Bob Mugaas, University of Minnesota Extension Educator
Resources for turfgrass species selectionDetermining and then finding the best lawn grasses for one's lawn is not as simple a task as it might seem. However, there are at least a couple readily available resources about turfgrass species and varieties that are helpful when trying to determine which ones will do well in this area. One of those is a local source from right here at the University of Minnesota. The other is a national database with extensive information about turfgrass varieties. Let's see how we might utilize these to make a list of suitable varieties for this area. We'll begin with a look at the national database known as NTEP, the National Turfgrass Evaluation Program. Their information can be accessed at: http://www.ntep.org. This program is a cooperative effort between the non-profit National Turfgrass Federation, Inc. and the United States Department of Agriculture. It conducts comprehensive evaluations of turfgrass species and varieties across the country in cooperation with researchers at state Universities. (See Photo 1. U of MN NTEP perennial ryegrass trial). The evaluation data is submitted to NTEP for review, analysis and publishing. As a result of that effort, reports are created and made available to anyone wishing to view that data at no cost. For more information about the organization and what resources they can provide, visit their website mentioned above. They also provide a variety of helps to help navigate through the information.
Determining well adapted varieties
Variety information provided by NTEP as well as many others use a very common statistic known as 'least significant difference' or LSD to help aid the user in determining which varieties show true differences from the others being evaluated. This statistic also comes with a probability value that indicates these differences would occur either less than 5% or even less than 1% of the time strictly by chance. Hence, that gives us some level of certainty that the observed differences were not just a chance occurrence but are real differences between the varieties. Fortunately, the LSD and its probability value will have already been determined by NTEP or by the organization doing the data analysis. It is usually found at the bottom of charts that list various variety ratings for a particular characteristic.
For example, when looking down a column of numerical rankings, there can easily be questions about which of these varieties are really superior to all or any other varieties. That would be a perfectly fair question and hence, the need for a tool to help separate the top performing varieties from the rest of the pack. This is where we use the LSD statistic.
Here's how it works. Let's suppose we have the following (fictitious) ranking of bluegrass varieties based on their overall turfgrass quality during the year. In other words, which of these varieties consistently exhibited the best overall turfgrass quality? In this case the rating scale is 1 to 9 with 1 being virtually dead and the lowest quality and 9 being the very best overall turfgrass quality. In most cases, a rating of 6.0 or above would be considered acceptable for a home lawn situation.
Variety Turfgrass Quality
Variety A 7.1
Variety G 7.0
Variety Y 6.8
Variety B 6.8
Variety M 6.7
Variety J 6.4
Variety S 5.7
Variety C 5.6
Variety R 4.4
Variety H 3.7
As noted above, these are all fictitious numbers but they will serve to illustrate how to use the real data presented by NTEP or individually by Universities such as here in Minnesota. Remember the LSD statistic indicates what the minimum difference is between the averages of the different varieties for them to be considered truly better or different than the others. In this case, all of those varieties separated by a difference of less than 1.3 would be considered similar in their turfgrass quality even though they may not have exactly the same average value. Thus, in our example, varieties A, G, Y, B, M, and J would all be considered similar in turfgrass quality. Hence, there should be little to no difference among the first six varieties relative to overall turfgrass quality.
In general, you work from the top down when determining LSD groupings. It should be apparent that you could start anywhere in the column and create a set of varieties that are not significantly different from each other in turfgrass quality. For example, using our LSD of 1.3, we could justifiably say that varieties M, J, S and C are not significantly different from each other. That would be a true statement but, there would be some question as to how meaningful that particular group of varieties would be. In general, we are looking for varieties that rank at or near the top for our particular characteristic, not necessarily those in the middle of the pack. Hence, the reason for beginning at the top and working down the column rather than working from the bottom up or starting randomly in the middle of the column. One can also use the LSD statistic to compare one variety with another rather than creating a particular group of varieties as was just done in the above example.Our own University of Minnesota Turfgrass Program website, www.turf.umn.edu, also has variety evaluation information. Likewise the evaluation data presented in our various research reports is arranged in the same manner as that of NTEP and utilizes the LSD statistic to separate significant differences between particular cultivars or groups of cultivars. Thus, if you would like to know how particular turfgrass varieties performed in this area, check out this website for that information as well as other information about the University of Minnesota Turfgrass program.
Hopefully, this information will encourage you to do some investigating into the turfgrass varieties that have the potential to perform well in this area and in your particular situation. Next month, we'll discuss understanding a grass seed label and what to do when the varieties you're looking for aren't listed on any of the packages of seed you've examined.