This post, as promised, presents a quick overview of the various plants used in bioswale and rain garden environments. It is not as simple as just throwing a few water tolerant plants in the ground. Careful plant choice and placement play key roles in successful “wet” landscapes.
Plant selection for these projects is driven by several key factors including the following:
Obviously, the basic site conditions play a huge role. Factors like sun exposure, soil depth, physical and chemical properties and moisture holding capacity can vary, so need to be understood for successful plantings.
What is the intended function of the project? For many projects, the landscape’s performance, including infiltration, pollutant removal and evapotranspiration rate will determine its success.
But, there can also be safety issues, which may require added protection such as surrounding hedges. Finally, aesthetics play a role since many working landscapes sit in neighborhoods and other public areas. While visible, they can seen as an amenity, and even provide some recreational opportunities.
No landscape is going to be maintenance free; so long term needs should be studied. This is one area where the plant material choice can have dramatically different cost impacts.
Finally, recognize each site’s natural water regime. Check the depth, frequency and duration of soil saturation, which will vary daily, seasonally or annually. For instance, Portland, Oregon, is considered a “wet” climate, but the summer is extremely dry. Plants in these urban, constructed wetland must survive extreme variations. A similar garden in Atlanta, Georgia, or Columbus, Ohio, would get significant summer rain.
Actually every rain garden or bioswale has its own “zones” that have different requirements, according to the Virginia Department of Forestry’s Rain Gardens Technical Guide. The guide points out that the center, and deepest, part of the garden best grows the very wet to wet-loving plants. Meanwhile, the middle of the garden’s side takes wet to dry plants, while the upper rim takes drier types of vegetation.
The guide lists other factors affecting the choice of the plants for rain gardens:
• Decide on objectives, such which wildlife you want to attract, then decide on the varieties you would plant to attract those species. [Refer to reference list below]
• The rain garden’s location affects use of fruit-bearing plants and trees, since if it is near the driveway or walkway, it could create messes and maintenance issues. Trees next to a power line or too close to a house are not good choices.
• If the bioswale are near enough to receive runoff from a road that gets chemical treatments for ice in winter, choose plants that tolerant salt.
And then there is actual selection of species and varieties…and a common question, should we plant natives compared to introduced, commercial varieties?
Why Native Plants?
The majority of the web sites that deal with bioswales or rain gardens are also now recommending using natives. So, why is this the accepted trend?
As Withrow-Robison and Johnson point out in the OSU publication Selecting Native Plant Materials for Restoration Projects, “selecting appropriate plant materials for restoration projects helps make any of these projects more successful. They state that, “‘appropriate’ means choosing species that are suitable for the site, are grown from locally adapted sources, and have a solid genetic composition.” In many cases, this leads to using native species.
So, what is a “native plant?” Most definitions say a “native plant” occurs naturally or has existed for many years in an area, and they can be trees, flowers, grasses or any other plants. “Local adapted sources” can mean those plants have adapted to a very limited range, living in unusual environments, under very harsh climates, or growing in unique soil conditions. Yet, while some had a very limited range, many others live in diverse areas or easily adapt to different surroundings.
So, to summarize the strengths of using natives in bioswales and rain gardens.
• First, native plants are better adapted to the local climate. Once planted and established, do tend not to need extra water or fertilizer.
• Secondly, many are deep rooted, allowing them to survive droughts. This is especially important in the Northwest, where the normal wet weather can disappear for several months during the summer months.
• Third, native plants provide habitat and food for native wildlife and, are thus very attractive to the diverse native bees, butterflies, beetles and birds, all important pollinators.
These plants, which include many wildflowers, sedges, rushes, ferns, shrubs and small trees, grow on the edges of natural wetland, also have root systems that enhance infiltration, moisture redistribution, and diverse microbial populations involved in biofiltration.
A key point to remember is that rain gardens, unlike a water garden, will be dry most of the time. Plant selection should include those that tolerate short periods of inundation, but not require constant standing water. In areas that will have moist, well-drained soil, select plants with moderate moisture requirements. For drier sites like the edge of your rain garden, plant species with low or moderate moisture requirements.
Meanwhile, any perennial plants need to be hardy in your growing zone.
Each region has growers of appropriate native and related plants for rain gardens and bioswales.
In fact, some successful growers will collect seed their own seed from the local area. For example, one Oregon native plant producer has collected seed for plants such as snowberry (Symphoricarpos albus), salmonberry (Rubus spectabilis) and twinberry (Lonicera involucrate) in the immediate area, using on a couple of mother plants for each. Another grower collects all her Pacific dogwood (Cornus nuttallii) from two trees growing at a nearby park.
See references below for several recommendation lists.
These three urban plant technologies are just part of a wider set of alternative Best Management Practices (BMPs). Many are simple, practical designs, but provide effective storm water management. Some even add aesthetic enhancements to the urban, suburban, and rural landscapes. They can be cost effective to build while providing long-term sustainability for city infrastructure and conservation of a city’s water resources. These include filter strips, grassed swale, green roof, and infiltration basin, planters and trenches.
So, as the cost savings are identified, the demand for specific plant materials should increase. At this point, the trend seems to be moving toward regionalized, native plant materials. Since there are a number of operations already propagating this niche, they may have the best opportunity to benefit from this particular green movement.
The following references are available online and have been updated relatively recently, so they contain more current research and data regarding various plant choices.
• Rain Gardens Technical Guide Virginia Department of Forestry
• Selecting Native Plant Materials for Restoration Projects by B. Withrow-Robinson and R. Johnson, OSU publication EM 8885-E, November 2006.
• Plants for Stormwater Design
Native plant database and suppliers directory for North America.
• Rain Gardens Technical Guide – Virginia Department of Forestry
900 Natural Resources Drive, Suite 800, Charlottesville, Virginia 22903
Phone: (434) 977-6555 – Fax: (434) 296-2369
VDOF P00127; 05/2008
• Brooklyn Botanic Gardens, Rain Garden Plants.
This web site offers regionalized lists of suggested plants for rain gardens. Not as extensive as other sites, its easy to use breakdown is a good starting place in identifying plants for an effective design palette. www.bbg.org/gar2/topics/design/2004sp_raingardens.html
• 10,000 Rain Gardens (www.rainkc.com) has an extensive site that features a diverse list of plants for rain garden situations. It also has a search feature that allows criteria selection from five categories, so a nursery could focus first on what it is already growing, expand to closely related varieties, and then look for new opportunities that would fit within existing production systems.
• Bluestem Services: (www.bluestemservices.com) Has numerous plants lists, but two feature nearly 100 plants for rain gardens and wetlands.