Planning and Planting a Rain Garden

The Purpose of a Rain Garden

A rain garden is landscape feature that helps reduce flooding, reduce stress on municipal water processing, and reduce pollution. During a rain storm the water that runs off the hard surfaces in the environment is call Water Runoff. The hard surfaces can be roofs, patio, lawn, driveway and roads, etc. The amount and intensity of water flowing from these surfaces during a large rain event is immense and sudden, and contributes to flooding and pollution, and overloads municipal water systems. The purpose of a rain garden is to capture excess water runoff from these hard surfaces to keep it from flowing into the water system, lakes and rivers. Rain gardens are comprised of plants with large root systems that can withstand the inundation, slow the entry of water into the water system, and filter the water before it enters the water table.

Rain Garden Location

Rain gardens should be installed at least 10 feet away from the foundation of your house or building, and in-line with a downspout and/or slight downslope to intercept the rooftop and ground-level runoff. Rain gardens should be planted in full sun. Some shade is OK, but the garden itself should be situated beyond the tree drip-line (not beneath a tree).

The Ideal Location for a Rain Garden is:

At least ten feet from the house,
In a gently sloping area,
At least eight feet from any well,
At least five feet from any septic system,
Not directly over a sewer lateral or buried utilities,
Not where tree roots may make it difficult to dig or where digging could damage a tree,
Not in areas that receive heavy foot-traffic.

To Function well, a rain garden should be situated on a flat or gently sloping area of the yard. The depth should be between four and eight inches deep. The depth allows the rain garden to hold water during periods of heavy rainfall. (A rain garden which is more than eight inches deep might pond the water for too long.) Clay soils need the most depth, and very dry sandy (fast draining) soils need less depth.

The planting basin area should be level.

Rain gardens situated on steeper slopes will require more digging to level the garden area. STEEP slopes, over 12 degrees, may require a terraced rain garden. It makes sense to situate a rain garden on either a level or slightly sloped area, if at all possible. Gardens on steeper slopes, and gardens placed farther away from the main source of water runnoff, need to be larger than the estimates given here.

Sizing Your Rain Garden

What Size of Rain Garden Do I Need?

To arrive at the size for your rain garden you need to estimate the amount of water that will flow into the garden. Your rain garden is meant to receive water from hard surfaces that shed “rainwater runoff.” These surface areas (from roofs, driveways and lawn/hillside) are the measurements you need to figure the best size for your garden.

Rule of Thumb:
The garden area should = 25% of your drainage area.

Example:
If 400 square feet of your roof surface drains to a downspout, then garden area for that downspout might be about 100 sqare feet. (400 x .25 = 100)

You might make it LARGER than 25% if:

The soil contains a good deal of clay (slow draining),
you want to insure that you infiltrate even largest storms, or
you’d simply like a larger garden

You might make it SMALLER than 25% if:

The soil is very sandy (quick draining),
parts of the drainage surface area are permeable surfaces (e.g., lawn), or
you don’t have enough space

Rain Garden sizes are approximate. Your yard may not have enough room to capture 100% of the roof water runoff. By focusing on just a section of your roof, you can plant a smaller garden and capture water from just that portion of your roof.

Estimate the Roof Area that Drains into the Garden

To estimate the roof area that drains into the garden, multiply the footprint of the house by the percent of the roof area that sends rain to the downspout. If there is a nearby impervious area like a patio or driveway that also drains into the rain garden, add that additional surface area to the calculation of your roof area. (Impervious areas may need to be connected to your rain garden by constructing a shallow ditch across the lawn.) When the rain garden is closer to the house (at least 10 ft away) almost all of the water entering the garden will come from the roof downspout.

Choose Your Rain Garden

200-250 SQ FT AREA:
20’ x 10’ = 200 square feet of rainwater runoff surface area.
32 Plant Rain Garden

300-400 SQ FT AREA:
15’ X 25’ = 375 square feet of rainwater runoff surface area.
64 Plant Rain Garden

500-700 SQ FT AREA:
20’ X 30’ = 600 square feet of rainwater runoff surface area.
128 Plant Rain Garden

Our Pre-Planned Rain Gardens come in three sizes: 32, 64 and 128 plants. We’ve done most of the planning for you. The gardens come with plants, a plant list, and a planning design that includes a QR code that links to these instructions.

Instead of a pre-planned garden, You may prefer to select your plants from our list of Plants for Rain Gardens, and design the garden yourself. This way you'll be able to customize the size, and maybe even include a shrub or two.

Preparing the Site

If your rain garden will be situated where there is an existing lawn, you’ll first need to remove the sod layer. Killing the grass will make sod removal easier, and will ensure that long taproots of weeds don’t sprout within your rain garden. An herbicide can be used to spray/kill the lawn area, but a more environmentally friendly approach is to smother the lawn until all growth dies. This usually takes a full growing season. Use black plastic sheeting anchored with landscape pins, or sheets of cardboard weighted down with rocks, to completely cover the planting area. Sod-cutters, available for rent from hardware stores, make it easier to cut and remove sod, which can be composted or used in the berm.

Contructing the Planting Bed

Digging the rain garden
size, shape
water in, direction
berm side
flat and even on the bottom (level)

Clay soils need 7-8” depth
loam medim soils 6 -7” depth
and very dry sandy 4-5” depth