 1/3 has been found to reside between cultivars. And thus, if we assume that there are roughly 200 cultivars of creeping bentgrass in today’s market (there really aren’t that many, but it helps to make the numbers work better), then the total number of genetically unique individuals that might exit within the Penncross cultivar would roughly be on the order of 1 googol (or 1 x 10100; that is a 1 followed by 100 zeros). Thus, there is way more number of genetically unique individuals of Penncross creeping bentgrass, or any cultivar of creeping bentgrass for that matter, than there are stars in the universe.

The point of this exercise is to illustrate the importance of managing your young seedling plants during establishment. If, for example, you plant creeping bentgrass, which possesses 6 million seeds per pound, at a recommended rate of say 1 lb. per 1,000 sq. ft. on an average sized golf course putting green of 5,000 sq. ft., then your initial establishment will give you 30 million genetically unique seedlings. Over time, it is unrealistic to believe that all 30 million seedlings will survive. Some seedlings will succumb to disease, other will die from heat or cold, and many will simply be outcompeted by their neighboring brothers and sisters. There is immense competition for the limited resources of light, nutrients, water and simply space to grow on a golf course putting green. Therefore, over 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.

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.