- November 21, 2017
The hurricanes and tropical storms events of 2017 wereopen_in_new historic. Images from impacted communities can invoke strong feelings. As we look around our homes and communities–both those impacted and those that were unscathed–we may ask ourselves, what can I do?
It is likely that some storms will continue to bring heavy winds and precipitation that may threaten infrastructure no matter what we do.open_in_new Absorbing all the rain water from a mega storm like Harvey may not be possible, but in urban areas landscaping features like rain gardens can absorb rain from smaller events and decrease the frequency of floods in high risk neighborhoods.
In densely urban areas, the high percentage of impervious landscaping–where stormwater cannot absorb into the ground and instead runs off and pools at the lowest point–is a serious contributor to urban floodingopen_in_new. For instance, a study in coastal Texas found that for each square meter of impervious surface added to the landscape, flooding water damaged property costing thousands of U.S. dollars.open_in_new
We have written about storm water management systems such as rain barrels, bioswales and rain gardens–all of these are excellent options. A well-designed rain garden seems to provide the greatest impact for reducing stormwater runoff for individual homeownersopen_in_new. Here we will attempt to provide a detailed step-by-step guide for adding a rain garden to your landscape.
No matter your location, rain gardens can have a positive impact. One place they may be especially helpful are in communities that are upstream from low-lying areas that are susceptible to large, damaging storms. Sometimes the landscaping choices made by homeowners miles away can help to mitigate flooding damage downstream.
Planning a rain garden is an exercise in balance. Specifically, you are trying to balance the amount of rain that falls on a property with the amount of rain that stays on the property long enough to be absorbed.
The ground can only absorb so much water at once, and depending on your square footage of impervious surface area (roof, cement driveway, etc), you may need to build a large rain garden, or more than one rain garden, to balance all the water running off the impervious areas. A habitat map is a great way to help you figure out how to strike a balance by helping you measure the total area of your impervious surfaces.
Using a map from a Habitat Network user, we’ll walk you through how to use our metrics to roughly calculate how many square feet of rain garden(s) you may need. The general rule of thumb is that rain gardens should be five to ten percent the size of the impervious landscape that is generating runoff. We recommend, then, a rain garden should be 7% (0.07%) of your total impervious surface. The Habitat Overview in the mapping tool calculates the square feet of your impervious surface if you hover over the buildings and pavement. Then you can add these together for your total impervious square footage and multiply by 0.07 (or 7%). Use the image as a guide for your own calculations.
Now that you know how much square footage is required to offset your impervious surface in rain gardens, take a walk around your property and decide whether the layout of your downspouts, pavement, and other features means one large rain garden or multiple rain gardens. Keep in mind you may not be able to provide a 100% offset. Or, like the example above of this Habitat Network user, the Shady Maple House is offsetting their entire impervious surface, plus an additional 87.8 sqft.
Your landscape may have other features that will help absorb runoff from impervious surfaces‒mature trees! Consider that during a one inch rainfall on a 1200 sqft. house approximately 750 gallons of water runs off the impervious roof. Depending on the species of tree, anywhere between 25-200 gallons of water can be absorbed by a single mature tree in twenty-four hoursopen_in_new. So, while planting and maintaining trees can help manage stormwater, a property will still need some way to retain water on site to help with absorption.
Found the perfect location? Excellent. Now we’ll walk you through the process of creating a rain garden. For the purposes of this article, we will stick to 100-300 square foot gardens that a homeowner could create on their own without the use of heavy equipment. For larger gardens or sites that are more complicated, a local landscaper may be worth engaging in the process.
Step 1: Find a perfect spot using these guidelines
- A site that is down slope from the buildings and more than 10 feet from the building’s foundation.
- A location that uses the downspout from your roof. Two or more downspouts can mitigate approximately 50% of water run off in most storm events.open_in_new
- The area should stop and hold the water before it enters a sewer, road, or sidewalk.
- A site that is approximately 100 to 300 square feet (9.3 and 27.9 square meters).open_in_new
- Has soil that drains well or can be amended to drain well as infiltration capacity is a key to success.open_in_new
- An ideal location for adding forb-rich, broad-leafed flowering perennial plants (such as black-eyed susans or milkweed), as there is evidence to suggest these provide the best retention and detention services in rain gardens.open_in_new
- Choose a location in full sun for six or more hours a day.
- A site that avoids large trees with complex root systems or utility lines that could be damaged when digging out the garden.
Step 2: Choosing a shape for the garden and test your soil
Helpful Tools: String or rope, shovel, watering can
Mark-out the shape of the garden using rope, twine, extra garden hose, flour, or other materials that can be placed on top of the surface where the garden will be. This will provide a visual outline for your project.
A bean-like shape with a gradual slope towards the center is common for rain gardens. Similar to a catcher’s mitt, the garden will be elevated on one side (the finger cushion area) and slope towards the middle (palm) to help water move to the deeper center of the garden to maximize infiltration.
Defined: A berm is simply a raised or terraced portion of the rain garden.
A small berm will be created on the downhill side of the garden or if the spot is flat, a berm can be built around the entire area or on a preferred side of the garden. The water will move away from the elevated berm side so you’ll want to plan accordingly.
To test the soil, locate the middle of your garden and dig an 8-12 inch hole and fill it with water. Monitor this hole over 24-48 hours for drainage.
Ideally, the water will drain in 24 hours. If the water is still in the hole at the end of 24 hours one of two things need to happen for your rain garden to be effective at absorbing stormwater: 1) A new site can be located, or 2) you can plan to mix in sand to the soil to increase the rate of infiltration.
Desert Rain Gardens
Research suggests that the best rain gardens utilize plants such as broad-leafed flowering plants (forbs) compared to lawn and open_in_newbare earth. This, however, may not be as practical in a desert environment. Consider utilizing rocks and sand in the substrate depending on the native plants you are using. Some flowering plants to consider for your desert garden: arroyo lupine (Lupinus succulentus) or mojave lupine (Lupinus sparsiflorus), california poppy (Eschscholzia californica), desert marigold (Baileya multiradiata), mountain phlox (Phlox austromontana), or eaton’s penstemon (Penstemon eatonii). The general construction of the rain garden follows the same principles provided in this article.
Step 3: Digging out the garden and adding soil
Helpful Tools: Pick-ax (or digging iron), shovel, garden hoe, and wheelbarrow (or bucket)
Following the established outline for the garden, dig down and remove six to eight inches of the soil with the deepest part of the garden located in the center. If the soil is compact, a pick-ax might be best used to break-up the soil before removing it with shovel and wheelbarrow.
The removed soil can be used to provide the base of the berm, or the elevated side of the rain garden. The berm acts to help direct the water towards the deeper center of the rain garden.
The height of the berm can vary but should be approximately six to eight inches above the ground level or 12-16 inches higher than the lowest point of the rain garden. The berm is a combination of the removed soil from your hole plus extra amended soil on top to provide more nutrition to rain garden plants (see below for recommendations for type of soil to use.)
If the site for your garden is flat, you may choose to build a berm around the entire garden, essentially creating a circular depression. The berm also provides a “dryer” area in your rain garden. If the soil you remove from rain garden is clay-rich, you may need to amend the soil in the berm with sand so that absorption is still maximized in the berm.
Most soil types will require adding new soil to your rain garden to support the native plants you will be adding. The composition of the soil that you add to your rain garden can vary slightly depending on of your existing soil. Ideally the newly added soil is:
50% sand + 25% top soil + 25% compost
This ratio will provide appropriate levels of infiltration while also providing sufficient nutrients to feed the growing plants.
If your original soil is rich in sand you can get away with adding a higher compost amount. Or, if you have very compact soils you may want to add more of a 60% sand, 20% topsoil and 20% compost. Use your best judgement based on your soil conditions. Premixed soils can often be purchased in large amounts from local landscaping companies.
Step 4: Planting your Rain Garden–Use Natives
Helpful: Tools: Native plants, hand shovel, gloves (if preferred)
Native plants are not only adapted to your area but provide better habitat for wildlife, which doubles your rain garden’s positive impact. Choose flowering plants, grasses, and shrubs that provide resources at different times of the year. Our planting palette article may help you with this process.
Plants that require drier soils should be planted in the berm, with water-loving plants in the lowest part of the garden. Using perennial flowering plants and shrubs in your rain garden will minimize the amount of annual work, as these plants will come back year after year. Make sure to provide enough “growing room” for each plant you add to the rain garden, generally 8-12 inches between native perennials.
A layer of mulch applied to the top after planting will help to give the garden a strong start by minimizing weeds and helping to hold in the moisture while the roots establish themselves. Make sure to thoroughly water the site right after planting to avoid plant-shock.
Rain gardens are one of those special landscaping features that are a triple win–beautiful, functional, and helpful for humans and wildlife. Any amount of water that can be absorbed back into the water table from impervious surfaces provides a helpful management feature in our built environments. What could our collective impact be if every community within 100 miles of a coastal area in United States installed a rain garden? Could we substantially minimize the flooding during some storm events? There’s only one way to find out…
Have a Rain Garden to Map?
In the toolshed, choose “second” for mapping a habitat. Rain gardens are lumped together with bioswales and bioretention cells–all of which are considered “Stormwater Management”. Once you have mapped this area, you can tell us more about this habitat, like the specific type of feature that it is, the run-off source, the % native plants used, and the run-off source size.