Welcome to the Chesapeake Ecology Center

The following chapter is excerpted from the newly released conservation landscaping primer entitled, Ecoscaping Back to the Future...Restoring Chesapeake Landscapes.. Click here to order your copy!

Rain gardens and xeriscapes often complement one another. If you create them together, you can even think of them as “yin and yang” gardening. Rain gardens are generally designed as shallow, saucer-shaped ponding areas planted with moisture-loving native plants; while xeriscapes (which are covered in the next chapter) can be ground level or mounded areas that tend to be drier and are planted with drought-tolerant native plants. Often, when grading is done to create one of these types of gardens, it presents conditions for creating the other type. For example, when a shallow depression is made to create a rain garden, the excess soil can be used to create a mounded xeriscape, which is a drier type of landscape arrangement. Leaving or creating depressions in the landscape promotes stormwater infiltration and reduces stormwater runoff.

Rain gardens are low-lying saucer-shaped garden areas that have absorbent soils which temporarily collect stormwater runoff, usually from a roof, pavement, or other impervious surface, and allow it to slowly percolate into the soil. This provides flood control, groundwater recharge, and water-cooling benefits, while the plants, soils, and associated microorganisms remove many types of pollutants—such as pesticides, oils, metals, and other contaminants—from stormwater runoff. A rain garden is one type of “bioretention” landscaping feature that is a depression into which water will flow.

Properly designed and maintained, rain gardens are attractive landscaping features that function like a natural moist garden, moist meadow, or light forest ecosystem. They can look as naturalistic or as formal as you like.

Our man-made system of curbs, gutters, and stormdrains quickly carries stormwater runoff directly to local streams, rivers, and the Chesapeake Bay without any natural filtering process. “We tend to think that large industrial polluters cause most water pollution, but this is not the case. We are the real culprits. CAStudies by the US Environmental Protection Agency have determined that up to 70% of the pollution in our surface waters is carried there by stormwater runoff. Some studies show that about 50% ofCAsuch pollution comes from individuals and homeowners, due to yard care, yard waste, and chemical pollution from household activities.”18

Rain gardens serve several important functions in conservation landscaping. Stormwater pouring off hot roofs, pavement, and other impervious surfaces is temporarily captured, cooled, and allowed to percolate into the ground. This provides flood control, groundwater recharge, and water-cooling benefits. Stormwater is able to reach a more healthy temperature for fish and other wildlife by the time it slowly makes its way to the Bay. In addition, this living filter system removes many types of pollutants from stormwater runoff. Nutrients such as nitrogen and phosphorus, which would otherwise contribute to algae blooms and other problems, are taken up by the plants in the garden. Sediment that carries pollution and buries plants and animals in the Bay is trapped before it can reach a waterway. Rain gardens also tend to become wildlife oases with colors, fragrances, and the sights and sounds of songbirds and butterflies regularly visiting your garden.

By creating a rain garden and keeping most of the rain that falls on your site contained on site—the way nature intended—you can help improve water quality in local streams, rivers, and ultimately the Chesapeake Bay. Rain gardens save water, reduce pollution, and help wildlife—all at the same time.

There is usually an existing drainage pattern for most landscaped sites (even if it is not very noticeable), and it may be best to go with the flow. Determine the direction of runoff and low spots where water collects, either by observing the flow of water during rainfall or by running hose water on the ground. If the low areas are away and downhill from your building foundation, they would be good places to locate rain gardens. If you have many low spots, you can choose those that are closest to the downspouts from your roof (but at least 10’ away from a building) or nearest to a paved driveway.

Many rain gardens are designed to contain runoff from a roof downspout. You can even bury an extension to the downspout and have it discharge right into the rain garden. For single-family homes, rain gardens are typically between 150 to 400 square feet. Any size rain garden is better than no rain garden, and the bigger the better. Ideally, all of the rainwater that falls on a piece of land would stay on-site and be captured by rain gardens and rain barrels.

• Different parts of your roof drain to different downspouts. Determine which part of the roof, and the square footage, will drain to each downspout, that will then drain to the area sited for your rain garden. Ideally, all downspouts and impervious surface runoff from driveways, etc., would be channeled to one or more appropriately sized rain gardens.

• For sandy soil, your rain garden should be 20-30% of the drain area. For example, if your roof and driveway measure 1200 square feet and all the rain from them will be used, your rain garden should be 20 to 30% of that, or 240-360 square feet (e.g. 10’ X 24’).

• For clay soil, your rain garden should be about 60% of the drain area. (Clay absorbs water very poorly; the varieties of rain garden plants that do well in clay take at least three years to get established. Soil replacement may be the best choice in clay soils.)

• If you improve your soil drainage and replace your soil with rain garden mix (50-60% sand, 20-30% topsoil, 20-30% compost), your rain garden should generally be about 20-30% of the square footage of your drain area.

Areas where standing water collects and doesn’t soak into the ground within a day or two have poor infiltration. This is likely due to either heavy clay soil or the compaction of soil, for example, during construction. Compaction of buildingCAsite soilsCAmakes the need for your rain garden even more critical.

As with most gardens, an important first step for rain gardens is to test the soil where you plan to install your garden. (See Chapter X “Resources” for information on soil testing.) Extreme pH, presence of a clay layer, or other surprises may need to be corrected. (See Chapter IV for additional information.) Amend the soil, only if necessary, based on conditions. If you are landscaping with native plants, this step may not be necessary. Remember, native plants are adapted to local soil conditions, provided the local soil has not been significantly degraded or changed.

It is a good idea to check the percolation of the soil in the area you have chosen for your rain garden. Dig a one-foot deep test hole and fill it with water. If water remains in it for more than 24 hours, you may need to amend the soil or choose another location. If standing water remains in a rain garden for more than a few days, it could become a breeding place for mosquitoes. The fastest breeding mosquitoes take at least four days to reproduce; therefore, this should be the outside range for water to remain standing.

Examine the slopes on your site, and be sure that any overflow water will be channeled away from structures or other sensitive features. Install rain gardens at least 10 feet from a building foundation to keep basements from getting wet. If you have a septic system, stay away from the drainfield. It doesn’t need any added water. Be sure that you consider where excess water will flow out of the garden in a heavy storm. Try not to direct too much water to the roots of trees that don’t like to live in wet soil; and don’t excavate too close to large trees, or you may damage their root system.

Before you dig in any area, be sure to call Miss Utility (1-800-257-7777, from 7 AM to 5 PM, unless it is an emergency, in MD, DE, and DC) no less than two business days ahead to locate any underground service lines or utilities that may be located where you intend to plant. (See Chapter IV for additional information.)

Site preparation may be a simple process or it can take more effort than planting the garden. First, define the borders and shape of your garden at the location you have selected using non-toxic spray paint or a garden hose. (See Chapter IV.)

Turf grass usually needs to be removed by hand, or with a sod cutter or bobcat, or it will need to be covered for several months until it is fully dead. Removing turf grass is a convenient way to start a rain garden, since it will help create a 3- to 6-inch deep depression needed to collect the runoff. Check the soil for obstructions such as asphalt, large rocks, or unusual soil texture (e.g. clay, sand, excessive compaction). A rock bar can be helpful in probing the soil. In some cases obstructions need to be removed or curbs need to be cut to allow water to flow into the garden.

Heavily compacted soil will need to be dug up and loosened to a depth of two feet, or amended for better drainage (see the formula above). Extra soil left over from creating the depression for the rain garden and amending the soil (if necessary) can be used in another part of your yard to create a mounded xeriscape garden.

Grade the surface of your prepared rain garden bed by creating a saucer-shaped depression. This will improve the effectiveness of the garden to hold and treat the runoff as it sheets across the surface of the garden. When preparing large gardens you may want to consider hiring a contractor with a bobcat to remove turf grass and shape the planting beds to create depressions and knolls or other pleasing contours to your property. Remember, driving any kind of heavy equipment on your land will compact the soil, and reduce its ability to absorb water. Be aware that extensive grading will require a permit.

If your land is sloped and all the water drains away (the condition for some of the rain gardens at the CEC), create terraces in the slope. Install berms on the down-slope side of each terrace to slow the water down and allow it to weep into the next cell. In larger rain gardens, berms such as biologs, made of coconut fiber, work well. Biologs will slowly decompose over several years and eventually become earthen berms. You may want to enlist the help of an engineer to help determine the number and spacing of the berms needed to control the water flow.

Site considerations: If you are mitigating a large area of impervious surface runoff with a rain garden, it is useful to have the water travel across a grassy swale and/or gravel bed area before it reaches the plants. The swale can help remove silt from the stormwater, and the gravel bed can help slow the water so that it doesn’t wash away the first plants as it reaches the garden. Install a permeable layer such as weed-blocker cloth under the gravel so that it doesn’t sink into the underlying soil.

A drain system may be necessary if a lot of water needs to be infiltrated or the soil contains clay. An under-drain system of gravel and perforated pipe (“French drain”) may be helpful. Rain gardens are sometimes constructed to absorb and filter a certain amount of rain, and the filtered water is then piped to another location through the underdrain system.

(Adapted from an illustration from the Wisconsin DNR and U. of Wisconsin-Extension)

Rain gardens can be installed in the front, back, or side yard. Design with the end in mind, and consider how the rain garden can be integrated into existing and future landscaping. Locate your rain garden to capture water from your rooftop channeled by downspouts, from other impervious surfaces such as driveways and sidewalks, or to capture runoff from lawn areas. Consider views from different vantage points, including inside the house or, for example, from a patio where you can take advantage of the sights and sounds of a wide variety of wildlife attracted to your native plants.

Once your planting bed is ready, it is time to plant. You can prepare a rain garden bed, then cover it with mulch, and as time permits, plant through the mulch; or, you can install the plants, then mulch. The choice is yours. Water plants as soon as you install them.

Choosing the appropriate native plants will help insure the success of your rain garden. Estimate how often the plants in each part of the rain garden will be inundated, and choose plants accordingly. Some plants are able to withstand moist and dry conditions, and they would do well along the drier edges of a rain garden. Be sure that the plants that receive the most water are well-adapted to those conditions. Consider the amount of light the plants will receive as well. Choose sturdy plants to be closest to the strongest flows of water.

There are many good references on which native plants will do well in a rain garden. Perennial native plants are recommended because they will be easy to care for, good for local wildlife, and provide lasting beauty year after year. (See the lists of suggested plants on the following pages and in Chapter VIII.)

Spring and fall are good planting times for your rain garden. As you design the garden, determine the height of plants you would like, and consider adding some trees and shrubs to the design. The look of several different plant layers is more natural, and larger plants absorb lots of water.

After the garden is installed, top-dress it with 2- to 3-inches of organic mulch, such as leaf mulch or composted wood chips. Mulch is especially useful in rain gardens since it helps to retain moisture, suppresses weed growth, absorbs some of the pollutants in the stormwater runoff, and provides an attractive top dressing. Be sure not to bury perennials or the trunks of trees or shrubs with mulch. Weed regularly. A nicely prepared rain garden is a great place for invasive plantsCAto start growing in the newly loosened soil.

Water your newly planted rain garden regularly until it becomes established, unless Mother Nature does the job for you. Aside from watering, the primary maintenance needed for a native plant rain garden will be weeding and the addition of mulch to the surface about once a year. Remember, the lower maintenance requirements of native plants does not mean NO maintenance.

Over time, adjustments may need to be made to optimize the effectiveness of your rain garden. Plants may need to be relocated and/or water flow adjusted, for example, by adding, modifying, or moving earthen, biolog or gravel berms.

The SRA Rock ‘N Rain Garden infiltrates stormwater from the upper parking lot at Adams Academy at Adams Park Middle School. This rain garden was adopted by the Severn River Association (SRA), who assisted with the installation; and they help maintain the garden. Rock ‘N Rain refers to the rock berms placed at intervals to break the water flow draining from the adjoining parking lot. (Two other CEC rain gardens use biolog berms to break the water flow.)

In the 1950s—when Adams Academy at Adams Park (formerly Adams Elementary and The Learning Center) was built—the Anne Arundel County School System cut a 120-foot long swale and lined it with asphalt to drain the water as quickly as possible off the parking lot to nearby College Creek. Today’s best management practice is to do the opposite, that is, to break the water flow, allow it to cool and soak into the ground to recharge groundwater, and allow the water to flow through native vegetation to remove pollutants. After the difficult process of removing asphalt—by hand and with a bobcat—from the 1,800-square-foot area, we amended the hard clay soil with 1/3 sand, and 1/3 top soil. John Flood rototilled the planting bed. (Other than this garden, we’ve purposely avoided amending the soil. For other gardens, we’ve used what is in place and installed plants appropriate for conditions.)

Carol Jelich designed the garden. Members of the Severn River Association, Adams Academy students, and other volunteers planted the garden. The garden has flourished since its installation in the spring of 2003, and includes the following plants:

• Tussock Sedge (Carex stricta)

• Switchgrass (Panicum virgatum)

• Foxglove Beardtongue (Penstemon digitalis)

• Narrow Leaf Mountain Mint (Pycnanthemum tenuifolium)

• Swamp Milkweed (Asclepias incarnata)

• Swamp Sunflower (Helianthus angustifolius)

• Joe-Pye Weed (Eupatorium fistulosum)

• Virginia Wild Rye (Elymus virginicus)

• Turtlehead (Chelone glabra)

• Inkberry (Ilex glabra)
• Sweet Pepperbush or SummersweetCA(Clethra alnifolia)

• Silky Dogwood (Cornus amomum)

• River Birch (Betula nigra)

• Red Chokeberry (Photinia pyrifolia) or (Aronia arbutifolia)

Case study: Biolog Rain Garden (2,000 square feet)

At the lowest point of the front parking lot, near the entrance to Adams Academy at Adams Park Middle School grounds, the Anne Arundel County School System cut a 100-foot long swale and lined the area with asphalt in the 1950’s. The asphalt swale sloped toward College Creek in order to shoot water from the parking lot as quickly as possible to the creek.

Today, we recognize that the opposite process is preferable for protecting the Bay and its tributaries. Instead of an asphalt chute, we designed this rain garden to break the water flow, allow it to cool and soak into the ground to recharge groundwater, and allow the water to flow through native vegetation to remove pollutants. Like the Rock ‘N Rain Garden, the Biolog Rain Garden saves water, reduces pollution, and helps wildlife—all at the same time. The rain garden infiltrates stormwater runoff from the front, lower parking lot. Named for the devices used here to slow the flow of water, this garden uses biologs made from rolled, biodegradable, coconut fiber.

Rob Schnabel, restoration specialist with the Chesapeake Bay Foundation, designed the garden. The asphalt was removed with the help of a bobcat, the assistance of the Anne Arundel County Stewardship Workgroup, and many others. John Flood rototilled the garden; and Vince Leggett, Bill Sanders, and Zora Lathan planted the garden.

• Cattail (Typha latifolia)(native, but very aggressive)

This rather large rain garden is an attractive addition to our collection of gardens. Unlike the other four rain gardens at the CEC, the Sweetbay Magnolia Rain Garden has a wide frontage on the front parking lot and several curb cuts to allow runoff to enter its bioretention area. It also features a low berm on the back side and ironstone boulders (for visual interest) at the downhill side.

Like our other rain gardens, this garden provides not only wildlife benefits with its attractive flowers and inviting fruits, it also contributes water quality advantages through its filtering action. Stormwater is purified by the plants and soil in the garden, and it is allowed to soak into the ground and recharge groundwater. The addition of this rain garden significantly lessens the impact on the Biolog Rain Garden, which is occasionally inundated with water since it is at the lowest point on the grounds in the front of the school.

Darnell Hall, Marvin Redding, Bill Sanders, Vince Leggett, members of the Anne Arundel County Stewardship Workgroup, and others, assisted Zora Lathan and Thistle Cone in preparing the site. Adams Academy students, Naval Academy Midshipmen, Anne Arundel Green Party members, and other volunteers planted the garden. Zora Lathan designed the garden, which includes:

The Highbush Blueberry Rain Garden provides not only delicious blueberries for wildlife and humans, but includes other moisture-loving plants with aesthetic and habitat benefits. The garden was tricky to install due to the asphalt swale that was there previously. A large-sized area of asphalt was removed with the help of a group of Midshipmen from the Naval Academy. However, since this was too much for volunteers to do entirely by hand, a bobcat was used to remove the bulk of the material. Students from Adams Academy, as well as other volunteers, planted this garden.

Three biologs (rolled, biodegradable, coconut fiber) were added to the Highbush Blueberry Rain Garden to help slow the flow of water down the slope. These provided separate sections for plantings as noted in the plant list below. The garden is the sole filtration device for the entire back parking lot for Adams Academy at Adams Park Middle School.

This site was designed by Adams Academy students with assistance from Zora Lathan and Thistle Cone.

(1st section)
• Foxglove Beardtongue (Penstemon digitalis)

• Switchgrass (Panicum virgatum)

• Highbush Blueberry (Vaccinium corymbosum)

(2nd section)
• Highbush Blueberry (Vaccinium corymbosum)
• Black Chokeberry (Photinia melanocarpa) or (Aronia melanocarpa)

• Button Bush (Cephalanthus occidentalis)
• Winterberry (Ilex verticillata)

• Pawpaw (Asimina triloba)

(3rd section)
• Highbush Blueberry (Vaccinium corymbosum)
• Silky Dogwood (Cornus amomum)
• Inkberry (Ilex glabra)
• Northern Bayberry (Morella pensylvanica) or (Myrica pensylvanica)

• Cinnamon Fern (Osmunda cinnamomea)

(4th section)
• Highbush Blueberry (Vaccinium corymbosum)
• American Cranberry (Vaccinium macrocarpon)