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Development of salt tolerant grasses for roadside use.
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2011-09-01
[PDF-2.95 MB]
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Edition:Final; 2007-2010.
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Abstract:Roadsides in Rhode Island and elsewhere are planted to mowed turfgrass in order to prevent erosion, improve aesthetics,
maintain visibility, and provide a safe means of stopping errant vehicles. However, there are a number of ways in which
mowed turfgrass is failing to meet the expectations of highway managers and the traveling public. Regular mowing and turf
maintenance is expensive. Use of non-native plants has resulted in problems with invasive species. Most importantly, the
seeded turfgrasses are not surviving in the roadside environment and so are failing to prevent erosion.
Solving this problem requires answering two questions: why are the seeded turfgrasses dying? And how can this be fixed
given severe limitations on funding? The purpose of this study was to seek answers to the first question, and then generate
possible solutions to the second. The initial hypothesis was that high levels of deicing salt were responsible for the damage.
We proposed to test this hypothesis by establishing test plots of 21 grass varieties selected for tolerance to salt in the highsalt
zone at two locations: I-95 North in Hopkinton, and I-295 North in Lincoln. The grass variety trial was combined with a
test of two potential low-cost soil amendments: yard waste compost and biosolids. We also proposed to monitor salt
deposition over two winters at six sites to determine how much salt the grass was being exposed to, and how long the salt
persisted in the root zone. Persistence is important because the grass is more sensitive to salt damage when it is actively
growing (April-October) than during the winter months. At the same time we proposed to survey the vegetation on
established, mowed roadsides to determine which plant species were surviving, as these might be good candidates for
inclusion in a modified seed mix.
We found that salt tolerant grasses showed no improvement in survival over the grass varieties already in use, even though
the salt tolerant grasses had shown significantly better survival in greenhouse salt screening trials. This suggested that the
salt was not the primary cause of vegetation failure; the finding that the salt did not persist in the root zone at levels high
enough to cause plant damage past early April further supported this hypothesis. Soil amendment, however, had a dramatic
effect on turfgrass survival. Perennial vegetation cover on plots amended with biosolids remained above 50% throughout the
two-year study, and beyond.
At the beginning of this study it was assumed that most of the vegetation surviving on mowed roadsides was the turfgrasses
seeded by RIDOT or weeds such as quack grass, sheep sorrel, and crab grass. However, in surveys of seven sites in mowed
areas along limited access highways in Rhode Island we found a total of 80 plant species, approximately half of which are
native to Rhode Island.. Our results suggest that the seed mix used by RIDOT is not persisting on the roadside, but that it is
gradually being replaced by native or naturalized species. Reductions in mowing would further this process, as they would
permit more of these naturally-occurring plants to mature seed, filling in gaps created by vehicles and other disturbances.
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