Departments of transportation often provide mixture recommendations for various roadside plantings.
In Minnesota, specifications are given by the Minnesota Department of Transportation (Mn/DOT) for a number of different turf plantings, including salt-tolerant turf for roadsides. Even though these recommendations exist—and are sometimes followed—contractors have had very little success with roadside turf plantings in Minnesota’s harsh roadside environment. The combination of high levels of salt stress following winter and heat and drought stress during the summer creates a tremendous challenge. Failed installations have both economic costs (time, labor) and environmental costs (soil erosion).
There are a number of problems that can cause turf loss in these situations, with salt damage to the turf being among the most important. The Department of Horticultural Science at the University of Minnesota was awarded a grant by the Minnesota Local Road Research Board to address this important problem. Through this research project, we have begun to identify grasses that are better adapted to the harsh roadside environment found in Minnesota and similar climates.
Only the winter hardy
The first task was to evaluate the best available material for salt tolerance on a Minnesota roadside. When selecting varieties of grass to use, researchers considered if the grass was winter hardy, if it was known to have at least some salt tolerance (not necessarily in a roadside environment) and if it was likely to survive in a low-input environment such as that found on most roadsides. Collaborators and seed companies were contacted, and current scientific literature was reviewed to come up with a list of 75 varieties, representing a total of 14 grass species. In fall 2010, 75 grasses were planted along roadsides at four sites: (1) Larpenteur Avenue along Roselawn Cemetery (Roseville, Minn.), (2) I-35W in Minneapolis, (3) MnROAD research site (Albertville, Minn.) and (4) Gortner Avenue on the campus of the University of Minnesota (St. Paul, Minn.). The 35W site experienced high levels of salt damage that resulted in the death of most plots, and many of the plots that did survive were damaged by unexpected machinery activity at the site. Conversely, the Gortner Avenue site, on a low-volume street, did not experience sufficient salt stress to see differences between grass varieties. The remaining two sites provided usable results.
Assessment of salt tolerance began during the second week of April 2011. Salt tolerance was determined by visually estimating the proportion of existing turf that remained alive and healthy. Rating and analysis of the data from the Roselawn Cemetery site was split into two groups: (1) Main plot—the area opposite the sidewalk from the road, and (2) Boulevard—the 3-ft-wide area between the sidewalk and the road. This was done due to assumed differences in salt loading and runoff patterns as well as measured differences in soil properties. This differentiation is important, as many salt-tolerant turfgrass installations in Minnesota are along roads that are next to sidewalks and paved trails.
Magnitude of salt damage was observed to have a wide range over all varieties and sites. Salt loading appeared highest within 4 to 6 ft of the road or shoulder on highway sites, and within 1 to 2 ft of roads and sidewalks on residential sites. With the exception of the boulevard at Roselawn Cemetery, turf plots extended well beyond this area. Thus, only the area between the road or sidewalk and the line formed by the mean extent of salt effect was considered in the rating of those plots, whereas the entire area of the Roselawn boulevard was taken into account.
The main plot at Roselawn showed a wide range of salt damage. Plots were rated on two different dates during the spring, and data from April 27, 2011, was chosen for analysis to avoid the confounding effects of mowing, which began shortly thereafter. Analysis of mean performance by species showed that all species of fine fescue (Festuca spp. including strong creeping red fescue, slender creeping red fescue, Chewings fescue, hard fescue, sheep fescue and blue hard fescue) performed the best, followed by alkaligrass (Puccinellia distans and Puccinellia maritima) and perennial ryegrass (Lolium perenne).
The boulevard section of Roselawn provided, arguably, the best overall data of all locations. Overall establishment was good, which provided a greater amount of turf on which to base the salt-tolerance evaluations. Plots were evaluated on the same dates as the Roselawn main plot section and data from April 27, 2011, was once again used for the statistical analysis. Top-performing species were sheep fescue, slender creeping red fescue, Chewings fescue, blue hard fescue, alkaligrass and strong creeping red fescue. It should be noted that alkaligrass, although it provides good survival under high salt conditions, rarely performs adequately as a mowed turf in our region and may not be the best option for areas where aesthetics are important.
The plots at the MnROAD Research Facility were located directly adjacent to I-94 and thus experienced a higher salt load than those at Roselawn. As such, the salt damage was more extensive. Initially it was observed that only the alkaligrass cultivars experienced no salt damage and were in fact thriving and spreading to adjacent plots. However, at a later rating date (May 13, 2011), significant differences could be observed between many of the cultivars. Means separation of species performance confirmed that alkaligrass performed the best, followed by slender creeping red fescue and creeping bentgrass (Agrostis stolonifera L.).
The better variety
Mn/DOT specifications are often mixtures of different grass species; therefore, after determining the best individual varieties for roadside performance, it was decided to evaluate the best grasses in various mixture combinations. Data was used from Phase 1, along with data from a greenhouse salinity tolerance screening trial, to determine the best variety to use for each of the species we found to be acceptable as a salt-tolerant roadside grass (Kentucky bluegrass also was included due to its ability to form a harvestable sod, even though it did not do well in Phase 1 testing).
Mixtures were seeded at two roadside sites (Larpenteur Avenue in St. Paul and Main Street in Centerville, Minn.) in fall 2011. Data collection began in spring 2012 and consisted of collecting digital images of each plot, which were subsequently evaluated for percent green ground cover.
Mixtures containing significant proportions of fine fescues performed the best during spring 2012 following recovery from the winter salt applications. Those that contained Kentucky bluegrass or creeping bentgrass in combination with large proportions of alkaligrass, despite its known salt tolerance, did not perform well. Slender creeping red fescue was found to be the common component between mixtures performing well for spring survival.
The summer of 2012 was characterized by greater-than-normal levels of heat stress, resulting in significant damage to the plots. Summer data showed no trends toward superior species, but the greatest proportion of green tissue in the surviving plots was observed in tall fescue. In spring 2013, plots were assessed to determine the proportion of ground still covered by turfgrass. Results differed from the previous year in that plots containing tall fescue suffered disproportionately due to the extreme ice cover experienced during winter 2012-13. Analysis of the cover data showed that including slender creeping red fescue, sheep fescue, hard fescue or alkaligrass in a mixture increased the odds of retaining at least 50% turf cover after two winters, whereas inclusion of other species reduced those odds. Together, the data indicate that seed mixtures containing fine fescues along with tall fescue in small proportions may have the best chance for survival on roadsides.
Waiting for the right one
In 2009, members of the Minnesota Turf Association (sod producers) began collaborating with Mn/DOT and the Minnesota Crop Improvement Association on the development of a sod quality-assurance program targeting salt-tolerant sod for roadsides. The hope was that this program would help contractors know the sod they were using along roadsides was correctly identified and was known to contain the salt-tolerant grasses specified by Mn/DOT.
The research by the Department of Horticultural Science began after the pilot for this program began, so researchers still have not seen new installations of sod that were made up entirely of their variety and mixture recommendations.
During the pilot of this program, when sod mixtures were made up of existing Mn/DOT specifications, a number of installation failures have been observed. It appears that many of these failures are the result of poor installation and establishment practices (laying sod during very high temperatures, failing to adequately water during the critical establishment period, etc.). There also may be other important factors causing these installations to fail. The Department of Horticultural Science has recently been awarded another grant from the Minnesota Local Road Research Board that will focus on identifying the reasons for sod and seed installation success and failure.
This next research project began on July 1, 2013. During the next year, researchers will assess 30 to 40 sites that have been seeded or sodded with salt-tolerant grasses within the last three years. At each site, data will be collected on the existing turf as well as site information such as soil compaction, soil temperature, soil nutrition and soil type. After collecting and analyzing this data, the most important factors affecting success or failure of these grasses along roadsides will be determined. The next step will be to develop research projects that address these factors. For instance, if watering during establishment appears to be one of the top barriers to success, researchers will determine the best and most reasonable approach to watering during this critical growth period.
The project is resulting in grass-variety recommendations for regions where salt application to roads during winter is common. Ultimately, the success of a roadside turfgrass installation will depend on a combination of factors, including starting with the best grasses available, managing those grasses properly during establishment and finally, the cooperation of grass seed and sod providers, government officials and contractors. WM
Departments of transportation often provide mixture recommendations for various roadside plantings.