How to Make a Rain Garden
You might be asking yourself “what is a rain garden?” A rain garden is a bowl shaped, perennial garden designed to absorb storm water run-off from impervious surfaces. They can be small and simple or large and complex. They can dramatically improve the impact that built environments have on the natural environment, especially in urban areas.
In densely populated areas the ground is covered by structures, pavements, cements and bricks. The result is an unnatural charge of precipitation to streams and rivers that has not followed the natural hydrologic process. In these conditions, rainwater and snowmelt rush over man made surfaces (including grass and turf) collecting pollutants such as oil, gas, fertilizers and pesticides. Rather than filtering through vegetation, these polluted waters, known as storm water runoff, deposit directly into surface water bodies. Studies indicate that up to 70% of pollution in streams, rivers and lakes was carried by storm water and about half of this pollution is from residential activities. In addition to being polluted, the volume of storm water runoff can burden streams and rivers leading to serious flooding.
It may seem like a small drop in the proverbial bucket, but any size rain garden has a positive impact on water quality. Any time rain or snow returns to the ground to be absorbed it alleviates some of the storm water runoff and creates a positive chain reaction.
To begin you need to find the right spot for your rain garden. As you start assessing the site, keep in mind that the rain garden is a type of bioretention: a system of water, soil and plants. The easiest way to choose a site is to observe current run off patterns and where run off collects. The low spots of your site may be away from foundations or they might be adjacent to downspouts or paved areas. These are all good places to put your rain garden. If your site is flat and there are no low spots apparent you can still plant a rain garden by “scalloping” the ground. If your site is steep you can employ either the scallop method, create a weep with a retaining wall or create a terrace like rice farmers do in steep territories.
Due to the “moisture” concept of a rain garden, it’s best to locate it away from foundations and septic system drain fields, both are places that don’t need extra water. It’s also important to be aware of any underground utilities that you don’t want to disturb – call your local service providers before you dig if you are unsure.
Once you have chosen a spot, the next step is evaluating the soil. Certain nearby trees can give you your first indication of soil quality. Some trees that will affect your soil quality are pines which are high in tannins and black walnuts which give off the natural chemical juglone. Some plants that you may want to use in your garden are sensitive to both of these chemicals. If either of these trees is close to your garden spot you should consider moving before investing too much energy. In general, it’s best to place your garden away from large trees so as not to disturb existing root systems and they might not like the new moisture conditions created by your rain garden.
After you have selected a good spot for your rain garden you’ll need to evaluate the soil. There are three important factors: fertility, drainage and pH. Drainage is a key component to the rain garden concept and the soils need to be well suited to absorb and drain water. Sandy soils drain very well but are not fertile. Clay is fertile but does not drain well. If you have a sandy soil in your chosen site you can improve the fertility with compost (another great way to improve the environment!). If you have clay soils you will need to replace the soil with a mix of sand, topsoil and compost. Your local garden center can help you get the proper mix (usually 50-60% sand, 20-30% topsoil, 20-30% compost). They can also provide you with tips or resources for pH testing in your area.
If you have your garden spot picked and your soils ready, it’s time to design your garden and start choosing the right plants. As mentioned earlier, any size rain garden has a positive impact on local water quality. However, if you would like your rain garden to offset your home’s storm water run off it should be between 150-400 square feet for a typical one family residence or cover this area with several rain gardens.
You can predefine the size and shape of your rain garden by using a hose. You can also use non-toxic spray paint or stake out the edges with twine. Next, remove any grass or turf from within your defined area (be sure to throw it on the compost pile!). Whether using existing soil or replacing the soil, it’s important to loosen the soils to a depth of two feet for planting and water absorption. Remove the soils and pile nearby.
There are going to be various zones in your garden just like in natural bioretention systems. These will range from very wet to dry. Creating a dip in the middle of your excavated garden is the start of this zone creation and is where water will collect as it slowly absorbs into the ground. Next, grade the exposed surface so that water will spread out over as large an area as possible. Although you have removed two feet of soil, you can graduate the depth from the edges to the middle depression. Use your replacement soil or improved existing soil to fill in the planting area. Be sure to keep the soil nice and loose.
Now it’s time to plant! It is highly recommended to use native plants in your rain garden for several reasons. Native plants will thrive in the given environment without chemicals or fertilizers, they are adapted to local seasonal changes and they enhance the natural ecosystem because local bugs and birds know them. There is a distinction between native and wild. A nursery can provide you with native plants that will add to your site’s ecosystem. Obtaining plants from the wild will disturb the ecosystem, the opposite of your project’s intention.
Choose plants native to your region and suited to your gardens conditions and zones (very wet, wet, dry). You can enlist the help of a local garden center, a knowledgeable friend or do your own research. And remember that in addition to filtering and absorbing storm water, the rain garden should be a visual delight, for you and any wildlife that might visit – such as butterflies and birds.
Once you have your garden planted there are a couple ways to keep it happy and effective. Mulching your rain garden will help protect the plants and discourage weeds. Be sure to supply sufficient water to your new plants until the garden is established. Keep an eye on the how rain affects your garden, it’s possible that the flow might be too heavy in some spots and erode your efforts. You can direct run off using stones or by redirecting storm drains.
Once your garden is established, keep it weeded to avoid invasive plants. Also, keep it free from extra sediment or debris that might collect from storm water run off. Avoid compaction of the rain garden soil in any way such as with heavy equipment or vehicles. In general, check in on your plants and remember that your rain garden has several zones. An unhappy plant may just need to live in a little wetter or dryer part of your garden.
As a perennial garden, this part of your property will be a seasonal reminder of your efforts to minimize your ecological footprint. All your efforts will pay off with a beautiful garden enjoyed by people, plants and animals and year to year your rain garden will work to keep rivers, streams and lakes clean. A beautiful solution to water pollution!
