What is a Rain Garden?
A rain garden is an attractive landscaping feature planted with perennial native plants. It is a bowl-shaped garden, designed to absorb stormwater run-off from impervious surfaces such as roofs and parking lots.
Rain gardens can be small, formal, home-owner style gardens, large complex bioretention gardens, or anywhere in between.
Why would you want a rain garden?
Rain is natural; stormwater is not. Government studies have shown that up to 70% of the pollution in our streams, rivers and lakes is carried there by stormwater. Although most people never think about stormwater, about half of the pollution that stormwater carries comes from things we do in our yards and gardens!
Planting a rain garden may seem like a small thing, but if you calculate the amount of rain that runs off your roof, you would be very surprised. That rain is supposed to soak into the ground, but instead heads down the street to the storm drain, carrying pollution with it.
Keeping rain where it falls, by putting it into a beautiful rain garden, is a natural solution. You not only get a lovely garden out of it, you have the added benefit of helping protect our rivers, streams and lakes from stormwater pollution. You can be part of a beautiful solution!
How to Get Started
- Avoid creating a rain garden too close to building foundations; this may lead to a leaky basement.
- If you can locate it at least 10 feet and downslope from the building, that should be good. Also, you must stay away from the drainfield if you have a septic system. They don’t need any added water.
- Be aware of rights of way and underground service lines or utilities.
- Avoid excavating or planting in these areas (this includes the drainage ditch in front of your house). You don’t want to accidentally dig up your phone line, and there may be restrictions to activities in rights of way. Before you dig, call ‘JULIE’, and have the area flagged. Hint: take pictures of the flagged areas so you have a record of underground utility locations.
- Re-directing the rain–there are a number of creative and attractive solutions if the rain doesn’t flow naturally to your chosen spot.
- You can install piping underground or send the rain along a constructed channel or swale. Treat these as part of your landscaping–they can be beautiful additions. You can also incorporate a rain barrel into the feature, and direct the overflow hose into your rain garden (this way, you can save your rain for a sunny day!). We will feature examples in our demonstration site area.
- If your land slopes, you can create a flat area for your rain garden in several ways.
- You can create a “weep” rain garden by building a small retaining wall and filling in behind it. The water will soak into the garden, filter through, and weep out the retaining wall. You can also dig out a “scallop” in the side of a hill for your rain garden.
- Black walnut trees growing by the garden may spell trouble, caused by juglone, a chemical exuded from their roots.
- Some plants are sensitive to juglone and will not grow well near walnut trees. If possible, locate your rain garden away from these trees.
- Don’t excavate an extensive rain garden under large trees.
- Trees have root systems that would have to be dug out in the excavation, and the health of the trees may be affected by damaging the roots. If the trees are not species that are adapted to rain garden conditions, directing ponds of water to their roots may also weaken or kill them. Trees enhance our communities and are very good at absorbing rain; protect them.
- Test your drainage!
- You can test your soil’s infiltration rate by digging a hole 8 inches wide and 8 inches deep. Pour a bucket of water into it and see how long it takes to sink in. The water needs to go down an inch per hour. If it takes longer than that, you will need to do additional site preparation to improve infiltration. Soil removal and replacement are often needed if your soil is clay. The recommended soil replacement mix is 50-60% sand, 20-30% topsoil, and 20-30% compost. Be sure no clay is in your replacement soil.
Beginning the Real Work
When you prepare the garden for planting, you must create a dip in the middle where water will collect as it sinks into the soil. There are various zones in a rain garden (very wet, wet to dry, and dry) and different kinds of plants will thrive at different zones.
You may also adjust the depth of the depression to the infiltration rate. The standard depth for the ponding area is six inches. If you have very poor drainage in your existing soils, and your garden preparations still leave you with slow absorption rates, make your depression shallower to reduce the water that gets trapped there. If your soil sucks up water, make your garden deeper to increase its storage capacity. It’s generally best to keep the bottom of your rain garden’s depression flat; saucer-shaped rather than bowl shaped. That way, the rainwater will always spread out as much as possible.
To be certain that your rain garden will function properly, simply replace the soil with the recommended rain garden mix: 50-60% sand, 20-30% topsoil (no clay), and 20-30% compost. This mix allows water to soak in and supports the growth of healthy plants. If the soil is very heavy and/or a lot of water will need to be infiltrated, an under-drain system of gravel and perforated pipe (French drain) may be helpful. This will enable the garden to absorb more rain. Sometimes a rain garden is constructed to absorb and filter a certain amount of rain, and the filtered water is then piped to another location through the underdrain system.
Designing the Pond Area
You can’t have a rain garden that is too large. However, any size garden will make a difference, even a small one. The ideal situation is to create a garden that will absorb all the rain that would otherwise flow away from your yard. To calculate the most useful size of a smaller garden, here’s how:
- Figure out what kind of soil you have.
- Estimate the area from which your garden will get rain. Multiply width times length of your rooftop, to get square feet. Add the square feet of paved areas. Remember, though, that different parts of your roof drain to different downspouts. You want to estimate only the square footage that will drain into your rain garden. Don’t forget roof overhangs.
- For sandy soil, your rain garden should be 20-30% of the drain area. For example, if your roof and driveway measures 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. (ex: 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.
Preparing the Site
Sometimes creating a rain garden is as simple as directing rain to an absorbent swale in your yard and perhaps putting in a few plants. If you have no convenient place to direct your roof runoff, or you want a formal garden design, then it’s time to get down and dirty.
Soils on developed land have been compacted by heavy construction equipment. The soils will need to be dug up and loosened to a depth of two feet, or replaced with rain garden soil mix (50-60% sand, 20-30% topsoil, 20-30% compost), not only to prepare for planting the garden but so rain can soak in. If you have extra soil left over after this loosening process, use it in another part of your yard.
Preparing a rain garden is just as much work as creating any other perennial garden. Get out your shovel and dig to produce beautiful results.
Defining the Borders
First, define the borders and shape of your garden at the location you have selected. When we prepare a rain garden, we outline the area to be dug by spraying with non-toxic soccer-field paint. Another method is to lay a hose along the shape of the garden, then dig along the hose. This gives a nice flowing border to the garden area. Or, you could simply choose a rectangle as the shape of your garden.
If the area is lawn, you will have to remove the turf. You can either use this either in another area of your yard, or it can be composted to help improve your soils. There is a special tool for removing turf that can make this task easier.
If you are not replacing your soil, double-digging to a depth of two feet is recommended. Remove the soil from the hole and pile it to the side; then dig and loosen the earth in a new hole, and pile it loosely into the first hole. This is the time to mix compost into the soil, if you are doing soil improvements. Continue until the whole garden is prepared. A great tool for this purpose is a digging fork. Try it, you’ll like it!
To be certain that your rain garden will function properly, simply replace the soil with the recommended rain garden mix: 50-60% sand, 20-30% topsoil (no clay), and 20-30% compost. This will give your new plants a great start, and the soil mix is designed to soak up rain. If your site soils are clay, soil replacement is probably in order. You may also want to add a reservoir of gravel at the bottom of the garden bed, or add tiles or an underdrain that leads to another area. This will avoid having your rain garden become waterlogged. The idea is to create a living sponge of soil, plants, roots and mulch, not a soggy bog.
How much soil is needed for replacement?
1 cubic yard=27 cubic feet.
A rain garden that is 2 feet deep X 5 feet wide X 10 feet long will need 100 cubic feet of replacement soil, or about 3.7 cubic yards.
Grading the Pond Area
Grade the surface of your prepared rain garden bed in such a way that the water entering it can spread out over a large flat area and soak into the soil. This may involve removing a lot of soil. The depth of the dip should be about six inches. The depth an be graduated from the edges of the garden to the middle. This avoids creating a crater that people can trip and fall into.
When your pond area is ready and the soil is nice and loose, it’s time to plant. You can prepare a rain garden bed and then cover it with mulch until later; then, plant through the mulch. Or, you can plant immediately, then mulch the plants. The choice is yours. The sooner the plants are in, the faster your rain garden will get established.
Why use native plants in planting rain gardens?
We recommend using native plants in your rain garden for a number of reasons. Our garden designs are a combination of native species and of non-native plants (for example, daylilies do very well in rain gardens). Although many non-natives also do well, there are some unique benefits to using natives that you should consider.
Native plants can tough it out.
Plants that are native to your area are uniquely adapted to thriving in the local weather, soils, and ecosystems. They have been working at it for years (thousands, in fact). They live through droughts and downpours, and survive the winters without special care. Fertilizer will make them grow bigger, but they grow beautifully without it. Pests munch on them and the plants bounce right back without chemical pesticide sprays. Just think of the reduced chemical load in your yard! Much healthier for you, children and pets.
Native plants attract beautiful creatures.
Native plants also have relationships with local butterflies, insects, birds, animals and other plants that they have developed by living together over thousands of years. Planting natives in your landscape helps provide habitat for local wildlife. How long has it been since your yard was decorated by butterflies and birds?
Native plants have deep roots.
Native plants are great helpers for protecting water quality in your neighborhood. The deep roots of many established native plants increase the ability of soil to hold water. For example, Blue False Indigo, Baptisia australis, grows only 3-5 feet tall, but the roots may go down 25 feet! These fantastic roots create deep channels in the soil for rain to soak into. Some of the roots die each year, and new roots grow. The decomposing roots enrich soil, making it more fertile and absorbent. The root systems also hold soil together and help prevent erosion.
Many of the native plants that do well in rain gardens are not available from traditional plant nurseries and retailers. One must instead contact certified, inspected professional nurseries that specialize in native species.
Native Plants for Rain Gardens
We recommend our beautiful native species for rain gardens, or a combination of cultivated varieties and natives. The advantage of using natives is that they will thrive with little care, once established. Although native plants species are low maintenance, that doesn’t mean NO maintenance. Designing the garden for easy maintenance is part of your planning process. If you use or adapt existing garden site plans, keep the following guidelines in mind:
Put the right plant in the right place.
Choose plants or a design suited to the existing conditions of the site. For example; if a garden is in deep shade then plants that require sun are not going to thrive. Some plants are adaptable as to soils, and some are not. Choose with the needs of the plants in mind.
If a plant is not happy, move it.
Rain gardens also have three zones; very wet, wet-to-dry, and dry areas at the edges. Our rain garden plant database will let you identify which zone each plant does best in, but you will have to evaluate your own garden to see where in your garden the plants you select will thrive. If a plant is not doing well in one location of the garden, it’s a good idea to move it. Sometimes the wettest areas are not where you thought they would be!
If you use a design with tall plants, think about how this will affect the view.
Some prairie plants can grow ten feet tall. This could be a good thing, or it could inconveniently block the view of street traffic from your driveway.
Mulch makes things easier.
Plan to mulch your rain garden, which will keep it moist, protect your plants, discourage weeds, and make weeds easy to remove. How much mulch? A cubic yard of mulch will cover approximately 100 square feet 3″ deep. A 10 X 20 (200 square feet) rain garden will require 2 cubic yards of mulch.
A rain garden is a GARDEN.
Remember that a rain garden is not merely a functioning infiltration system. Rain gardens can and should be beautiful, attractive improvement to your property. Choose your plantings to delight, and arrange them so that they are pleasing to your eye.
Rain gardens also tend to become wildlife oases. You can expect and should plan for songbirds, butterflies, colors, fragrances, and sounds. Depending on your neighborhood, you may have squirrels, rabbits, or deer visiting your garden regularly.