Promoting Native Roadside Plant Communities and Ensuring Successful Vegetation Establishment Practices

About the report:

The loss of vegetation from roadside activities can lead to erosion and an increased sediment load in stormwater ponds. Current VDOT procedures regarding approved seed blends and establishment practices have led to inconsistent vegetation establishment and greatly rely on introduced species. Growing concerns regarding the threat of introduced, invasive species have increased the promotion of native plants in landscapes. One example is VDOT’s participation in the Candidate Conservation Agreement for monarchs fostering a desire to better understand factors that may improve milkweed abundance. Native seed blends, however, have failed to produce soil stabilization or long-term establishment in the past, presumably because of erroneous species selection, seed dormancy, and competitive displacement by weedy vegetation. This study was conducted to (1) identify and document potential procedural improvements for successful roadside vegetation establishment in Virginia; (2) propose candidate native plants for VDOT see blend consideration based on a statewide plant community assessment on Virginia roadsides; and (3) summarize the literature on availability, cost, and establishment success of candidate native species.

A review of VDOT’s vegetation establishment practices indicates that procedural inconsistencies related to the development of Roadside Development Sheets and recent restrictions on fertilizer application may be contributing to vegetation establishment failures. A statewide plant community assessment evaluated 490 sites and identified 616 unique plant species among the 67,330 plants surveyed. The Shannon Diversity Index was calculated for 2,450 10-m transects, indicating that plant biodiversity was higher on low-maintenance distal backslopes compared with high-maintenance road edges, shoulders, and ditches. Plant biodiversity was also higher on secondary roads than on primary roads. The unique introduced species encountered were relatively stable across Virginia’s seven ecoregions, but unique native species were more ecosystem dependent. Unique native species increased from 114 species on the road edge and shoulder to 281 species on the distal backslope. The likelihood of encountering a native plant increases from 1 in 4 on the road edge to 1 in 2 on the distal backslope.

Among the native plants that were most frequently encountered, seeds were often unavailable or price prohibitive. Andropogon virginicus, Tridens flavus, Dichanthelium clandestinum, Tripsacum dactyloides, and Sorghastrum nutans have desirable attributes as native roadside grasses and are among the top 20most commonly encountered native grasses on Virginia roadsides. The average cost of the seed for these grasses was $59 per pound compared with $2.40 per pound for tall fescue. Among grasses that are currently not commercially available, Setaria parviflora, Eragrostis pectinacean, Dichanthelium laxiflorum, and Panicum anceps are among the top 10 most commonly encountered native grasses and have characteristics that would be desirable for roadside vegetation. At least one milkweed species was observed at 37 out of 490 sites statewide (7.6%). The report recommends that VDOT explore opportunities to improve understanding of procedural policy and to implement procedural improvements, including revisions to the roadside development sheet. Additional opportunities for research include testing native plants for establishment and long-term dominance.

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