InSPIRE/low impact/habitat/ground cover
Low-Impact Development Strategies
Native Vegetation and Pollinator Habitat
During the design process, developers should consider the ground cover intended to be used on site as these decisions can have an impact on future O&M considerations. Common ground cover types include: leaving the earth bare after construction; limiting vegetation growth, either through installation of an imperviable mat or applying pre-emergent herbicide to the site; covering the site with gravel; seeding with low growth or native vegetation; and some combination of the above.
The type of ground cover suitable for each site can be highly dependent on site-specific characteristics, such as previous land use and climate. For example, some solar sites in desert regions often employ either bare ground or gravel cover. The dry climate and remote locations lead to issues with obtaining water for establishing vegetation and there are less issues of invasive weeds due to the lack of water. However, the O&M considerations in the desert are different as dusty, dry climates can facilitate the need for washing panels often which can provide a water source for weeds and impact site performance. In other, more temperate climates (e.g. Minnesota, Oregon, etc.) issues arise with invasive species that move in after ground disturbance and O&M considerations include steps to prevent certain vegetation growth. Industry members indicated that permitting requirements and vegetation establishment costs are the primary drivers of the choice of ground cover for solar sites.
Industry representatives have identified numerous benefits of establishing native and/or low-growth vegetation, including:
- Potential for reduced Stormwater Pollution Prevention (SWPP) permitting and land needed for storm water management. This can reduce construction costs and open otherwise unusable land for PV generation
- Protection of beneficial vegetation in the region that can provide benefits to local communities and species
- Provision of habitat for multiple species
- If pollinator-dependent crops are near the solar site, pollinator-friendly ground cover can create habitat and promote increased agricultural yields4
- Native vegetation may provide nutrient recharge for previous agricultural lands and lead to enhancement of soils for future agricultural uses
- Reduced O&M costs over time (after vegetation is established) from reduced mowing events and reduced herbicide application
- Reduced dust on modules and cleaning reductions by establishing ground cover
Following construction all waste, construction materials, and debris from construction activities will be removed from the site. Temporarily disturbed areas and any areas of bare ground should be re-vegetated with appropriate seed mixes identified for the site-specific area. Beneficial vegetation could include, depending on the location: short grasses or flowers, low-growing forbs, low-growing wetland seed mixes or some other low-growing perennial cover.
Site preparation for re-vegetation and seeding activities generally occur after completing construction cleanup of the solar project. Soil underneath the panels can be tilled with a disc, field cultivator, or chisel plow to prepare the soil. In some cases, mulch can be applied. Prior to seeding, the ground should be sufficiently soft to allow for seed penetration but still provide surface soil stability. The soil should be prepared as appropriate to the seed mix type and site conditions.
Re-vegetation should be timed such that seeding/plantings are done at a time congruent with the area’s growing schedule/season, to the extent practicable with the construction schedule. If final construction occurs in winter months, reseeding augmentation may be necessary in the following spring, depending on local conditions.
Seeding of a site can often be successfully accomplished using broadcast seeding techniques. In small areas, seeds can be uniformly distributed by a mechanical, hand-operated seeder; or by hand. Larger areas can utilize tractors or other equipment that broadcasts seeds. Following seeding, the surface should be raked with tractor equipment, a cultipacker, or hand raked, as necessary.
A temporary cover-crop seed mix may be broadcast-seeded to provide temporary cover and reduce the potential for noxious weed invasion while native vegetation becomes established.
Solar developers should work with regional vegetation professionals to develop a diverse and appropriate seeding plan for each site. Seed mixes can vary throughout the site underneath the solar array, depending on site conditions (e.g., parts of wetlands or differing soil types), aesthetics, or for experimental purposes. Seed mixes might also vary throughout an entire project site, with different mixes on the site perimeter, on the solar array perimeter, and underneath the solar arrays.
The preferred method for re-vegetation of disturbed areas within wetlands is reliance on re-vegetation by resident hydrophytic plant communities. In some situations, a disturbed area may be dominated by native wetland plant species with rhizomatous root systems. In these cases, these root systems may be able to recolonize small areas of disturbance rapidly.
Seed Mixes should be locally sourced, when possible, and purchased on a Pure Live Seed (“PLS”) basis for seeding re-vegetation areas. Seed tags should identify purity, germination, date tested, total weight, PLS weight, weed seed content, and the supplier’s name and contact information.
Seeds should be used within 12 months of testing. The seeds should also be certified by the supplier as being free of noxious weeds.
Different ground cover choices may impact solar site design. For example, if a developer chooses native vegetation ground cover the racking system may have to be raised to accommodate vegetation height. Leaving the ground bare or installing gravel may change the stormwater characteristics on the site requiring larger retention ponds to manage stormwater runoff. In contrast, establishing vegetation could reduce the water runoff coefficients and allow the developer to reduce stormwater mitigation efforts, which can reduce the installed cost and require the developer to purchase less land.
When selecting ground cover for the solar site, future O&M measures should be balanced with the cost of ground cover establishment. Installing low growth grass will reduce the number of mowing events needed for the year compared to other vegetation types but may require a few years of management and water upfront to establish the grass. Installation of gravel on the site will inhibit vegetation growth but will require operators to use herbicide to control weeds on site. Mowing events on gravel installations can introduce risks of rocks being caught up in the mower and potentially flinging these rocks into solar modules, damaging equipment. Installation of pollinator friendly habitat may limit the time of year for mowing events and cause operators to reduce herbicide use that can lead to complications in managing vegetation. Gravel and bare ground sites may also increase dust issues and require the operator to perform costly module washing efforts more frequently. Several industry members noted the high expense of landscaping fabric designed to control weeds and the lack of a long term weed abatement solution it can provide.
During conversations and informal survey industry members identified several best practices for ground cover. Numerous installers mentioned that establishment of low-growth vegetation to reduce O&M on the site. When establishing native and low-growth vegetation, industry members prioritized working with local botanists and other resources such as local BLM offices and agricultural extension offices to design native, low growth seed mix tailored to each site and soil type within the site. Working with local botanists and resource can increase the likelihood of vegetation establishment at the site. If possible and feasible, prioritize native pollinator-friendly vegetation to provide benefits to nearby agricultural crops and promote goodwill in the local community.
While revegetation was the most favored ground cover, many survey respondents noted significant challenges with reestablishing vegetation, with failures including:
- Reseeding before piles and racking were installed which prevented establishment
- Hydroseeding in extremely hot climates, which did not take and required ongoing maintenance from resulting erosion
- Rainfall not meeting predictions, which prevented seeding from establishing
- Lack of irrigation water to establish vegetation
- Difficulty re-establishing native vegetation after it is removed from a site due to competition from weeds and other invasive species
Applying gravel as a ground cover was widely identified by industry interviewees as expensive and problematic as it creates uneven work surfaces, changes runoff coefficients, and does not provide a long term weed abatement solution. Gravel applications were described as requiring either regular application of herbicides, which can be restricted by local regulations, or mechanical weed control, which can kick up rocks and damage modules. However, survey respondents mentioned that gravel may be more feasible at certain sites that are not beneficial for establishing vegetation.
“Revegetation is not only possible but can achieve ground cover sufficient to control erosion and to begin to restore wildlife habitat. Further, successful establishment of low-growing warm-season grasses also achieves the goal of retaining short vegetation to avoid interference with panels and minimize fuel load for potential wildfires.” (Beatty et al. 2017)
“Low growth or beneficial vegetation underneath modules reduces the need for mowing and can prevent erosion, dust, or reduce the amount of stormwater prevention measures needed for the entire site.”
“Not vegetating after construction increased project cost due to increased drainage/run-off. Roughly 20% increase in civil costs due to changing drainage and for controlling sedimentation and run-off.”