How is a rain garden different from any other garden?

The main difference is that they’re created in man-made or natural depressions, rather than being flat or mounded on the surface. Their purpose is to create a space where stormwater runoff pools temporarily, preventing polluted water from entering our streams and creeks. Deep-rooted native prairie plants, especially those that thrive in moist or wet soils, are often used and sometimes soil amendment is added.

So is this a water garden or a pond? Do I need a pump, aquatic plants, and fish?

Nope. Rain gardens do not have permanent water. Instead, they’re designed to temporarily hold runoff during storms. A properly designed garden should drain completely within 24-48 hours. Most of the time the soil will be moist, but not wet, and of course we periodically have long, dry periods without any rain whatsoever.

I’ve got this low spot in my yard where water always pools for days at a time. Would a rain garden help soak up the water?

Not necessarily. Adding more water to an area that’s already poorly-drained will create a bigger problem. Though rain gardens can be planted in depressions that naturally receive runoff, it’s important that they drain within a day or two. Rain garden plants can withstand brief periods of inundation, but most can’t handle standing water for a long period of time. In addition, we don’t want to create a mosquito breeding area.

Plants do absorb and hold water in their leaves, stems, and roots. Water continually moves up and out of the plant through the processes of transpiration, and roots create channels that encourage water infiltration. So plants can help, moderately, with drainage problems- but it’s not enough to compensate for environmental variables like a high water table or wetland soils. Consider working with your landscape, not against it; some areas will hold water during rainy times because of underlying conditions and there isn’t much that can be done. Consider creating habitat where grass isn’t an option!

In these situations, we don’t recommend adding more water to a wet area, but would encourage you to utilize plants that can withstand these conditions. Hibiscus, red-twig dogwood, switchgrasses, big bluestem, buttonbush, and soft rush are examples.

See our plant lists for rain gardens, poorly drained areas, and wetlands, wet areas, and ponds. 

Will the rain garden attract mosquitoes?

Not if it’s designed well. Rain gardens drain completely within a couple of days, too short a period for mosquito growth. In the warmest weather a mosquito can proceed from egg to adult in 5-7 days where there’s standing water. These temporary pools actually provide habitat and food for animals that eat mosquitos like frogs, toads, spiders, birds, dragonflies, and other insects.

Because household gutters often back up with debris and standing water, they can breed mosquitoes. When gutters wash developing larvae into rain barrels and rain gardens where they’re more visible, they often incorrectly receive the blame.

If my rain garden fills with polluted stormwater, won’t that kill the plants? When the water soaks into the ground, does it pollute the groundwater?

Many studies have shown rain gardens to be very effective at removing pollutants through biological processes.  Plants are really good at cleaning! It’s called phytoremediation. Sunflowers are used to clean up radioactive waste sites, and species of mustards can remove lead from contaminated garden sites. Wetlands are well-known for their ability to filter and degrade pollutants, and we all know that rain forests help provide clean air.

Here’s the science:

  • Dissolved pollutants stick to particulates floating in stormwater or in the soil, particularly metals and soluble phosphorus
  • Plant uptake removes nutrients like nitrogen and phosphorus that contribute to algae blooms
  • Microbial processes (fungi, bacteria) break down organics and pathogens into simpler, or less harmful, compounds
  • Exposure to sunlight and dryness removes pathogens, and encourage volatilization
  • Sedimentation and filtration remove suspended solids (like silt and sand) and capture debris

How large and deep does it have to be?

Size is a function of the area draining to your garden, how quickly your soil absorbs water, and the volume of runoff to be captured. There are many different regional guides, but we like The Blue Thumb Guide to Raingardens’ specifications because of its simplicity and ease of use for the homeowner. These gardens are designed to hold the first inch of rainfall- this is important, as this “first flush” tends to pick up lots of pollutants. Since most storms produce under  1” of rainfall, rain gardens can handle the majority of water they receive.

This guide recommends that your drainage area be divided by the number of inches of water per day that soaks into the ground.  Most residential rain gardens are between 4-8” deep, and 100-300 square feet. Larger, deeper rain gardens are discouraged, as they may be hazardous to pedestrians, (with too steep a drop), or create too large an area for water to spread out well. For larger areas, consider a chain of of connected, smaller rain gardens where water flows from one to another as they fill with water.

More importantly, the garden should fit in your landscape and be an attractive asset for you.

My clay soil is too heavy and it won’t absorb any water. Should I add sand so it drains better?

For the residential gardener, we recommend using native soils rather than replacing them. It may be necessary to incorporate coarse, fibrous organic matter like compost, bark fines, or peat to help break up heavy clay. These materials add structure and pore space without excess nutrients.

Replacing soil is expensive, and many mixes contain sand. While it may seem like a good idea, consider the following:

  • Our native soils vary from loamy to heavy clay. Plants that grow well locally aren’t adapted to sandy soils.
  • When small clay and silt particles settle into large pore space created by sand, they clog. Over time, this concrete-like substance can no longer absorb water.
  • Drainage that’s too rapid defeats the purpose of a basin that is designed to slowly clean water through the process of filtration.
  • Even if the rain garden basin itself contains an engineered soil mix, infiltration is ultimately dependent on the surrounding soils as water moves down and laterally. It’s important that soils “mesh” so plant roots can successfully move from one medium to the next.

 

The texture of a soil and its percentage of sand, silt, and clay particles dictate how fast water will move through it. To see whether your soil can accommodate a rain garden, do a percolation test.    Larger gardens such as those in commercial areas may need an underdrain and soil replacement, particularly in urban areas where the soil is compacted.

There are some things you can do to improve drainage, but be mindful that rain gardens are not always a good solution. If your soil doesn’t absorb standing water within two days at most, look for a different location.

Where should I put it?

Do:

  • Construct the rain garden at least 10 feet from buildings to prevent seepage into foundations
  • Choose a location with full or partial sun
  • Look for a relatively flat or gently sloped area
  • Construct the rain garden at least 25 feet from a septic tank, septic drain field or well head
  • Have a nearby water source
  • Opt for plants native to Ohio

Avoid:

  • Slopes greater than 12%
  • Areas that hold water for more than two days where soils exhibit very slow filtration
  • Placement over underground utilities, shallow water tables
  • Locations under trees, as their canopy creates excess shade, and digging may disturb tree roots

How do I design it?

  • Shape: Most rain gardens are longer than wide, perpendicular to the flow of water. They can be many shapes; an often seen shape is that of a kidney bean, where water enters at the notch and spreads from there.
  • Planting zones: Rain gardens have 3 zones: the middle, where water typically pools the longest, should have vegetation that tolerates wetter conditions. The intermediate zone, between the middle and outer edge, features vegetation that tolerates occasional standing water. Finally, the perimeter of the rain garden will have plants that prefer drier conditions.Some general rules of thumb for any gardener: group species together in odd numbers (this makes weeding easier and allows each clump to stand out); consider form, texture, and color; and put larger plants in the rear where they’ll act as a screen and won’t overshadow shorter plants.
  • Berm and sides: The basin of the garden should be level. If you’re working on a slope, you can use the soil you dig out to create a berm (earthen mound) on the lower side. This keeps the water in, preventing overflows and allowing water to stage up before it draws down slowly. You can plant drought-tolerant, low plants on the berm, such as sedum or turfgrass. (Be aware that grass can spread into gardens unless you use edging.)The sides of your rain garden will be gently sloped (rather than a vertical step) to avoid erosion, unpleasant drop-offs where someone could twist an ankle, and ample space for different planting zones. A 3:1 ratio is recommended: for example, if the garden pool is 6” deep, the side slopes are 3 times that long at 18”.
  • Inlet and outlet: Water can enter your garden a variety of ways: rain chains, downspout, grass swale, creek bed, and driveways. Consider an outlet if overflow may be an issue for your lawn or neighbor. Some communities require that disconnected downspouts remain “in-line” so that they must reconnect to the existing stormwater system if intercepted by a rain garden or rain barrel.
  • Soil preparation: If using an excavator, do not allow it to sit on the garden where it will compact the soils. Overdig the garden, loosening or “fluffing” the subsoils and leaving them in the basin. Avoid “smearing” clay which creates as smooth, impermeable layer- rather, use bucket tines to roughen soil layers so water and roots can permeate. Use a rototiller to incorporate 2-3” of organic matter into the soil, taking care to avoid over-tilling that can destroy soil structure. Be mindful that excavation creates much more soil that you might anticipate (soil that’s compact in-ground picks up air and volume when dug out) so you must have a plan for where to put it.

How will creating a rain garden benefit the environment?

There are lots of ways. Keeping water where it falls is the most effective way to reduce runoff that can degrade the quality of streams. Reducing lawn area and adding natives means less air (and noise) pollution through mowing, and less chemical application. And habitat is vitally important as we continue to develop and modify land.

How much does it cost to install a rain garden?

Estimates vary between $3-5 per square foot (DIY) to $10-15 per square foot if it’s professionally installed.  There’s a great deal of variation depending on the complexity of the work. Plants are usually the most expensive item. You can save money by purchasing smaller plants, but they will take longer to fill in.

Is this something I can tackle myself?

Yes. However, digging out a rain garden can be quite laborious, especially if your soil is very heavy and hard. Tools and equipment typical to rain garden installation include: mini-excavator, heavy-duty tiller, soil tamper, line level, tape measure, and hand tools like shovels, rakes, and trowels. You won’t necessarily need heavy equipment, but it sure can help! Many of these items can be rented at hardware stores.

If disconnecting a downspout or creating a dry streambed, you’ll need additional items such as a saw, landscape fabric, and splash block.

Make sure you check your local zoning code to see that disconnection is okay. In some cases, you can intercept flow but are expected to have the overflow reconnected to your existing curb-and-gutter system. If you live in a more rural area with few close neighbors, overflow may not be an issue.

If you haven’t alienated your friends with constant babble about water quality and native plants, consider asking them to help you dig your rain garden! It’s a great way to start a dialogue and attract attention to conservation measures that homeowners can incorporate.

How do I maintain my rain garden?

Much the same as any other garden, with some exceptions:

  • Inflow and outflow: Young plants may not be able to withstand lots of water right away; it’s wise to create an overflow exit during heavy rains so your plants aren’t overwhelmed during the first growing season while they’re getting established. Maintain flow paths free of debris including sediment, pebbles and leaves.
  • Watering: Water deeply immediately after planting any garden to prevent root shock. During the first season, young establishing plants need 1” of water per week. As plants mature, they’ll only need supplemental watering during extremely dry spells- this is especially true for tough prairie plants that have deep root systems.
  • Weeding: At least twice a year, during spring and fall. Prevent weeds from spreading by removing seed heads and covering bare soil with mulch.
  • Mulch: use heavier, double-shredded mulch rather than lighter materials, because it will lock together rather than float away when the garden fills with water. It should be about 2” thick.
  • Erosion: Placing rocks at the entry to your rain garden can slow the speed of water into your garden so young plants aren’t uprooted.
  • Fertilizer, herbicide, and pesticides: They aren’t needed; fertilizer can encourage spindly, leggy growth on natives, and create a favorable environment for weeds. Remember that your garden functions to help keep water clean! With proper maintenance, your plants should thrive and fill in to out-compete weeds. Some insect damage on your plants? That’s okay! (We all need to eat.)

What are native plants, and why would I use them in my rain garden?

Native plants are those that existed in the landscape pre-settlement, and they’ve weathered many winters, survived heat, drought, rain, and temperature fluctuations that characterize the lower Midwest. They’ve also developed complex relationships with insects (bees, butterflies, beetles, and flies) that are critical to ecosystem function, serving as pollinators and decomposers, and an important food source especially for amphibians, reptiles, and birds.

Perennial prairie plants work especially well in rain gardens because their deep roots continuously create channels for water to move through. These same long roots can find water during hot summers, making natives a low-maintenance selection that require little water.

Should I use cultivars of native plants?

Cultivars, or cultivated varieties, are selected and/or bred for certain characteristics like flower size, height, and mildew-resistance. Native plant enthusiasts are sometimes wary of cultivars, and it can be a confusing issue.

Characteristics exhibited by cultivars may be appealing to us, but these same traits may render the plant less than useful to the animals that rely on it. For example, purple coneflower now comes in dozens of shapes, sizes, and colors. Some are sterile and don’t produce seed (bad news for goldfinches), and the double-flowered forms make it difficult for pollinators to reach pollen and nectar. Some plants produce extra large berries that look nice but are too big for birds to eat, and others’ pretty foliage results in chemical changes that warrant them inferior host plants for insects. In addition, because cultivars are often cloned, they aren’t representative of genetic diversity.

Ultimately, the choice is that of the homeowner or landscaper.

Where can I get native plants and guidance about my rain garden design?

We’ve noticed that gardeners have been asking about native plants and rain gardens. The Gardening for Clean Water Project connects the landscaper and nursery to its conservation-minded customers by providing technical training for employees, and educational materials for gardeners. This grant-funded program was awarded by Ohio EPA’s Environmental Education Fund for years 2013-14.

Learn which nurseries and garden centers are participating under our Project: Nursery Outreach category.

Will Franklin Soil and Water help me install my rain garden?

No. Franklin Soil and Water’s rain garden staff mainly consists of one 100 lb. weakling who gets winded opening jars.

We are happy to assist with planning your rain garden, and you are required to consult with us if taking advantage of a cost-share in your community. You may be eligible, and you can contact us for an up-to-date list of participating communities. Here’s an example of a cost-share application.

We can’t dig or plant your rain garden (barring workshops or community projects), but there are service providers who can.

Are you available for presentations about rain gardens?

Yes. Garden clubs, watershed groups, and community organizations regularly request presentations. A speaker fee of $150 is generally required, unless it aligns with our working partnerships. In some cases, donations are permitted in lieu of a set fee. These donations would go to Franklin Soil and Water’s Conservation Fund and Mini-Grant Program.