Water-Sensitive Landscaping: a Growing Opportunity

By Andrea Ludwig, Associate Professor of Ecological Engineering and Extension Specialist, University of Tennessee Department of Biosystems Engineering & Soil Science, Dennis Gregg, Extension Associate, University of Tennessee Extension and Natalie Landry, Project Manager, Knox County Stormwater

While in some seasons it is difficult to recall, Tennessee is a wet state, with average annual rainfall from 48 inches to over 70 inches in places, which is well above the national average of 30 inches. These conditions present challenges for landscape managers and developers. Two forces are creating a market for landscaping solutions that address rainwater run-off from developed land, also called “stormwater”. The first is a growing awareness that unmanaged stormwater has damaging impacts on natural resources. To address this, new regulations developed by the Tennessee Department of Environment and Conservation are enforced by local governments seeking to capture pollutants carried by stormwater before they end up in streams and to reduce flooding of streets and lowlying areas. In response to these new rules, proactive municipalities and community groups have encouraged landscaping solutions that use plants and improved soil to soak in water where appropriate, rather than traditional reliance on pipes and concrete drainage channels that contribute to flooding. Rain gardens and vegetated swales are two commonly used landscaping features that add visual interest to a property while providing a valuable function and addressing natural resource needs.

Second, growing consumer interest in sustainability is touching all market sectors; from buildings and appliances to the clothes we wear and the food we eat. By definition, something that is sustainable keeps its value and quality while avoiding depletion of resources. It is no surprise then that sustainability is a marketable quality within landscape design and management. In this year’s American Society of Landscape Architects survey of over 850 disciplinary professionals, participants reported a high consumer demand for sustainability. Eight out of the top ten “hot topics” in landscape design currently have a sustainability or conservation element. Rain gardens, reduced lawn area, native plants, and adapted drought-tolerant plants all made the list and each of these are elements of a sustainable developed landscape. The University of Tennessee is partnering with local governments and the Tennessee Nursery and Landscape Association to assist landscapers in helping their clients by creating beautiful, functional home landscapes that integrate water-sensitive landscaping. Educational resources and training are available for landscape professionals and will inform how to develop skills in landscape design and installation using natural water features or stormwater flows. This series will focus on how to effectively integrate water-sensitive sustainable landscaping elements (including rain gardens, water gardens (Figure 1), and streamside plantings) into residential and commercial landscaping. The goal is to create landscape features that please the customer and address environmental needs while also meeting regulatory expectations. Rain gardens and water gardens both use rainwater runoff to nurture a specially selected plant palette. Streamside plantings (or buffers) create a natural transition from the water’s edge into the yard or commercial space. Such buffers also provide an important stabilizing function for the streambank itself. Integrating these features into a landscape will lead to new opportunities for plant selection and less maintenance across time.

FIGURE 1: A 30 ft by 40 ft water garden at the University of Tennessee Gardens(Knoxville, Tennessee) flourishes with color in late summer, attractsbeneficial insects and is the birth place of scores of tree frogs eachyear. Pictured here are rosemallow (Hibiscus moscheutos ssp.lasiocarpos), Lizard’s Tail (Saururus cernuus), Blue Vervain (Verbenahastata), and Pickerelweed (Pondetaria cordata).

FIGURE 1 (Detail)

In practice, sustainable landscaping reflects a functional natural environment by creating a resilient and healthy landscape. When planted correctly, plant species that are adapted to local soil and climate conditions will have the best long-term success, so choosing regionally native plants is a basic concept. Plants that can tolerate saturated soils, or short periods of flooding, represent a special subset of native plants and many of these species may be less familiar to many landscapers. See the list of “Plants that Work” for more details.

FIGURE 1 (Detail)

FIGURE 1 (Detail)

Implementing these best practice approaches may mark a transition from traditional drainage strategies, which were aimed to avoid such saturated soil conditions. The stimulus to make this change, however, stems from our learning that over-reliance on traditional drainage techniques can have undesirable consequences, especially in urbanized areas. Quickly moving an excess of stormwater from the land into streams has three immediate negative impacts. First, any pollutants on the surface end up in the stream and negatively impact stream health; first locally, and then regionally. Second, this rapid increase in runoff water reaching the stream is like discharging a fire hose in the stream channel: these channels erode both down and sideways while trying to contain the new higher volumes, with the subsequent channel damage requiring expensive stabilization efforts to protect neighboring properties. Finally, (and this third consequence sometimes surprises people) a rapid-draining landscape system generally results in both lower stream levels and greater localized need for increased irrigation. This occurs because the stormwater was moved away so quickly, that the soils failed to have time to capture the rain: it simply couldn’t soak in to benefit the local plant life and environment.

FIGURE 1 (Detail)

FIGURE 1 (Detail)

To reap all the benefits of a functionally sustainable landscape, the way in which water moves through the landscape (or hydrology) needs to be returned to balance. The new training opportunities that are being shared will cover practical aspects of how to address challenges like soil compaction and drainage issues in residential landscapes, as in the rain garden project at Lakemoor Hills neighborhood in Knoxville (depicted in Figure 2).

FIGURE 2: A leaf litter and aged hardwood chip compost amendment was incorporated into this rain garden bed in Knox County using a Harley rake to loosen compacted soils and boost plant growth.

In a wet state with average annual rainfall that is often well above the national 30-inch average, managing Tennessee’s stormwater runoff as a resource in our landscaping can create both unique landscape features and provide opportunities for plants not often used. As we experience an increase in extreme weather conditions (that include intense downpours, extended dry conditions or drought, etc.), having resilient landscapes is even more important. Embracing landscaping with water is a way to meet this growing demand.

The 2019 training course schedule is under development and will be posted at https://ag.tennessee.edu/watersheds/Pages/ default.aspx. For more information, contact Dr. Andrea Ludwig, Associate Professor of Ecological Engineering and Extension Specialist with the University of Tennessee Department of Biosystems Engineering & Soil Science (aludwig@utk.edu).

Plants that Work

These US native plants pull double-duty in the landscape, providing both visual interest and added functions that keep our landscapes naturally resilient and rejuvenated. Use these plants along creek banks and drainages, in waterlogged areas near gutter downspouts, in rain gardens, or in water gardens.

Hold Strong

These shrubs are adapted to streambanks, where they experience wet and dry soil conditions and help hold the bank in place with their strong root structures. All may be pruned in late spring to maintain desired shape and spread.

Ninebark (Physocarpus opulifolius) Photo 1 – Growing 5–8 feet tall, this upright, spreading, deciduous shrub provides small pink or white flower clusters from May to June and has exfoliating bark that provides winter interest. It grows in full sun to part shade and can take harsh conditions. Cultivars of note include ‘Ruby Spice’, which only grows to 3–5 feet with red spring foliage, and ‘Diabolo’, with purple foliage.

PHOTO 1: Ninebark ‘Ruby Spice’

Photo Credit: William Klingeman, The University of Tennessee

American Elderberry (Sambucus canadensis) Photo 2 –Growing 5–12 feet tall, this suckering, deciduous shrub flowers in white clusters from June to July and bears black berries in late summer that are attractive to birds.

PHOTO 2: American Elderberry

Photo Credit: William Morgan, Obed Watershed Community Association

Witchhazel (Hamamelis virginiana) Photo 3 – Growing 15–20 feet tall, this deciduous shrub or small tree bears clusters of fragrant bright yellow flowers at the stem at a unique time, from October to December, and leaves turn a showy yellow in fall.

PHOTO 3: Witchazel

Photo Credit: William Morgan, Obed Watershed Community Association

Soak In

These plants can tolerate periods of saturated soils and are ideal for downspout rain gardens or areas that collect water when it rains.

Sweet Pepperbush (Clethra alnifolia) Photo 4 – Best in the center/wettest part of a rain garden, this small, compact shrub grows 3–8 feet tall and produces fragrant, white panicled flowers in mid to late summer, attracting butterflies and bees.

PHOTO 4: Sweet Pepperbush

Photo Credit: Sam Rogers, The University of Tennessee

Smooth Hydrangea (Hydrangea arborescens) Photo 5 –Best along the outer edge of rain gardens, this loose, widelybranched deciduous shrub grows 3–6 feet tall and bears tiny white flowers in flattened clusters from May to July.

PHOTO 5: Smooth Hydrangea

Photo Credit: Andrea Ludwig, The University of Tennessee

Coralberry (Symphoricarpos orbiculatus) Photo 6 – This dense, suckering deciduous shrub grows 2–5 feet tall, has pinkish white flowers from June to July, and produces clusters of round coral-red berries in fall that persist throughout winter.

PHOTO 6: Coralberry

Photo Credit: Andrea Ludwig, The University of Tennessee

Inkberry (Ilex glabra) Photo 7 – This upright-rounded, broadleaf evergreen shrub grows 5–8 feet tall, tolerates shade and wet soils, and females produce black berries if pollinated, attracting birds and providing winter interest.

PHOTO 7: Inkberry

Photo Credit: Andrea Ludwig, The University of Tennessee

Tennessee Coneflower (Echinacea tennesseensis) Photo 8 –This perennial is on the Federal Endangered Species List within its native habitat yet is now commercially available from licensed growers. It grows 1.5–2 feet tall and blooms purple with an orange center cone from June to August, attracting birds and butterflies.

PHOTO 8: Tennessee Coneflower

Photo Credit: Sam Rogers, The University of Tennessee

New England Aster (Symphyotrichum novae-angliae) Photo 9 – This perennial grows 3-6 feet tall and has deep pinkish purple flowers from August to September borne on stout, upright stems. Flowers, which attract butterflies and native bees, may set abundant seed that are self-sown around plantings.

PHOTO 9: New England Aster

Photo Credit: Sam Rogers, The University of Tennessee

Cherokee Sedge (Carex cherokeensis) Photo 10 – This spreading, clump form sedge grows up to one foot tall, sends up wheat-like seed spikes in the fall, and thrives in moist soils.

PHOTO 10: Cherokee Sedge

Photo Credit: Sam Rogers, The University of Tennessee

Indian woodoats (Chasmanthium latifolium) Photo 11 –Also called River Oats, this clump-forming ornamental grass grows 2–5 feet tall in full sun to part shade, is tolerant of poor soils, and has an interesting drooping seed head that flutters in the breeze.

PHOTO 11: Indian woodoats

Photo Credit: William Morgan, Obed Watershed Community Association

Cinnamon Fern (Osmundastrum cinnamomeum) Photo 12 –This southeast native fern can tolerate heavy shade and wet soils, grows to be 2–3 feet tall or taller with constant moisture, and produces “fiddleheads” from its base that persist through summer and turn yellow in fall.

PHOTO 12: Cinnamon Fern

Photo Credit: Dennis Gregg, The University of Tennessee

Filter Out

These wetland plants bring a splash of color to water gardens while also filtering out toxins and pollution from rainwater runoff.

Rosemallow (Hibiscus moscheutos ssp. lasiocarpos) Photo 13 – Best along the edge of a water garden, this vigorous, woody perennial grows 3–7 feet tall sporting white or pink flowers from July to October.

PHOTO 13: Rosemallow

Photo Credit: William Morgan, Obed Watershed Community Association

Blue Flag (Iris versicolor) and Copper (Iris fulva) Irises Photo 14 – These clump-forming perennials grow 1.5–2.5 feet tall, will spread with rhizomes along the edge of a water garden, and offer purple and coppery pink flowers from May to June.

PHOTO 14: Blue Flag Iris

Photo Credit: William Morgan, Obed Watershed Community Association

Broadleaf/Common Arrowhead (Sagittaria latifolia) Photo 15 – This perennial grows 1–4 feet tall, spreads through tubers, and is noted for its arrowhead-shaped leaves and whorls of threepetaled, white flowers.

PHOTO 15: Arrowhead

Photo Credit: Andrea Ludwig, The University of Tennessee