Choose The Right Place for Your Rain Garden

A rain garden is one type of “bioretention”—a system of pond area, soil, plants and mulch that will retain water and soak it up instead of letting it run off of your property (even though your “pond” will be dry most of the time).  So the most basic things are the “pond,” or depression into which water will flow, and the soils that will absorb the water.

Your property has an existing drainage pattern (even though it may not be very noticeable), and it will usually be easiest to take advantage of that. Note the direction of runoff and low spots where water collects. If you are not sure where these are, and it's not raining, find them by running hose water on the ground. If these spots are away and downhill from your building foundations, they will be good places for your rain garden.

If there is a spot on your property where standing water collects, this area has poor infiltration. You may think it is thus the wrong spot for your rain garden. However, if you could collect water anywhere on your property, these areas would likely have poor infiltration also. This is because soil is compacted during construction in order to prevent sinkholes and to support building foundations and other structures. This general compaction of building site soils makes the need for your rain garden even more critical.

If you have many low spots, you can choose those that are closest to the downspouts from your roof or nearest to a paved driveway.

If the layout of your land is such that you don’t like the natural low spots (or they are not very low), you can create your own by excavating earth—anywhere that you can get rainwater to flow, either directly or even by installing piping. For preparing large gardens or in very hard soil, it may be worth your while to hire an excavator to bring in a machine for a few hours. Have the excavator use the removed soil to create a knoll or other pleasing contour to your property. Be aware that driving any kind of heavy equipment on your land will compact the soil, and reduce its ability to absorb water.

If your land is sloped and all the water drains away, you can still build a rain garden by designing a level "scallop" or small terrace in your slope, and creating a rain garden there.

A few hints for choosing a spot:

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 “Miss Dig”, 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, due to 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. If you can’t, we have a list of juglone-resistant plants (Can't read pdf files? Click here to download Adobe Acrobat Reader).

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.

The next step is to evaluate your soil.