Raingardens

One of the goals of the Hinkson Creek Watershed Restoration Project is to encourage landowners to create raingardens on their property by providing education and cost-share incentives. For information on what a raingarden is and how to build one, read the guide below or some of the reference material provided at the end of the article.

On April 9, 2005, the 40 attendees of the Hinkson Creek Watershed Restoration Project's raingarden workshop dug, mulched, planted, and filled a raingarden for the kids at Rockbridge Elementary School in an hour and a half.

Most of you will not have 40 people helping out on creating your raingarden, and it probably won't be finished within 2 hours. But it can be done in a weekend, and it will start functioning immediately afterwards.

Raingardens! A How-To (and Why) Guide

In order to protect and restore water quality, the EPA distributes “319” grants to groups interested in addressing stream degradation. Efforts are now underway to heal some of the impaired urban watersheds by landscaping with native plants and creating raingardens (among other activities). The Hinkson Creek Watershed Restoration Project, situated in Columbia , endeavors to repair some of the damage of urbanization through physical restoration as well as educational activities.

Problems with Stormwater Quantity

Historically, precipitation falling on parcel of land in Missouri would be first met with a forest canopy or the lofty vegetation of a tallgrass prairie. Nowadays, in some urbanized areas of the Hinkson watershed, for example, a raindrop may be as likely (36%) to be met with pavement than tree or plant. In presettlement times, a raindrop may also make contact with mid- and low level vegetation, followed by a layer of leaf/grass litter, which would transition into an organic horizon within the soil.   Each raindrop that is intercepted by vegetation loses its energy that may otherwise cause erosion. In contrast, the urban raindrop meets no such resistance, and often flows unimpeded from its initial contact with the pavement into the gutter, and then to stream.

This lack of resistance, or residence time, within the upper portions of a watershed translates to more water spilling into streams within a shorter period of time, rather than recharging groundwater. This is often referred to as making creeks more “flashy”, in reference to their ability to cause flash floods.   Developments may increase surface flow volumes up to 43%, and reduce subsurface absorption to 32% (50% subsurface absorption in forest areas)   (Prince George’s County, 1999). The effect of the greater impervious cover is to cause urban stream channels   to erode, causing the sloughing of trees into the channel, which often jam culvert pipes and bridges, causing increased local flooding. Less noticeable is the reduced amount of baseflow during times when it is not raining, meaning that once perennially-flowing urban streams tend to dry up because of lack of seepage from stored groundwater.  

  …And Quality

A “first flush” event is defined as the first half inch of runoff from an impervious surface, and is expected to carry with it most of the pollutant load associated with stormwater. In terms of a typical storm, the “first flush” represents only a small portion of the quantity of water, but a greater percentage of the quantity of pollutants (Prince George’s County, 1999) . The individual pollutants in this “first flush” can vary widely depending on their original source, but can be generally categorized into a few broad categories:

Sediment is the most common pollutant in stormwater runoff.. Sediments enter streams by erosion from new construction sites and other areas in which the topsoil is disturbed.   Excess sediment turns streams cloudy, clogs fish gills, decreases available spawning areas, and reduces aquatic insect habitat. Other pollutants tend to attach to soil particles.

Oils and greases originate from oil spots on parking lots and the overflowing grease containers of restaurants. Oils can coat fish gills, and restrict the amount of oxygen entering the water from the surface.  

Metals such as lead, copper, zinc and cadmium, are commonly found in urban runoff, originating from scrap yards, paints, and auto emissions. Metals can concentrate in bottom sediments, impacting the reproductive cycles of bottom-dwelling fish species.

Nutrients such as phosphorus and nitrogen are needed by plants, but high levels of these nutrients within streams can be detrimental to aquatic life. Fertilizers, lawn clippings, food wastes, and animal feces can deplete the oxygen in the water of slow moving streams as they decay.    Excess nutrient levels can over-stimulate the growth of algae, which can also lower dissolved oxygen levels when it dies.

Toxic substances such as insecticides and herbicides are particularly dangerous to the aquatic environment. Incorrect or excessive application of these chemicals, or application of these chemicals shortly before a storm, can result in toxic runoff from agricultural lands, golf courses, and residential lawns. Initial investigation into the sources of Hinkson Creek problems revealed some toxicity was caused by improper storage of pesticides on uncovered lots.

Fecal coliform bacteria in water may indicate the presence of pathogenic (disease-causing) bacteria and viruses. Animal wastes and failing septic systems can contribute fecal coliform bacteria, making streams unfit for bodily contact.

How Raingardens Work

Raingardens are essentially man-made shallow depressions on the landscape that hold and slowly release surface run-off water, thus mitigating the effects of   impervious surfaces. All raingardens have one source of water: surficial stormwater run-off. However, raingardens may lose their water via percolation into soil, evaporation, or (detained) surface run-off.   Some are designed to dry out occasionally (“dry raingardens”), while others fluctuate water levels, but always maintain a permanent pool (“wet raingardens”), see Figures 1-3.   Ecologically, raingardens mimic small wetlands that have been lost in our modern graded-and-compacted landscape.    Functionally, raingardens may also simulate municipal wastewater treatment systems.

Even if plants were not present, raingardens act as settling basins for the “first flush”, as well as suspended sediments and associated pollutants. As any gardener knows, plants use nutrients that are conveyed to their roots via watering. Additionally, the r oots and stems of wetland vegetation provide a home for naturally occurring micro-organisms, which feed on those nutrients as well (City of Columbia , 2002).   Columbia uses both Typha latifolia and T. angustifolia to treat its municipal waste, but literature indicates that a wide variety of native plants are effective at removing pollutants from standing water. For instance, Arrowhead ( Sagittaria latifolia) can uptake high levels of phosphorous and metals from sediment; Elodea canadensis has been found useful in breaking down oil, and its growth is even stimulated by its presence in small concentrations   (Center for Watershed Protection, 2000; Whitley et al., 1990 ) .

Rain garden installation and placement

One can engineer a raingarden to excruciating detail, but one can never predict the weather. A droughty year can dry out a raingarden that was planned to always be wet. A wet spring may make for perfect mosquito habitat in the raingarden you planned to dry out in two days. Evaporation can be hard to predict because it depends on the temperature, wind, and shade. The elevation of water in your raingarden can depend on the saturation of nearby soil or whether a burrowing crayfish has taken up residence. There are two general solutions for any problem that may arise, and they both require planning: One is to have a robust design which uses plants that are adapted to a variety of conditions, the other is to integrate water control structures that are easy to manipulate. In any event, there will always be a certain amount of maintenance and tweaking that will be needed after the initial installation.  

Figure 4.     Average daily rainfall for Columbia , MO for the last 30 years of record.

                  Source: National Weather Service data provided by Weather Underground .

Hydrology

Missouri is a water rich state. Annually, we receive about 40” of precipitation in Columbia , the peak occurring in spring, the lowest amount occurring in the winter (see Figure 4).   Our net evaporative loss equals our precipitation, so theoretically, a deep enough hole that has an impermeable bottom should have the same amount of water in it from one year to the next. Of course, the water level in that hole (or your raingarden) will vary with precipitation as in Figure 5. The one big difference between a rain gauge (the source of this graph’s data) and your raingarden is watershed size. A watershed is the area that drains to a waterbody. A typical raingarden’s watershed may include the rooftop , driveway and backyard of a residence. The ratio of the watershed surface area to the surface area of a pond or raingarden is the watershed:basin ratio.   This number can be used to multiply the amount of rain received during a storm to get an idea of how much the water level in your raingarden will rise. For example, a raingarden placed downhill of a parking lot with a 10:1 watershed:basin ratio can be expected to fill up with 5 inches of water after getting ½ inch of rainfall (1/2 inch rain X   10 = 5 inches of water elevation change).                                 

Figure 5.         A graphic illustration of the frequency and intensity of rainfall in Columbia , MO in 2003.

                      Source:   National Weather Service data provided by Weather Underground .

Since not everyone’s watershed will be totally impervious like the example above, the amount of rainfall actually making it into your raingarden may be substantially less, depending on how porous your soil is. So how deep do you have to dig to maintain water year-round? In my experience, a 1.5 foot deep raingarden with clay soil and a watershed:basin ratio of greater than 10:1 doesn’t dry out, a 4” deep raingarden in the same area dries out periodically in the summer.

Siting

The typical configuration of a raingarden consists of a small dam placed in areas that receive overland flow, and the material for the dam is usually excavated from the area uphill, forming a basin. Raingardens in the suburban environment should be situated in places that normally receive a fair amount of runoff, such as swales, areas by gutter downspouts, or areas receiving runoff from parking areas. Raingardens shouldn’t be situated near a house (particularly uphill) because the moist soil around them may damage foundations.   Before you dig, you should do some initial investigation. Make sure that you call 1-800 DIG RITE, which is a free service that locates underground utilities in your yard that could damper your plans for digging.

Holding water

An important preliminary step is to test your soil. Clay soils will hold water longer and lend themselves to permanent pool situations; conversely sandy soils will percolate water rapidly, and are better suited to ‘ephemeral’ rain gardens. To determine whether you have clay soil, wet a handful and knead it until it is uniformly wet. Squeeze the mud between thumb and forefinger, forming a ribbon. If you can make a ribbon more than 2” in length, your soil is clay.   You may also simply dig a test hole as deep as you plan to dig your raingarden, fill it with water, and monitor how long it takes to dry out.   This will give you a rough idea of your water retention time. It is important to note that clay soil liner that is exposed to the air may shrink and crack, allowing future rain to literally “slip between the cracks” if your pond dries completely.

Liners can be placed in wet raingardens to retain water for a greater length of time. Liners can be of the artificial variety (plastic or rubber derived), or they can be a layer of imported clay and/or rock. Synthetic liners are discouraged in making raingardens, because they prevent surface water from percolating into the soil and do not provide good habitat for wetland plants, which are two key elements of the raingarden design.

Soil compaction is another good way to hold water on an area for a longer amount of time. Simply taking a heavy board and pounding on the soil underfoot will close off pores of the soil that otherwise would have drained the raingarden. In larger projects this can be accomplished with vehicle tires. Note that the sides of the raingarden should also be compacted to the level that you want to retain water.   In order to prevent sustained moist conditions downhill of a raingarden with a permanent pool, you should compact the soil under the “dam” to make it more impermeable.

An important consideration after siting the raingarden is figuring out what to do with the excess soil. In most situations, the soil can be formed into a berm on the downhill side, allowing more water to be detained. Any exposed soil should be covered in case of rain, for the soil is now very vulnerable to erosion.

Planting

The depth of the excavation will affect the suitability of the site for certain plants. The deepest part of the depression will retain water longest, and thus only plants adapted to prolonged wet conditions will survive. A large raingarden may have distinct “rings” of vegetation zones, similar to prairie potholes found farther north.  

Raingarden plantings can be established in a couple ways. One can transplant cuttings, bulbs, or mature plants. This option is fairly expensive, but the survival rate is good, and you can pick where the plants will grow (for the first season, anyways). A cheaper alternative is to plant seed, but there is a reduced amount of survival, and it will take a while before you enjoy the blooms on the forbs. This option requires more maintenance initially, because seedlings will be competing with weed seedlings, and more attention may have to be paid to weeding in order to get a desired outcome. Another option is to collect soil or litter from wet prairies/wetlands and add them to the raingarden soils.   This is an economical method for letting mother nature establish a wetland, but you risk getting a monocrop of reed canarygrass or some other undesirable plant.   The last method is to dig a depression and let it go. This is probably not appropriate for suburban applications because a viable seedbank does not exist and nosy neighbors do. If there is a decent natural seed source nearby, wind and animals may deliver the seeds to the raingarden, if they’re not already present in the soil.

It is important to choose plants that will thrive under the moisture conditions you provide them, because they not only must tolerate these conditions, they also must out-compete potential weeds.   To find out which plants are suitable, you can search MDC’s Grow Native website and search for “bog” plants, or read   MDC’s publication “Water Plants for Missouri Ponds”. Note that many of the plants described as “unsuitable” for ponds are perfectly fine for raingardens, because you don’t have to worry about tangling your fishing line.

Heading off Problems

Unwanted Plants

It is important to consider potential maintenance problems when designing your raingarden.   Missouri is blessed with a climate which is hospitable to many plants, unfortunately, some of these plants are weeds you didn’t intend to grow.

Solutions: Generally, the more water retention time in your raingarden, the less problems you will have with common lawn weeds. Most suburban weed seedlings die off after 4-5 days of continual inundation, which is handy for weed maintenance.   Placing large flat stones at the bottom of ponds will also restrict the areas in which weeds can invade. Weeding the raingarden when it is still moist is very easy since entire plants can be removed by hand.

Algae and Duckweed

Continuous inundation for a week or longer may produce algae blooms, if the “fruits” somehow get into your raingarden. Similarly, continuous inundation may also provide ideal conditions for duckweed or watermeal, which can blanket the water surface and may not be desired by homeowners. This can happen if you transplant a plant from an afflicted pond, and some of the seeds/fruits hitch a ride within transplanted soil.

Solutions: To prevent any problems, simply quarantine transplants in a bucket of water for a week in a partly- sunny area and see if any algae or duckweeds appear, then remove any that appear. The best way to combat these simple plants is to deny them light, nutrients, or water. If your raingarden dries out completely several times a year, this will kill off ‘mature’ duckweed and algae, but their seeds/spores will likely persist in the mud, and reproduce when the water level rises. Duckweed can reproduce in 4 days, so a raingarden that dries out occasionally may always have a small population of duckweed or algae, but not enough to be a nuisance. You can shade out algae by adding a dye such as AquaShade to the water. This is most effective with deeper water, and is a usual remedy for ponds.   Shading by other plants is very effective, and doesn’t require any exertion on your part. Since duckweed and algae will only occur on very wet raingardens, emergent and floating plants such as water primrose, water lotus, and arrow-head will shade them out. Keep in mind that these nuisance plants survive because of nutrients in the water. Eliminating the source of nutrients in the watershed will eliminate the duckweed and algae. Once established, algae can be fished out with a rake and composted easily enough, but it takes a net to scoop out duckweed or watermeal

Nuisance Animals

Keep in mind that what you view as “raingarden” might be viewed as “birdbath” by a passing flock, or “romping area” by your neighbor’s unleashed dog. My initial intentions when constructing my raingarden was to attract birds.   I achieved this goal, but was rewarded with bird poop which fertilized my water and caused an algal bloom. Roaming neighborhood dogs (and rabbits, squirrels, cats, and probably raccoons and possums) will probably take drinks from your raingarden when handy. Unfortunately, some dogs will also play in the water, which can damage or uproot plants.  

Solutions : If excessive nutrients seem to be entering your raingarden, you can provide a separate bird bath that will lure away much of your problem.   Animals tend to follow the path of least resistance, so if you don’t want a trampled path through your landscaping, provide a rock access area to the water so that nothing will be damaged when Fido gets thirsty.   As a last resort, you can put up fencing or natural barriers to repel unwanted guests. I’ve found blackberries to be effective against dogs and neighbor’s kids.

Mosquitoes

If water stands in your raingarden for a week or more, there is a chance that mosquitoes may develop into adults during that period.

Solutions : Reports vary as to how long it takes to grow an adult mosquito in the continental US. The fastest I’ve read is 4 days, but I believe over a week is most accurate for our state. As insurance, burying aquarium tubing or a pipe with a removable cap under the “dam” of a raingarden can allow you to control the amount and duration of water within the raingarden.   Engineering your raingarden to not have standing water for more than a week   is probably the easiest solution. But if you want to have more of a “water feature”, having a permanent pool of water can provide habitat for mosquito predators such as dragonflies, damselflies, amphibians, and fish, which have worked reliably in the past. Mosquito problems seem to occur when raingardens stay wet for an intermediate amount of time, perhaps 2 weeks (I’ve found birdbaths, dogbowls, rain-gutters, and flower pots to fall into this category in the spring). Finally, Bacillus thuringiensis israelensis (Bti) is a mosquito-specific natural control that can be purchased as small disks that can be placed in areas of standing water.

Enjoy!

Raingardens are a learning experience that can connect a suburban backyard with some of the processes of nature. The amount of time that is saved by not mowing these areas should be spent observing them. By establishing what amounts to small wetlands within suburban settings, we can reclaim at least part of the environment that was historically present in Missouri . These small showy water features can provide an important habitat network for birds, reptiles and amphibians, as well as helping the water quality of our streams.

For more information on raingarden ideas and construction details, check out these on-line pamphlets:

http://www.dnr.state.wi.us/org/water/wm/dsfm/shore/documents/rgmanual.pdf (Wisconsin Department of Natural Resources)

http://herpcenter.ipfw.edu/index.htm?http://herpcenter.ipfw.edu/outreach/VernalPonds/&2 (USDA Forest Service)

In addition, the Missouri Department of Conservation has recently published “ Native Plant Rain Gardens ”, available at MDC offices.

Literature Cited:

City of Columbia . 2002.   City of Columbia , Missouri ’s Constructed Wetlands Wastewater Treatment Project. http://www.gocolumbiamo.com/PublicWorks/Sewer/wetlands_1.html

City of Springfield , MO. 2003 Choose Environmental Excellence Newsletter. Vol 4, No. 1: Springfield , MO. http://www.springfieldmogov.org/community/cee_spring03.html#spring

Currier, M. 2004. personal communication of as-yet unpublished “Prairie” section of revised edition of The Terrestrial Natural Communities of Missouri

Floore, T. 2004. Mosquito Information. Public Health Entomology Research & Education Center Florida Agricultural & Mechanical University http://www.mosquito.org/info.php

King County , Washington Department of Natural Resources and Parks. 2003.   The Science of Stormwater.   http://dnr.metrokc.gov/wlr/stormwater/StormwaterScience.htm#pollutants )

Nelson, P. 1987. The Terrestrial Natural Communities of Missouri . Missouri Department of Natural Resources. 197p.

Prince George ’s County, Maryland . 1999. Low-Impact Development Hydrologic Analysis,

Department of Environmental Resources, Prince George ’s County, Maryland .

  http://www.epa.gov/owow/nps/lid/lid_hydr.pdf

Center for Watershed Protection. 2000. Broad-Leaf Arrowhead: A Workhorse of the Wetland. Article 99 in The Practice of Watershed Protection . Center For Watershed Protection Ellicott City , MD . http://www.cwp.org/Practice_Articles.htm

Weather Underground website for Columbia

http://www.wunderground.com/NORMS/DisplayNORMS.asp?AirportCode

=KCOU&StateCode=MO&SafeCityName=Columbia&Units=none&IATA=COU&normals=on

Weaver, J. E. 1968 . Prairie plants and their environment : a fifty-year study in the Midwest . University of Nebraska Press. 276p.

Whitley, J. R., B. Basett, J. G. Dillard, and R. A Haefner. 1990. Water Plants for Missouri Ponds. Conservation Commission of the State of Missouri

Wilson , C. 2002 Lawn and disorder: A 'natural' view of landscaping. 04/11/2002   USA TODAY

Yatskievych, G. A. 1999. Steyermark’s Flora of Missouri.   Missouri Dept. of Conservation in cooperation with Missouri Botanical Garden Press, 951 pp