statistically different from three other mixtures in the trial including the MNDOT standard mixture, which had a mean maximum tensile load of 219.26 N. All of the mixtures in the weakest statistical grouping were seeded with either 20 or 40 percent alkaligrass, by seed count. Alkaligrass has primarily a bunch-type growth habit and some species in the Puccinellia genus have been shown to not compete well with other turfgrasses, especially slender creeping red fescue, under dry conditions.
Mixture 30 (40 percent ALK; 20 percent KBG; 40 percent SHF) required the least amount of work to tear at both sites, with an average of 3.32 Nm and 8.21 Nm, which was not significantly different from 17 other mixtures in the trial including the MNDOT mixture at the Rosemount site. None of the mixtures in the top statistical grouping for work to tear at Rosemount contained less than 90 percent fine fescues in the final plant community and just two of them contained less than 90 percent fine fescues at the St. Paul site. Chewings fescue was the most commonly-seeded fine fescue species for mixtures in the top statistical grouping at Rosemount and hard fescue was the most commonly- seeded fine fescue in the top group at St. Paul. Once again, mixtures comprising identical final species compositions produced different work to tear values.
Sod strips were cut from the research plots using a Turfco KisCutter sod cutter. Photo courtesy of Eric Watkins, PhD
Just three of the mixtures in the top statistical grouping for maximum tensile load contained less than 90 percent fine fescue in the final plant community, and none contained more than 19 percent Kentucky bluegrass. Perhaps most interesting, mixtures that resulted in identical final plant community compositions sometimes produced varying maximum tensile loads. For example, 7 mixtures all comprised 100 percent fine fescues in the final plant community but were seeded with quite different original seed mixtures. Maximum tensile loads for these mixtures ranged from 300.07 to 464.38 N and spanned several different statistical groupings. Tis result indicates that several establishment and maintenance factors influence final sod strength besides species composition and growth types.
When measuring the amount of work to tear sod in Newton-meters (Nm), Mixture 13 (20 percent KBG; 40 percent HDF; 40 percent CHF) required the greatest amount of work (16.57 Nm) at the St. Paul site, but was not different than 12 other mixtures. At the Rosemount site, Mixture 32 (10 percent STCRF; 40 percent CBG; 10 percent TF; 40 percent CHF) had the highest required work to tear (38.86 Nm), which was not different than 15 other mixtures.
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Mixture 43 (40 percent STCRF; 20 percent CBG; 20 percent HDF; 20 percent CHF) produced the greatest mean thatch depth of 9.7 mm and was similar to four other mixtures in the trial, while Mixture 23 (20 percent ALK; 40 percent SHF; 40 percent TF) produced the lowest mean thatch depth of 2.1 mm and was similar to eight other mixtures in the trial. Sod maximum tensile load and work required to tear sod both had a positive correlation with thatch depth for the combined site data. However, despite the significant positive correlation of thatch depth with maximum tensile load and work to tear, the small coefficient values suggest that thatch depth is likely not a major determining factor of sod strength for practical applications.
It is notable that, averaged across both sites, 75 percent of the mixtures in the trial resulted in final species compositions that comprised more than 90 percent fine fescue species. Moreover, it is likely that thinning of other species, such as tall fescue or alkaligrass, would create an effectively lower seeding rate for mixtures containing less fine fescue in the initial seed mixture and, thus, potentially weaker sod. Change in the proportion of fine
Te sod tensile strength testing apparatus and data acquisition system is shown here at the St. Paul site. Photo courtesy of Joshua Friell, PhD
TPI Turf News September/October 2018
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