By Mary Free, Extension Master Gardener
Part 2. Residential Land Use: Impervious Surfaces
Fragmentation, degradation, and loss of natural habitats caused by the development of residential areas threaten pollinators. Impervious surfaces, such as buildings, parking lots, and roads, not only reduce the land areas available for wildlife habitat but stop rainwater from soaking into the soil. Instead, they produce runoff that carries surface sediments and pollutants, like nitrogen from synthetic fertilizers, into stormwater drains and local waterbodies.
Impervious surfaces also create a “heat island effect” by absorbing heat during the day and releasing it more slowly in the evening. In urban areas resulting temperatures could be as much as 7°F higher during the day and up to 5°F higher at night than in more rural regions. This increases energy consumption to cool buildings, increases air pollutants, compromises human health, and creates microclimates in which plants may leaf out earlier and freeze later, putting them out of sync with their pollinators’ life cycles. Research conducted in Raleigh, North Carolina showed that for every 1.8°F increase in temperature due to urban warming, bee abundance declined by about 41%. Although increasing the density of flowering plants in urban areas leads to more bee species, that alone will not fully offset the bees lost to urban warming because “floral resources benefit large bees more than small bees” (Hamblin et al. 2018).
DID YOU KNOW?
Research has shown that areas with impervious surface coverage greater than 50% have led to a decrease in pollinator populations and pollination services (Wenzel, 2020).
• In the City of Alexandria, impervious surfaces cover 44% of the land, a 3.45% increase from 2001 to 2021.
• In Arlington County, 37% of the land is covered by impervious surfaces, up 3.71% since 2001.
You can find land coverage (developed, impervious surfaces, agricultural, forested, wetlands, etc.) information for your locality at the Multi-Resolution Land Characteristics (MRLC) Consortium.
Individual property owners have the most flexibility to take steps to address the problems associated with impervious surfaces. However, those who live in condominiums or in areas with homeowner associations can work with their governing bodies on implementing some of the recommended actions below.
What You Can Do
- Use permeable paving materials for driveways, parking areas, paths, and patios that allow up to 95% of surface rainwater to penetrate to the soil below. Reduce the size of paved areas.
- Collect rainwater from rooftops by installing rain barrels or cisterns under downspouts, which will reduce runoff, flooding, and erosion. Direct the overflow to gardens and lawns. Use water reserves for thirsty plants and lawns and to fill birdbaths, which will conserve drinking water and lower costs for municipal water.
- Create a rain garden, which is a shallow depression with a filter bed that simulates the run-off treatment of natural areas. It is topped with native plants attractive to pollinators. For a list of suitable plants, see MGNV’s Native Plants for Wet Conditions Best Bets Fact Sheet and video.
- Plant more native trees, which absorb water runoff, reduce air pollutants, and help mitigate heat island effects as well as serve as host plants for pollinators and other insects and provide food, nesting, and protection for other wildlife.
Note: Click on images to see enlarged photos, captions, and photo attributions. On a mobile phone, click on the information symbol (circle with a letter ℹ︎ symbol).
DID YOU KNOW?
Tree canopy is an essential part of the urban landscape. 90% of keystone plant species—those with a disproportionately large effect on the diversity and stability of their ecosystems—are woody trees and shrubs. View the list of the Top Plants for Food Web Support.
• Alexandria’s 2018 Tree Canopy Assessment showed that 32.5% of the city’s land was covered by tree canopy.
• Arlington’s 2017 Urban Tree Canopy Assessment showed that 41% of the county (excluding the DOD and airport properties) was covered by tree canopy.
In both Alexandria and Arlington, residential land has the largest amount of canopy acreage and thus presents the greatest potential for expanding the tree canopy. Residential property owners can add to the canopy and benefit wildlife as well by planting more native trees on their land, especially those with high wildlife value such as (listed in order of lepidoptera support): medium to tall trees—oaks (various including white), black cherry, black willow, river birch, red maple, bitternut and shagbark hickories, and black gum or smaller trees—downy serviceberry, and flowering or pagoda dogwoods. Visit MGNV’s Tried and True Native Plant Selections for the Mid-Atlantic for fact sheets on more native trees.
Free (or reduced cost) Tree Programs
• MGNV has a Small Trees Make Big Canopies program where you can register in spring (April, May) or fall (September, October, November) to receive free native saplings (or donate some).
• Alexandria residents can request that a tree be planted between October and May on City-owned land in front of their home at no cost to them. Some years the city also sells trees in the fall that can be purchased at reduced prices.
• Arlington County sponsors an annual free tree giveaway for its residents. This fall, the County will distribute 500 young native trees—one per residential property based on registration, which begins September. 3, 2024.
- Create a conservation landscape by converting lawn or hard surfaces into a mulched garden, often situated slightly below the existing ground level, to capture runoff from rooftops, driveways, and patios. Use a combination of native plants that provide a succession of flowers and fruits throughout the seasons providing food and habitat for pollinators and wildlife. Be sure to include some “keystone plant genera,” such as Solidago (goldenrod) and Symphyotrichum (aster), each of which supports about 100 lepidopteran species. Do not forget to look at your foundation plantings to determine if pollinators would benefit from the addition of native shrubs and groundcovers. For a list of suitable plants and other suggestions, see MGNV’s Native Plants to Attract Pollinators Best Bets Fact Sheet and Small Space Gardening for Pollinators video as well as Gardening to Attract Butterflies and Moths.
- Eliminate (preferably) or minimize the use of pesticides and synthetic fertilizers and avoid purchasing plants pre-treated with neonicotinoids. Pesticides kill not only pests but also pollinators and can affect human health through direct or residual contact. Plant tissue absorbs neonicotinoid insecticides, which, in trees especially, can linger long after application, harming pollinators.
- See Part 4. Residential Land Use: Invasive Species for information on what to do about invasive species on your property.
DID YOU KNOW?
In 2022, Alexandria approved a Stormwater Utility (SWU) Credit Manual that provides property (residential, condo associations, and non-residential) owners with information on eligible practices that can reduce SWU fees up to 50%. Practices to protect stormwater quality and reduce flooding impacts include: permeable pavement, rain barrels, rain gardens, new tree planting, mature tree preservation, conservation landscaping, no fertilizer pledge, dry floodproofing practices, etc.
In 2023, Arlington published a Stormwater Credit Manual, which provides a menu of options that single family residential, multi-family residential, and non-residential owners can take to limit the impact of stormwater from their property and receive up to 35% credit toward their SWU fee.
There may be local regulations governing certain improvements to your property. Follow SWU guidelines to qualify for credits.
Next up: Part 3. Residential Land Use: Lawns
Additional MGNV Resources
If you would like to learn more about stormwater management solutions or how to create sustainable or climate conscious landscapes and inviting habitats for pollinators, then the following MGNV resources may help:
- Case Studies in Stormwater Management video
- Stormwater Management Solutions in Alexandria and in Arlington, Part 2 and Part 3 videos
- Climate Conscious Gardening videos
- Creating a Monarch Waystation
- Tried and True Native Tree Selections for the Mid-Atlantic fact sheets
References
Hamblin AL, Youngsteadt E, & Frank SD. 2018. Wild bee abundance declines with urban warming, regardless of floral density. Urban Ecosystems 21: 419–428. doi:10.1007/s11252-018-0731-4.
Keystone Plant Guides. Homegrown National Park™. (accessed May 6, 2024).
Laws A. 2019. Mitigating The Effects Of Heat On Urban Pollinators. Xerces Society for Invertebrate Conservation.
Learn About Heat Islands. Last Updated August 28, 2023. U.S. Environmental Protection Agency. (accessed May 13, 2024).
Stormwater Utility Fee Credit Manual For Residential and Non-Residential Properties. December 2018. Revised October 2022, Alexandria, Virginia.
Stormwater Credit Manual. April 2023. Arlington County, Virginia.
Wenzel, A., Grass, I., Belavadi, V.V. & Tscharntke, T. 2020. How urbanization is driving pollinator diversity and pollination – A systematic review. Biological Conservation, Volume 241.