Cool-season turfgrass mixtures for Minnesota roadsides

Abstract

Roadsides present significant challenges to maintaining functional and sustainable vegetation due to the presence of multiple stresses that can be extreme. These stresses may include drought, heat, disease, and in cold-weather climates exposure to deicing salts used during winter road maintenance practices. Mixtures of cool-season turfgrasses can be used to create high quality roadside vegetation that can withstand these stresses. The current specification for roadside turfgrass in Minnesota included outdated cultivars and was in need of reassessment. The purpose of this research was to identify the best cultivars for use on roadsides in Minnesota and to create a suitable mixture of those cultivars that would maximize establishment and survival.In the first part of this research, cultivars of cool-season turfgrass were assessed for their ability to establish and survive on roadsides in Minnesota. This was accomplished by visually assessing fall-seeded plots on roadsides for establishment and, subsequently, survival the following spring. Successful establishment and survival were related to edaphic characteristic; specifically, soil compaction and moisture.In cold weather climates like that in Minnesota, salt tolerance is a required trait of roadside vegetation due to the application of deicing salts in the winter. As such, the same cultivars were directly assessed for salt tolerance in nutrient solution culture amended with sodium chloride to 4, 14, and 24 dS m<super>-1</super>. Assessment of salt tolerance was accomplished using digital image analysis to quantify percent green tissue remaining following the different severities of salt exposure. Following exposure to the lowest level of salt stress, no significant differences were observed between cultivars in the trial. Following moderate salt stress, significant differences were identified between cultivars of turfgrass, including between cultivars within species. Specifically, between cultivars of Kentucky bluegrass (<italics>Poa pratensis</italics> L.) and perennial ryegrass (<italics>Lolium perenne</italics> L.). However, under extreme salt stress no significant differences existed between cultivars within any given species, and trends among the mean tolerance of each species were dominant. Overall, cultivars of tall fescue (<italics>Festuca arundinacea</italics> Schreb.) and slender creeping red fescue (<italics>Festuca rubra</italics> L. ssp. <italics>litoralis</italics>) retained the greatest amount of green tissue. An improved method of evaluating turfgrass seed mixtures was needed before creation of an ideal mixture for Minnesota roadsides was possible. A novel approach to design and analysis of seed mixture experiments was introduced. The method used a plant community-level approach to identify the optimal species mixture. To demonstrate the method, a simple four-species mixture experiment was established in a growth chamber using a simplex-centroid design of seed mixture proportions. Polynomial regression analysis and augmented Lagrangian numerical optimization were used to predict a mixture that would maximize total dry clippings biomass over 19 wk. From this experiment, the method was proven successful as the predicted optimal mixture of 83% perennial ryegrass (<italics>Lolium perenne</italics> L.) - 17% hard fescue [<italics>Festuca trachyphylla</italics> (Hack.) Krajina], by seed count, was shown to produce a similar or greater amount of clippings biomass as compared to all of the design point mixtures. That plant community-level approach to seed mixture design and analysis was applied to mixtures of nine turfgrass species on roadsides with the additional requirement that each mixture contain more than two species. A single cultivar was chosen to represent each species based on the previous evaluations of roadside establishment and survival as well as direct evaluation of salt tolerance. Mixtures were established at two roadside locations and evaluated for green canopy cover and weed encroachment over two years. Data from digital image analysis and grid-intersect counts indicated that inclusion of tall fescue in the seed mixture significantly decreased the probability of retaining at least 60% cover after two years. In contrast, inclusion of hard fescue, sheep fescue (<italics>Festuca ovina</italics> L.), and slender creeping red fescue each increased the probability of retaining at least 60% cover. A 2:2:1 mixture by seed count of those three species, respectively, was predicted to produce the greatest percent green cover after two years of exposure to roadside environmental stresses. That mixture was deemed best for establishment on roadsides in Minnesota.