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Rain Garden FAQs


What is a rain garden?

A rain garden is a garden of native shrubs, perennials, and flowers planted in a small depression. It is designed to temporarily hold and soak in rain water runoff that flows from roofs, driveways, patios, lawns, or other hard (impervious) surfaces, preventing it from entering the storm sewer system. Soil and plant roots use natural processes to improve water quality by filtering pollutants. Rain gardens are effective in removing up to 90 percent of nutrients and chemicals, and up to 80 percent of sediments from the storm water runoff. Compared to a conventional lawn, rain gardens allow for 30 percent more water to soak into the ground. The water is held by the garden and allowed to slowly infiltrate the soil, providing an important role in recharging ground water supplies and reducing storm water runoff volumes to local streams. A rain garden is not a pond or wetland, but is dry most of the time and typically holds water for not more than two days during and following a rainfall event.


Why are we promoting rain gardens?

A rain garden is an attractive garden with a special purpose — to improve local water quality and reduce the impacts of storm water on area streams. Communities around the country have experienced dramatic reductions in storm water pollution, due to many homeowners installing rain gardens on their properties. According to the US Environmental Protection Agency (EPA), 70 percent of all water pollution comes from pollutants carried in storm water runoff. A few examples of these pollutants include pet waste, fertilizers, oils and greases from automobiles, and trash (i.e. non-point pollution sources).

Rain gardens are effective in removing up to 90 percent of nutrient and chemical pollutants and up to 80 percent of sediments from storm waters flowing into them. This polluted water would otherwise reach nearby streams, rivers, and lakes untreated. Not only are rain gardens helpful to water quality, they also create beautiful additions to any landscape and can help reduce localized flooding or standing water in nearby streets. Constructing, installing, and maintaining a rain garden will help reduce pollution and flooding, and help keep our local water supplies and recreational areas healthy.


What are the benefits of rain gardens?

  • Improve water quality by filtering out pollutants
  • Provide localized storm water and flood control
  • Easy to maintain after establishment
  • Preserve and promote native vegetation
  • Attract beneficial birds, butterflies, and insects
  • Provide aesthetically pleasing landscaping
  • Provide a storm water management solution for homeowners who properly disconnect their downspouts from the sanitary or combined sewer system

What makes a rain garden different from any other perennial garden?

A rain garden is bowl or saucer shaped, not mounded or flat like other perennial gardens. Like a conventional garden, a rain garden is a beautiful form of landscaping; however, a rain garden is also designed with deep, loose soil specifically to collect and absorb rain that would otherwise run off your property, and/or to solve wet spot problems where water already is collecting.

In the design of a rain garden, often 6 to 12 inches of soil is removed and altered with tillage, compost, and sand to increase water infiltration. The type of alteration to the soil depends on the current soil type, so it is a good idea to conduct a simple test of your soil’s infiltration rate. 

Instead of using cultivated plants from Europe or Asia, a rain garden is planted primarily with deep-rooted, low-care, perennial plants native to your region, that have adapted over thousands of years to your local weather and environmental conditions. The deep roots create channels into your soil through which water may travel. The native plants provide habitat for local butterflies and other wildlife. The native plants do not need to be treated with chemical fertilizers, insecticides, or herbicides in order to thrive.


Why is storm water (rain water runoff) a problem?

We tend to think that large industrial polluters cause most water pollution, but this is not the case. We are the real culprits. The EPA has determined that up to 70 percent of the pollution in our surface waters is carried there by storm water runoff. Some studies show that about 50 percent of that pollution comes from individuals and homeowners due to yard care, yard waste, and chemical pollution from household activities. When it rains, water runs off of our roofs, parking lots, streets, and lawns instead of soaking into the soil the way it did before development. This water, along with everything it picks up along the way, ends up in storm sewers and ditches that discharge to streams, rivers, and lakes. Salt from roads, bacteria from pet waste, lawn nutrients, spilled gas, oil, and other pollutants are all washed into local waterways. Most modern American cities are built in such a way that when it rains, all of the water is directed into storm sewers via gutters, curbs, and ditches, and then the water flows directly into nearby creeks and streams without being treated or filtered.

In addition to carrying pollution, the storm water runoff is usually warm, causing a pulse of warmer water to flow down the stream. In a natural system, water enters a stream through a slow and steady release from groundwater. Groundwater has a fairly cool temperature, which allows water to hold more oxygen and keeps stream habitat stable. Many sensitive creatures, such as trout, cannot survive in a stream with fluctuating or warmer temperatures. While groundwater release is slow and fairly steady, storm water runoff occurs all at once. The large volumes of warm water flushing downstream cause erosion and flooding, carry dam-forming debris, and scour the stream bed.


What is an impervious surface?

An impervious surface is any surface that doesn’t allow rain water to penetrate into the soil, including roofs, roads, cement or gravel driveways, sidewalks, and most lawns (because the shallow, dense root system acts almost like cement). Impervious surfaces contribute to increased amounts of storm water runoff, increased non-point source water pollution, and the urban heat island effect.


How can I reduce impervious surface on my property?

 

  • Plant native plant gardens in your yard
  • Reduce the overall amount of concrete on your property
  • Consider alternative materials for patios and new driveways such as permeable pavers or pervious concrete
  • Construct a rain garden to collect storm water from your roof and driveway

 


Will rain gardens attract or breed mosquitoes?

No, not if properly constructed. Rain gardens are designed to absorb water, not to create ponds. Properly installed, your rain garden will not hold water long enough for mosquito larvae to complete their 7-to-12-day life cycle. A well-designed rain garden with mature plants will not have standing water in it after 48 hours; all the water will have soaked into the garden. In fact, rain gutters on homes are much more likely to produce mosquitoes than a rain garden.


Will a rain garden attract bees?

Fragrant flowering plants do attract a wide variety of birds, butterflies, and bees. Remember that 90 percent of insects are beneficial to gardening and rest assured that rain gardens are filled with busy pollinators pursuing nectar.


Do rain gardens have a wild and messy appearance?

No. Rain gardens do have a natural rather than a manicured appearance, but they need not look messy. You can keep a rain garden looking neat and attractive by keeping the edges well defined. Taller plants often have a more unkempt appearance, so use shorter plants if you want your garden to have a cleaner look.


Will my rain garden have standing water?

Rain gardens are designed to infiltrate water in about one day. If it rains several days in a row, it is possible that your rain garden may have standing water until the rain stops and the water has time to soak in. If designed and installed correctly, rain gardens typically do not have standing water for more than 48 hours. Be sure to test your soil type and infiltration rate, or percolation rate, before beginning your rain garden. Rain gardens may not be appropriate for all locations; high water tables, clay soils and bedrock locations may inhibit infiltration.  


Is a rain garden a pond?

Rain gardens are designed to hold water for no more than a couple of days. Unlike ponds, you don’t need costly pumps, electricity, or filters. In fact, a rain garden is a filter for the water that runs off of your property’s impervious surfaces.


Can the water in rain gardens be a hazard for small children?

During storms, rain gardens can fill with standing water but the typical amount of water is no more than 18 inches. This water will begin to recede immediately after the rain has stopped, emptying in a matter of minutes or hours. If standing water is a concern, you may add a few inches of pea gravel below the soil to hasten infiltration.


What types of plants are used in rain gardens?

As a rule, native vegetation should be incorporated into a rain garden. Native plants don’t require fertilizer, have good root systems, and are better at utilizing the water and nutrients available in their native soils than non-native species. Perennials, shrubs, wildflowers, or a mixture of all three can be planted. Avoid planting trees, as trees generally absorb more water than surrounding plants. We recommend species native to your region, but other cultivated non-native species can be very beautiful, too. Also, never plant invasive or noxious species in a rain garden, such as purple loosestrife, Japanese barberry, or Japanese knotweed. For more information on invasive or noxious species you need to avoid, visit PA-DCNR's Invasive Plants database or New York Invasive Species Information.

Despite what you may think, rain gardens don’t have to be planted with water-loving plants. Since rain gardens drain so quickly (ideally between 24 and 48 hours), the plants you put in only have to be able to tolerate lots of water for brief periods. They also need to be able to withstand periods of drought. Different areas of your garden should be planted with different kinds of plants as well. For example, the area near the top of the depression won’t be receiving as much water as the low-lying middle. If you’re not an experienced gardener or don’t have a lot of experience with native plants, just ask your local native plant nursery for suggestions. Remember to consider plant height, wildlife attraction, flowering, and sun and shade tolerance when choosing your plants.


Why native plants?

Native plants are ideal for landscaping for many reasons. Because they have adapted to our climate over thousands of years, they don’t need chemicals to help them grow, can tolerate our cold winters and hot summers, have very deep roots that allow them to be more drought resistant, have developed defenses against harmful native insects, and can serve as habitats for native wildlife (consider planting for butterflies, hummingbirds, or songbirds). The deep roots of native plants also make them ideal for rain gardens because they create channels in the soil which allow water to soak in quickly.

Be sure you are purchasing "propagated" native plants--sometimes, plants marked as "field grown" have been taken from the wild--including public and private conservation lands--replanted and resold.


Does this end up looking like an eyesore?

Most rain gardens look like a perennial garden and have neat edges that are mowed or trimmed, which gives the appearance of a neat and intentional garden. The owner will need to stay on top of weeding, especially in the first few years. Plants with shorter heights can also be selected to produce a more manicured look. Likewise a garden with fewer species may also result in a more manicured look.


Will a rain garden cause flooding in my basement?

No, not if it is properly located and designed. Rain gardens should be located at least 10 feet away from buildings so that water does not drain along foundations. Also, your rain garden should overflow away from buildings rather than toward them, so place the garden in the landscape accordingly.


Do rain gardens work in the wintertime?

Because plants are dormant, their activity is reduced, but the rain garden will still help slow down water movement and enable it to be absorbed into the ground in the winter. The ponding area provides storage for a certain amount of runoff even if the ground is frozen. Water may remain longer, particularly when the ground is frozen, but that’s not a problem in winter. Rain gardens do work best in the spring and summer, but that is when they are most needed to protect streams from polluted runoff and heavy storm volumes. If you leave dead seed heads standing or add a feeder, you will see a variety of songbirds frequenting your yard, seeking the important winter nutrition your rain garden can provide.

Keep in mind that cats have the potential to kill many birds in a year--be sure to keep your cats indoors if you plan to draw birds to your yard.


Are there rain gardens already out there?

There may be a rain garden near you. You can find a demonstration rain garden at Lackawanna College in Scranton, PA or at Lake Superior State Park in Bethel, NY.


Questions About Planting a Raingarden


Where should I place the rain garden?

Rain gardens are generally constructed on the downside of a gentle slope on your property to collect natural drainage and runoff from your lawn, roof, and driveway. The easiest way to read the slope and drainage of your yard is to watch water when it is raining. Take a few pictures to help you remember where water goes during and after a rainfall event. In a low area of your yard, a complex mix of plants and soil will absorb a lot of rain.  If you do not have a suitable site for the garden where water naturally flows, you can create a swale or pipe water to the rain garden, but it will take a bit of extra work.

Rain gardens are typically designed longer than they are wide and are perpendicular to the slope, in order to catch the maximum amount of rainfall. Rain gardens should be placed at least 10 feet away from building foundations. Generally you should avoid utility rights of way, where gas, phone, and telephone lines are located. Also keep the garden location away from septic drain fields, which don’t need any extra water.

Since the plants are aesthetically pleasing and attract birds and butterflies, locating a rain garden outside a picture window can provide the benefit of indoor enjoyment. On hot days in summer, many hummingbirds will be attracted to the plants of a rain garden. Every garden is site specific and unique; you can make it what you want to make it.


Should I locate the rain garden in the sun or the shade?

The best drainage will occur in full sun; however, many plants can be chosen to thrive in a part sun/full shade rain garden as well. The water will take a little longer to be absorbed, but the garden will function well in any sun exposure if the right plants are used.


I have a hard clay soil. Can I plant a rain garden in my yard?

Yes, but choose clay-loving plants and amend your soil. Typically, 6 to 12 inches of soil are removed and altered with tillage, compost, and sand to increase water infiltration and allow for more plant diversity. The type of alteration to the soil depends on the current soil type’s clay content, so it is a good idea to obtain a soil test. You can typically obtain soil test kits from your local county extensions, such as through Penn State Extension or Cornell Cooperative Extension. Your county conservation district might also be able to help.


How is a rain garden constructed?

They can be simple – just a green area of your yard where storm water goes, which requires little more than a shovel and some plants.  They might be more complex – involving excavating and re-building soil, sometimes requiring motorized equipment.  They also may be “industrial strength,” handling large volumes of water periodically, where large-scale storm water management regulations must be met.  These types of gardens require engineering plans and heavy machinery.


Can I build a rain garden myself?

Yes. View the publications listed here for planning, design, and construction information.  Although it may take a little more digging to create the depression, rain gardens are no harder to install than a traditional perennial garden.


Where can I find someone to build a rain garden for me?

Check in the Yellow Pages under Landscape Architects or Landscape Contractors. Remember that not all landscapers are experienced in building rain gardens, so ask lots of questions to be sure you’re hiring someone who will build you a quality rain garden.  Below are a few suggested questions to get you started:

  • What experience do you have with rain gardens?
  • Are you willing to work with homeowners?
  • Are you familiar with local requirements and permits (if there are any)?
  • Can you help me find an appropriate location and design for my rain garden?
  • Can you help with drainage, infiltration, and soil requirements for placing a rain garden on my property?

Where can I get drain tile, amended soils, and other supplies?

While drain tiles are not required for most rain garden projects, some homeowners want to direct drainage across or beneath a lawn to the rain garden. In this case, most home improvement stores stock drain tile. Drain tile is simply a flexible plastic hose that resembles an underground gutter.  If you are looking for amended soils to improve the infiltration rate of your rain garden, state websites for Pennsylvania and New York provide lists of soil and plant amendment licensees.


Do I have to use native plants?

You don’t have to use plants native to this area, but there are many advantages to doing so. Natives have adapted to our climate and are much better at handling the periodic inundation of water that goes along with a rain garden. They’ll also save you the time and money of replanting every year, and will offer much greater wildlife value. If you must use non-native plants, be careful to avoid known invasive species. Invasive species can crowd out and out-compete native species, creating a monoculture of one type of plant and potentially spreading into nearby native plant areas which then creates a habitat problem for local wildlife. Before planting exotic species, always check the list of common noxious/invasive plants for Pennsylvania or New York.


Where can I buy rain garden plants?

Some rain garden plants are carried by many local perennial nurseries, as native species are becoming more popular for home and commercial gardening. Other rain garden plants can be purchased from native plant nurseries. Check this database of native plant suppliers or iConservePA to locate sources. You may also choose to transplant some suitable plants in your yard, or you could get divisions or seeds from a friend or neighbor.

Large quantities of rain garden plants must be ordered far in advance, as suppliers do not usually keep them on hand unless they have a ready market for them.

Please do not collect your rain garden plants from wild populations. They may be growing everywhere, but they soon won’t be if they are removed from the local landscape. Purchase your plants from a reputable nursery that produces them in a sustainable way--the plants should be labeled as "propagated." Conversely, "field-grown" might mean that the plants were taken from wild populations, replanted, and re-sold.

Getting your neighbors involved and starting a community-wide native plant "trade" of cuttings or seeds is a great way to save money and get your garden growing, too.


Are there any special plants that will help me attract butterflies?

Yes, many rain garden plants are both beautiful and nutritious for hummingbirds, butterflies, bees, and more. Planting plans can be customized to attract birds and butterflies. For monarchs, the most important plant is milkweed, which they rely on to survive.


Can I grow fragrant plants?

Yes, some natives are sweetly fragrant. These often attract a vast array of wildlife (including humans). A great website for doing additional research on native species, including which are fragrant, is www.wildflower.org .


Can I just use flowers and not grasses?

Yes, but this will decrease the efficacy and depth of roots for infiltration. Many beautiful flowers depend on shorter grasses for support and nutrient uptake as well. We call these companion grasses. Some are short and provide a lot of visual interest by turning autumn colors and moving softly in a breeze.


Can I keep fish in the rain garden?

Rain gardens are designed to hold water for no more than a couple of days. Unlike ponds, you don’t need costly pumps, electricity, or filters. In fact, a rain garden is a filter for the water that runs off of your property’s impervious surfaces. Rain gardens are very short-term water storage features, and therefore are not conducive to fish.


How can I attract amphibians?

Amphibians will live in your rain garden if they are provided a chance to burrow down in the ground beneath the garden. Using a liner will deter them from living in your rain garden. Adding a long stick and/or stones for access in and out of puddles will facilitate tiny amphibian and dragonfly activity. These species eat mosquito larvae and adults in and around your yard that may be breeding in gutters or other standing water sources.


What is the cost of a rain garden?

The cost of a rain garden is dependent on the property’s soil type, the size of roof, driveway, and patio draining into a rain garden, and the types of plants chosen. If the soil is high in clay content, the garden may require adding a soil amendment to prevent standing water for more than 48 hours. For a self-built rain garden, expect to pay between $3 and $7 per square foot in plant costs and soil amendments. Digging the garden is the most time consuming task, as a depression needs to be created to catch the water and often 6 to 8 inches of additional soil depth is typically removed to add soil amendments. When working with a landscaping company to design and install a rain garden, the cost will significantly increase to around $10 to $15 per square foot. Plants are the single most expensive item so if you have friends or neighbors with native plant beds, consider asking them to split and donate some of their “good producers.” Remember, an important cost consideration is that like other perennial flower gardens, rain gardens are less expensive than replanting annuals every year.


Can a rain garden be too large or too small?

A rain garden should have an area about 20 percent the size of the roof, patio or pavement area draining into it. A typical residential rain garden is between 100 and 300 square feet. If a smaller rain garden than recommended for a lot is chosen by the landowner, the garden will still function. Any size rain garden can make a positive impact by infiltrating some storm water.  Rain gardens can vary in size and complexity depending on your site constraints and how you would like them to function. If you are still uncertain, simply seek help from a landscaper or other professional.


Questions about Existing Rain Gardens and Maintenance


Do I have to mulch my rain garden?

Organic mulch is recommended for formal rain garden designs. Mulch keeps the garden moist and able to absorb rain, makes the garden look tidy, and discourages weeds. An application of hardwood mulch will look good, and compost mulches will enrich the soil. Large rain gardens or bioretention systems that are planted from seed are not mulched.


What kind of maintenance is involved?

If native plants are used, rain gardens require less maintenance than a conventional garden.  They adapt well to their natural surroundings and do not need fertilizers or pesticides. During the first few years after the installation of a rain garden, the weeds will need to be removed periodically. After the plants in the rain garden are well established and have grown larger, they will eventually out-compete the weeds. During the first and second year, or during periods of little to no rainfall, occasional watering of the plants will be necessary.


Does a rain garden have to be burned like is recommended for other native plantings?

Though many native plants were subject to burns historically, cutting them to the ground in winter or early spring mimics the “burn cycle” and encourages growth for the next year. So consider this strategy for your rain garden if you want to encourage wildflower and forb growth. Early spring cutting is preferred over fall or winter, since brown, dry seed heads can provide important bird food over the winter.


Will native plants take over my yard (or my neighbors’ yards) like weeds?

No, many natives are not aggressive. In fact, many are struggling to compete with non native plants in the wild, so providing a habitat for them is good stewardship. Ironically, one challenge will be to keep turf grass species out of the rain garden. Choose rain garden plant varieties that are not aggressive and mow the area around the garden. NOTE: many native plants are highly sensitive to tiny traces of weed killers. You may see curling and damage to your new plants if you or a nearby neighbor sprays weed killer.


I have a lot of trees that drop a lot of seeds. Is this a problem for rain garden plants?

If at all possible, avoid placing the rain garden near a mature tree. Roots, seeds, and shade are challenges, but you can work with these if you make good choices. Try to stay flexible when digging, as roots may cause you to re-route a bit. You may need to keep up on weeding the seedlings more frequently, and you should choose shade-loving plants.


Will I need to water my rain garden during dry periods?

Maybe, but only if the weather is very dry for extended periods of time (greater than a month). The amount of water your rain garden needs will depend on the plants you choose. Native plants are adapted to a wide range of conditions, so they will only need watering in the driest seasons. Drought tolerant plants need to grow deep roots during the first two years to withstand dry periods. Overwatering may discourage this important growth. Those that are used to having their “toes” wet should be placed in the lowest part of the garden. Plants that like drier soil are placed on the banks of the garden and can withstand some wet and very dry conditions.


Do I need to fertilize my rain garden?

Native plants do not need special attention once they are established. They do not need to be fertilized or sprayed. Storm water carries many nutrients and therefore rain gardens are already fertilized regularly. One benefit of rain gardens is they help remove or take up excess nutrients/fertilizers in storm water. Fertilizing them with additional fertilizer would defeat the garden’s purpose as a storm water treatment method and would actually end up feeding the weeds more than the native plants you are trying to encourage.


Can I pile snow on my rain garden?

You can place a small amount of snow on your rain garden, but a large amount may compact the soils in the garden or damage the plants. A glacier on the garden in early spring may slow down the greening of your plants. It is better to place large amounts of shoveled snow next to the rain garden; as it melts, it will flow in and be absorbed.


What about road salt?

It is better to locate the rain garden away from direct salt discharge. There are some salt-tolerant plants that you might use if the only place you can locate the rain garden will be subject to salt spray and runoff from streets and sidewalks. For large parking lot and street applications, pre-treatment structures may be helpful, such as a sediment settling area. For larger rain gardens for commercial parking lots, an underdrain system would be best; the salty runoff is filtered through the rain garden, then carried away by the underdrain system. This prevents possible groundwater contamination with chlorides.


Can I order a sign for my rain garden?

A sign does a world of good in communicating to neighbors and onlookers. The National Wildlife Federation has a certification and sign program. Installing a rain garden with native plants and leaving the seeds over winter covers the checklist requirements for backyard habitat. To learn more, go to National Wildlife Federation Backyard Wildlife Habitat.


My rain garden overflows. What can I do to fix this?

In heavy rain events, this may occur. Spring and fall events also can provide repeated filling and delays in drainage. A few solutions to this problem:

  • Expand the garden to include the “high water mark.” This is the area your garden “claimed” during the heavy rain.
  • Add another rain garden down slope from the first one.
  • Decrease the amount of downspouts directed into the garden. Our rooftops can produce large quantities of runoff, and the rain garden may be too small to accommodate the amount of water you have routed to it.
  • Install rain barrels under downspouts so that some of the rain water is held there, sending less to the garden.
  • Wait until there is a dry spell and dig it deeper. Adding 1 or 2 inches of depth over 5 feet might accommodate enough of the excess water.
  • Install a small overflow drain by placing a short length of your drain tile in a high part of the garden. This allows overflow to run off, but only when the garden has filled.

The mulch floated out of place during a heavy rain. What can I do to prevent this?

The mulch is showing you where high water levels are. You can expand the garden, or just rake it back into place until the plants are dense enough to keep the soil and mulch in place. If it bothers you, you may wish to add boulders, stones or some large edging like pavers. These will increase the cost and maintenance, but they can be beautiful and functional.


My rain garden is holding water longer than 24 hours during heavy rains. How can I fix this?

During repeated rain events, the water may drain more slowly in the garden, which is not necessarily a problem. Rest assured that you are filling an important need by restoring the water table slowly. That said, even flooding is not necessarily a problem for your established rain garden. If it becomes an issue, you may wish to accommodate more water by:

  • Expanding the garden (width and/or depth).
  • Further amending the soil (especially if you have heavy clay soil).
  • Adding more wetland sedges and grasses that speed up water use.

My plants don’t stay where I put them. What can I do?

Gardening with native plants is not the same as traditional gardening. Remember the goal is to provide a dense vegetative cover that looks beautiful and treats storm water runoff. You can choose plant varieties that stay put, or you can let the plants choose where they flourish. If you feel you need to remove seeds of varieties that replant via seeds, simply cut off the seed heads once flowering is complete.


My downspout inlet has made a rut of erosion where it “shoots” out into the garden. What should I do?

This is an excellent opportunity to see storm water in action. Water from the drain tile can have a lot of pressure, so you may need to do one or more of the following:

  • Place a block or flat stone under the washed out area. This will help spread the flow to a wider pattern until the garden fills up.
  • Let that area be a deeper point in the garden and move the sediment to higher parts of the garden.
  • Keep leaf litter and asphalt debris out of the basin.
  • Occasionally reposition the drain tile, allowing it a lot of room for erosion.

 


Want a visual explanation of how rain gardens work?

Check out this rain garden site created by the University of Nebraska extension. Be sure to visit the rain garden animation at the bottom of the page. 

    Need more help selecting plants?

    The Lady Bird Johnson Wildflower Center website recommends native plants for each state and a variety of situations.

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