Despite the
proven environmental benefits of rain gardens, many people are reluctant to use
them because they can be unattractive. But a close examination of the
relationships between hydrology and vegetation in rain gardens suggests a
solution for improving their looks and their function. Rather than think of rain
gardens primarily as wet environments, we should design them as dry environments
that experience only brief wet periods. This shift in thinking increases
opportunities for ornamental planting without sacrificing environmental
performance.
Rain gardens are
one of the most frequently cited and promising strategies for managing
stormwater responsibly, and because of the ubiquitous presence of impervious
surfaces, these systems can be used on virtually any type of site. Rain gardens
come in many forms (and go by many names, such as bioswale, bioretention, and
bioinfiltration), but for the purposes of this article, the term “rain garden”
is essentially meant to describe a shallow depressional area designed to use the
natural capacities of soil and vegetation to retain, cleanse, and infiltrate
stormwater.
The Pros of the Rain Garden
Infiltration-based stormwater management strategies,
such as rain gardens, are crucial to downstream ecological health. Every parcel
of land interacts with water. If water infiltrates, it can be used as a resource
to nourish plants and replenish aquifers. When water runs off driveways, roads,
and compacted soils, however, it becomes a liability, carrying sediments and
pollutants downstream. The USEPA states that nonpoint sources, such as
stormwater runoff from an urbanized landscape, are the leading causes of urban
stream water-quality problems. To help, many designers are looking toward
landscape solutions to water-quality and flooding problems, altering land
surface functions to manipulate the way in which the land captures and absorbs
stormwater.
Many other stormwater management techniques address
only a portion of the problems caused by stormwater runoff. Rain gardens,
however, have the potential to solve all the problems of stormwater runoff
before they occur. Like other infiltration-based strategies, rain gardens
mitigate the hazardous stormwater runoff aspects of development by decreasing
peak flows responsible for storm surges and flooding. They reduce pollutant
discharges, minimize streambank erosion, replenish groundwater, and restore base
flows and aquatic habitats. Rain gardens can also offer real development cost
savings by eliminating expensive belowground stormwater infrastructure in favor
of combining stormwater management with ornamental landscapes.
Rain gardens can
also help with temperature pollution problems. In a completely natural setting,
water enters a stream or other water body almost entirely through groundwater
that provides steady flows at low temperatures. But when development introduces
impervious surfaces, higher temperatures often result as the runoff washes over
those warmer surfaces. Higher temperatures, in turn, cause the loss of a diverse
system of aquatic biota in receiving streams, ponds, and rivers that are
sensitive to the warmer water.
Because of
effects like these, traditional urban stormwater management has always viewed
water as a burden on the landscape. Water is typically taken away through
channels and pipes as quickly as possible to avoid flooding on site. But water
and ecological quality can be improved when water is allowed to infiltrate,
using it as a resource where it falls.
The (Perceived)
Cons of the Rain Garden
Attractive and
functional rain gardens are the exception, not the rule. Most rain garden
installations do not include those elements that are culturally accepted as
beautiful, like lush green lawns, flowering vegetation throughout the growing
season, clean lines, and a maintained appearance. As a result, people see these
landscapes as cluttered, unkempt, and unmanaged. Perceptions are just as
important as environmental performance. If rain gardens are not perceived as
attractive, cared-for environments, they will not be adopted during the design
phase or managed after installation. Although preferences vary from person to
person, a common theme for all is an appearance that communicates care to the
viewer.
People design
and manage landscapes as a reflection of who they are and how they want to be
perceived. Too often, rain gardens planted with water-loving species appear
unkempt and abandoned. Individual plants are often stressed and weak,
particularly in areas that experience hot and dry summers. The negative
perception of their ornamental character is an obstacle to their use in both new
and retrofit development projects. Because many rain gardens do not come close
to the ornamental quality of more traditional garden landscapes (especially from
the perspective of the general public, who may be largely unaware of the
environmental benefits), they are not a viable option in visually prominent
areas of a site such as in parking lots or at site and building entrances. In
high-visibility areas, environmental performance alone is not enough. Because
one cannot see the ecological functioning of the root systems, water
infiltrating through soil, and wildlife’s benefits from the landscape, it is
difficult to include an ecological assessment in our judgment of landscape’s
appearance. So rain gardens are not used, or are relegated to areas of the site
where their messy appearance will not offend.
Shifting the
Thinking
Rain gardens
collect and temporarily store rainwater; therefore, it is understandable that
many perceive them as wet environments and plant them with species adapted
accordingly. But water-loving plant species are not a good match for an
infiltration garden in areas that commonly experience long, dry periods between
rainstorms—which many areas of the country do during the summer months. This
effectively limits water at a time when the plants need it the most. These
moisture-loving plants often become stressed during this extended dry period
and, at best, enter dormancy until the rains come again or, at worst, simply
die. Supplemental irrigation can solve this problem, but this practice runs
counter to the rain garden philosophy that is otherwise respectful in the way it
manages water as a precious and limited resource.
Any single plant
species has a limited range of environmental conditions for which it is ideally
suited, and a slightly broader range that it will tolerate. One of the
challenges in rain garden design is creating a hydrology and soil moisture that
is preferred by a broad enough range of plants to achieve performance and
aesthetic objectives. A landscape with plants pushed to the limits of their
tolerated environmental conditions will not flourish and will look like a
collection of unkempt weeds.
A dry rain
garden regime with temporary wet periods during and shortly after storms is
significantly more hydrologically stable from a plant perspective (and within
the preference zone of many ornamental plants) than a wet rain garden regime
that regularly experiences long, dry periods between rainstorms. Rain gardens
planted with attractive, drought-tolerant species that thrive in dry summers and
easily manage brief rainy periods can be lush, can include wildflowers, and can
look like ornamental gardens, all without compromising their stormwater
management functions.
Dry Design: A Case
Study
A new 200-acre,
mixed-use project in Lynnfield, MA, is now in the process of designing rain
garden for the large ring of parking surrounding the retail and residential
blocks. A parking lot is one of the most challenging environments for an
aesthetically pleasing rain garden—and a parking lot in the variable, and often
severe, climate of New England is even more difficult to manage. The following
are some of the biggest obstacles the project team faces.
Water
Quality. The first flush of
runoff generated in the summer can be extremely hot and can carry oil and other
pollutants. In colder climates like that of Massachusetts, deicing salts and
their persistence in the soil are another concern. The first flush of pavement
runoff carries a relatively concentrated amount of oils and other contaminants
that accumulate on the surface of the pavement between storms. The combination
of heat and pollutants severely compromises the quality of water directed into
parking lot islands.
Tight
Space Constraints. The
geographical confines of a parking lot can also be a challenge, particularly in
retrofit projects where layout revisions are rarely feasible. The ratio of paved
areas to landscaped islands may make the storage of even small volumes of water
difficult or impractical. This can also make the management of the first flush
difficult, as this volume of water may overwhelm the rain garden, adding more
stress to the plants and diminishing their overall health and
attractiveness.
Soil
Compaction. One of the
greatest stressors on developed land is the disturbance of native soils during
construction. Often, heavy equipment causes severe soil compaction, changing the
soil structure drastically and preventing soils from functioning as they once
had. Soils subject to construction are generally highly compacted, low in
permeability, and poor in structure, and as a result infiltrate at slower rates
than before they were disturbed. In addition, soil compaction hinders plant
growth, because the soil has less room to replenish water, air, and
nutrients.
Visual
Prominence. Parking lots
are often one of the most visible areas of a site and part of the visitors’
arrival sequence. The aesthetic concerns of the developer, again, often take
precedence over environmental concerns. This Massachusetts development will form
the center of an affluent residential community, so an attractive appearance is
of utmost importance.
Anatomy of a Dry
Rain Garden
To solve these
issues, and to find the most viable and sustainable locations and conditions for
the rain gardens, the designers are researching, collecting, and testing a
number of site factors:
Required
Design Data. Two critical
pieces of information from the site’s existing conditions are required to
effectively design a rain garden: soil infiltration rate (saturated hydraulic
conductivity) and the elevation of seasonal-high
groundwater.
The soil
infiltration rate can be estimated based upon the soil type during preliminary
design, but once the locations of the rain gardens are proposed, testing should
be performed in each rain garden location to establish a more precise rate.
Different soil strata may have different rates. At the Lynnfield site, for
instance, some areas include a large amount of ledge, meaning water would not
infiltrate in those spots (and, thus, rain gardens wouldn’t work in those
areas).
It is important
for the assumed infiltration rate to be based on the most limiting soil layer
below the rain garden basin. A double-ring infiltrometer test is a very accurate
and widely used test to establish infiltration rates under saturated
conditions.
Seasonal-high
groundwater elevation is also needed to ensure that a minimum separation is
provided between the basin bottom and groundwater elevation to prevent
groundwater contamination. Soils are the primary method for filtering and
purifying contaminants. The microorganisms, clay, and organic particles that are
naturally found in virtually all soil types interact with and treat the pollutants commonly associated with
vehicular pavements. Because groundwater levels fluctuate throughout the year,
knowing the seasonal-high elevation is particularly important to ensure proper
treatment of these pollutants when groundwater is at its highest point. The
project team used such cues as soil reactions to high water and changes in soil
color to determine the high elevation and where the rain gardens would be best
suited.
In some cases,
groundwater-mounding analysis may be needed. When water infiltrates through the
soil and reaches the groundwater elevation, a mound of water forms before
equilibrium is reestablished. This phenomenon can also create a potential
water-quality concern if the shortened distance between the bottom of the rain
garden basin and the high groundwater elevation is less than
required.
First
Flush. The first flush of
runoff from pavement surfaces carries elevated levels of oils and other
pollutants that accumulate between rainstorms. Often overlooked is the high
temperature of the first flush during warm, sunny weather. In northern areas
like Massachusetts, the winter application of salts and sands presents
additional water-quality concerns. The poor water quality associated with the
first flush can be extremely stressful on rain garden plantings, pushing
ornamental plants to the limits of their tolerance zone. For this reason, the
most effective way of managing this water is to limit its exposure to the
plants’ root zone.
An infiltration
trench (potentially top-dressed with more ornamental stone) serves as an energy
dissipation and bypass strategy to keep the first flush away from the ornamental
landscape. As stated above, this does not create a water-quality concern as long
as there is adequate separation from the bottom of the trench to groundwater.
The infiltration trench can also serve as a trap for sands and other sediments.
When sediments are sequestered in this limited and easily accessible area, their
removal is a relatively easy part of a comprehensive maintenance program. In
Massachusetts, infiltration trenches are required for pretreatment. The project
team would have included them in any case, however, because they further
safeguard the rain garden from damage.
Ponding
Depth and Drain Time. Accounts
vary for appropriate ponding depth and drain time. Many seem to push the limit
of what individual species will tolerate, rather than adhering to what is within
their preference zone. To maintain a healthy stand of vegetation, adequate soil
aeration must be maintained. Dry rain garden drain time should be limited to
approximately 12 hours (preferably less) and ponding depth to approximately 6
inches. It may be feasible to increase the ponding depth in proportion to
decreasing drain time.
A rain garden
design should also take the differing infiltration rates of the soil layers into
account. The design team in Lynnfield specified the infiltration rate of the
topsoil as it compared to the lower layers to be sure infiltration and drain
time were consistent throughout.
This drain time
and ponding depth is not as limiting from a stormwater management perspective as
it may first appear. A rain garden that retains a relatively small volume of
water can have a surprisingly significant impact on a site’s hydrology. Small
rainfall events (approximately 1.5 inches or less) are the most common in many
parts of the country. As long as the rain garden can infiltrate most or all of
these smaller rainstorms, a significant percentage of yearly precipitation can
be returned to the groundwater system. Green roofs and permeable pavements are
two other stormwater management systems with limited retention volumes that also
demonstrate the importance of being able to manage small storm
events.
When space
constraints are particularly tight, or when limited infiltration rates require a
larger retention volume for the area available for the rain garden, a subsurface
stone reservoir can be incorporated into the rain garden design. Rather than
retain larger volumes of water at the surface, creating potential conflicts with
the capacities of the vegetation, the water is stored belowground in a
geotextile-lined reservoir filled with open-graded stone. Open-graded stone
(crushed stone with few or no fine particles) has a void space of approximately
35 to 40%. This is a very effective strategy, particularly in areas with low
groundwater elevations. The project team has designed such a reservoir for the
Lynnfield site, but it remains to be seen if it will be necessary, as the
infiltration rates may be fast enough.
Topsoil. To ensure adequate infiltration, the project
team is also specifying that the topsoil layer meets or exceeds the infiltration
rate of the subsoil. To meet the ponding depth and drain times described above,
a topsoil with high sand (60 to 80%) and limited clay (10 to 20%) content is
required.
Organic matter
is also a critical component of a healthy topsoil. Organic matter contributes to
soil aggregate formation, influences the amount of water available to plants,
stores nutrients, and sustains the growth of soil microbes. Aggregate formation
is particularly relevant to infiltration, as the spaces between aggregates have
a significant influence on soil permeability. The amount of organic matter
should be between 5 and 10% of the topsoil.
The high
percentage of sand, in addition to providing good drainage with high
infiltration rates, also provides good aeration to the plant root zone, which is
critically important to reestablish after the storm and temporary inundation
period has passed. This creates a natural fit between soil health, stormwater
management objectives, and ornamental plant requirements.
Once the design
team has set such criteria for the composition of the topsoil, they can leave it
to the contractors to determine if the onsite soil matches the criteria and can
be reused. If it doesn’t, the contractors will need to amend the topsoil, which
can add to the costs of the project.
Grass
Root Systems. From a soil
health and permeability perspective, grasses are the most important component of
a rain garden planting. Most of the biomass of grasses is belowground in the
roots, even at the height of the growing season. Approximately one-third of a
grass root system dies annually, which helps to maintain a good soil structure
and porosity (even through slowly accumulating sediment) by providing a
continuous source of organic matter. The death and decay of these extensive root
systems also contributes to an effective cycling of nutrients within the soil
system.
Grasses can be
categorized into two groups: warm-season and cool-season species. The
differences between these two groups relate to their differing processes of
photosynthesis. Warm-season grasses are very efficient at converting light
energy into chemical energy (sugars) at higher temperatures and are very drought
tolerant. Warm-season grasses thrive and grow during the hottest and driest
parts of the year. Cool-season grasses begin to put on new growth earlier in the
year, when soil temperatures are cooler and when there is more available soil
moisture. Although cool-season grasses can go dormant during the hotter, drier
summer months, they are green when the cool, moist weather returns in
fall.
In Lynnfield,
the design team is using both warm- and cool-season grass varieties. Switchgrass
and Little Bluestem, in particular, will dominate the garden in the heat of the
summer, because they stand up well to hot, sandy, dry environments. Although
those grasses are slow to establish until temperatures warm up, the design also
includes plants like Canada wild rye, which will bring some green to the site
earlier in the season.
Planting
Design. Drought-tolerant
native grasses, however, can look messy or unkempt in comparison to an orderly,
planned, conventional landscape. Although most people care about improving the
environment, they generally will not do so at the expense of the proper
appearance of their own landscape—especially on a retail site. If a landscape
meets the conventional expectation of a neat, tidy, and aesthetically pleasant
garden, one may assume that the owner cares about the quality of the experience
of the passersby.
Identifying the
important symbols in the landscape that communicate an aesthetic of care will
help make sure a rain garden is appreciated as an ornamental landscape. The
research of Joan Nassauer, FASLA, has suggested that certain landscape “cues to
care” are essential elements that communicate neighborliness, intent, and
stewardship of any landscape. These cues are symbols to the onlooker that the
land is being cared for, that there is human intent in the landscape, and that
this landscape is part of a plan.
In Lynnfield,
the design team has incorporated a solid stand of healthy, vigorously growing,
bunch-forming grasses and wildflowers to communicate an aesthetic of care. The
selected plants will grow no more than 3 to 4 feet in height, or short enough
for people to see clearly across, and a vehicular guardrail creates a
well-defined edge to the rain garden perimeter, enhancing its well-maintained
appearance.
The sidebar
describes several species that can become the backbone of an ornamental rain
garden landscape. All of these species have been selected for their ability to
withstand the tough conditions associated with parking lot environments. All of
them are commonly available in the landscape trade because of their ornamental
character and long seasonal interest. They are all tolerant of drought and salty
soils.
Construction. The soils under the rain garden must be
protected from compaction during construction to preserve soil structure and
infiltration rates. This is true for general site construction work, but also
specifically for the construction of the subsurface stone trench. A sand layer
placed below the stone trench helps to dissipate the energy of the stone being
dropped into place during construction, which might otherwise cause soil
compaction at the surface.
Rain garden
construction and planting should occur after adjacent contributing areas are
stabilized. If not, sediment can become a problem, particularly in the
subsurface stone trench, where it is not easily removed.
Establishment
and Maintenance. Rain garden
plantings can be installed as live plants or as seed. Planting live material can
give an instant ornamental effect to rain gardens when necessary. Seeding,
although much less expensive, is a slow method of establishing plants, which
take from two to three years to reach maturity. The Lynnfield site incorporates
a limited number of live plants for an instant effect but will be seeded over
time to balance the costs of adding live plants with the immediate appearance of
the garden.
One important
consideration when seeding is the management of annual and perennial weeds. If
weeds are allowed to get a significant foothold in rain gardens, they can become
extremely difficult to remove. Effective strategies to minimize weed competition
during seed establishment include the use of topsoil that is free of weeds and
weed seed, and seeding a nurse crop. A nurse crop consists of annual species,
such as annual rye and oats, which grow quickly to stabilize the soil and reduce
weed establishment without competing with the other grass and wildflower
seedlings. Applying an erosion control blanket provides further protection from
volunteer weeds, provides additional soil stability, and helps to retain soil
moisture during seedling establishment.
As for
maintenance, for the first full growing season the rain garden should be mowed
approximately once a month to maintain a height of 6 to 12 inches. The mowing
prevents weeds from setting seed, and it allows sunlight to reach the seedlings
that don’t grow much higher than 6 inches in the first growing season. Avoiding
shrubs also helps ease the mowing process. In Lynnfield, for example, the design
includes few shrubs and consolidates the live plantings to the ends of the
garden so maintenance crews do not have to worry about mowing over seeded and
live areas.
Long-term
maintenance consists of occasional sediment removal from the gravel trench and
annual mowing of the vegetation (burning is actually a more effective management
technique, but it is not typically a viable option). Mowing the previous year’s
growth down to the ground clears the way for the current season’s growth to
begin neatly and cleanly, and it also keeps weeds under control. Over time, a
thatch layer may develop at the soil surface that keeps the wildflowers from
self-seeding effectively, slowly leading to the dominance of the grasses. This
has no effect on the rain garden’s performance, but to keep the balance from an
aesthetic point of view, new wildflower plantings may be required every few
years.
Making Rain Gardens
Work for You
By rethinking rain
gardens as primarily dry environments, a stronger and more resilient system of
relationships can be established between vegetation, soil, and environmental
performance. This shift opens up new possibilities for incorporating ornamental,
attractive stormwater management systems in a variety of site locations and
regional climates. As more rain gardens are designed and implemented
successfully from both an aesthetic and environmental performance perspective,
we will be able to establish a positive standard that becomes the “norm” for
sustainable stormwater management.