Planting with a Purpose: Building, Planting, and Maintaining Your Rain Garden
Building Your Rain Garden
After planning you rain garden, the next step is to build, plant, and maintain it. Use string, rope, a garden hose, or builder’s chalk to mark out the edges of the planned garden. Double check that there aren’t any utilities in the way, and that you aren’t disturbing existing desirable vegetation. The roots of many trees extend as far out as the edge of the canopy.
Decide where you want the inflow and overflow to be. Mark those locations with stakes. The inflow is where water will enter the rain garden, and the overflow is where excess water will drain off. You can have more than one inflow, or no specific inflow.
A small, shallow rain garden can be excavated by hand, but if your rain garden needs to be larger or deeper, consider renting a mini-excavator. If using a mini-excavator, be careful not to drive it into the garden itself, as it can easily compact the soil, dramatically reducing drainage rate.
You will need to dig beyond the ponding depth to provide room for adding soil. The recommended soil depth is 12-24 inches. If your soil is poorer quality, such as a clay heavy soil, consider going with a soil depth of 24 inches, to allow more room for plant roots and water storage. You should dig a hole that is as deep as the soil depth plus the planned ponding depth plus 6 inches for overflow. The extra space for overflow might not be needed if you intend to create a berm, a raised line of soil. A berm is recommended for rain gardens on steeper slopes. However, sandy soil is ill-suited for a berm.
You’ll need to either import new soil or alter your current soil. Consider importing new soil if your soil is poor. You can alter soil by mixing in compost, to help your plants establish themselves. You’ll need to know how many cubic yards you want, since many soil companies measure deliveries in cubic yards. If adding compost, mix 2 parts soil for 1 part compost.
- First, convert the soil depth to feet. Divide the soil depth by 12 to get the number of feet.
- Then, multiple the soil depth (in feet) by the area of the rain garden (in square feet).
- Your answer will be the number of cubic feet you need. There are 27 cubic feet to 1 cubic yard, so divide the number of cubic feet by 27.
- Your answer will be the number of cubic yards you need.
- If composting, divide your number by 3 to get the number of cubic yards of compost.
Don’t add sand to clay-heavy soils in an attempt to improve soil quality, since this can create a concrete-like texture.
Your rain garden will need to be as level as possible overall, so that water doesn’t pool and spill over one side. If digging on a fairly flat surface (less than 5% slope), dig soil to the depth you decided on earlier. If digging on a steeper slope (5% or greater), then more care will need to be taken. The easiest way to dig out a steeper slope is to dig the downhill side to the desired depth, then create a flat bottom.
To create the overflow containment area, either construct a berm or dig down from the surface.
A berm should cover three of the rain garden’s edges – the downhill edge and the two sides perpendicular to the slope. A berm should be at least 6 inches tall and at least 12 inches wide along the downhill edge of the rain garden, tapering down from 6 inches to level with the ground uphill on the sides. Remove any vegetation under where the berm will go. Carefully compact the berm’s soil. If the overflow extends through the berm, line it with rocks to prevent erosion.
If choosing to dig down, start out 12 inches from the edge of the rain garden and construct a slope 6 inches down into the rain garden proper.
Install the inflow after excavating, but before adding in the soil. The inflow (or inflows) can consist of spout extensions, pipes, swales, runnels, or other features. Line the swale and the exit point of pipes and spout extensions with rock to slow down water entering the rain garden.
Carefully fill the rain garden with soil, gently walking over the soil every 6 inches to lightly tamp it down. Fill the soil until it is at the desired soil depth, checking that you’ve left room for the ponding depth and the overflow. Once done, check that the bottom of your rain garden is level.
Use washed/sediment-free round rocks (such as cobble or river rocks) that are a minimum of 2 inches in diameter to line the inflow and overflow. If the overflow passes through a berm, be careful to provide extra protection and extend the rock-lined overflow at least 4 feet past the berm to help prevent erosion. Consider edging the rain garden with the same rocks, to help prevent encroachment from the lawn and to nicely tie-in the overflow and inflow.
Planting Your Rain Garden
Rain gardens can be split into three zones for plants.
- Zone 1 is at the center and bottom. Plants in Zone 1 should be able to thrive in standing water for at least a few days at a time.
- Zone 2 is on the side slopes (of the overflow area and/or the berm). Plants in Zone 2 should be able to tolerate occasional standing water or water-logged soil.
- Zone 3 is on the perimeter of the rain garden, including the opposite slope of the berm. Plants in Zone 3 should be those that prefer drier conditions.
A successful rain garden planted with native species will attract beneficial insects and birds, further brightening your garden. The National Wildlife Federation has guidelines for developing wildlife habitats, and several states have lists of plants native to them. Your local plant nursery should also know what grows best in the area, when and how to best plant each species, and how best to care for specific plants.
Choose plants with a mixture of different textures, heights, and bloom times for visual interest. Include some evergreens in the mix to have year-round color. Plant according to each plant’s size at maturity, so that plants don’t overcrowd each other. If there are underground utilities nearby your rain garden, choose plants whose roots won’t threaten pipes.
Consider grouping plants by type and sticking plastic labels into the ground with the name of the plant. This will help you identify your plants when weeding, especially when they’re still young.
Maintaining Your Rain Garden
Water immediately after planting and continue to water as needed. Most plants will have instructions saying how often they need to be watered. Once plants have established themselves, you should only need to water during droughts or dry seasons if you’ve chosen native plants.
Maintain access to the bottom of the rain garden so you can still weed even after plants are mature. A few flat rocks can serve as stepping stones without compacting the soil. Weeding will be needed for at least the first two years. Weed by hand, removing only those plants you’re certain are weeds. Make sure you remove the weed’s roots. Even after plants have established themselves occasional weeding may still be needed.
Mulch the garden, especially when plants are still establishing themselves. Mulching helps retain soil moisture, control weeds, replenish soil nutrients, and prevent erosion.
Avoid using fertilizers, pesticides, or herbicides as these things can pollute your rainwater run off and damage other systems downstream. Natural compost is okay to use.
And last but certainly not least – enjoy! Get the whole family involved with planning your rain garden, discussing the different benefits and needs of different options. Think about how you might use the space. Do you want to add a bench for reading? Maybe a path of stepping stones for quiet reflection? Rain gardens can be a wonderful addition to any home, adding not only value at resale, but also significant value to your quality of life.
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