<b>Trees, Ornamental & Bedding Plant:</b> Trickle-down theory

Walters GardensRain gardens collect stormwater runoff from rooftops, driveways, lawns and parking lots and allow it to soak into the ground.

Pollutants such as sediment, fertilizers, pesticides and even oil, grease and heavy metals from roads are trapped by the soil and root systems, allowing clean water to infiltrate the subsoil and recharge the groundwater. Rain gardens also create habitat for wildlife.

Ideally, rain gardens are sited close to the source of the runoff and serve to slow the stormwater as it travels downhill, giving it more time to infiltrate and less opportunity to gain momentum and erosive power, according to The Low Impact Development Center. Rain gardens can be an excellent solution for both residential and commercial customers.

How it works
Below the surface of the garden, a number of processes are occurring that mimic the hydrologic action of a healthy, productive forest.

The garden acts as a small bioretention cell where stormwater is cleaned and reduced in volume once it enters the rain garden.

Nitrogen and phosphorus levels and overall sediment loads in the stormwater are reduced by the action of the plants and growing media on the water.

Rain gardens are simply low-lying, vegetated depressions – generally 3 to 6 inches deep. The depression should be a flat-bottom, saucer shape rather than a bowl shape so that rainwater runoff can sheet out throughout the garden for better infiltration.

This generally takes a few hours and shouldn’t take more than two days.

Plants for Rain Gardens
Rain gardens can look as informal or as formal as the client requires. There are hundreds of plants that will work well in a rain garden. The same principles apply to rain gardens as conventional landscape beds – right plant, right place.

Here's a sampling of plants to consider.

 

For more plant lists, visit www.rainscaping.org or rainkc.com.  

Planning Guidelines
There is usually an existing drainage pattern for most landscaped sites (even if it’s not very noticeable), and it’s generally best to go with the flow, according to the Chesapeake Bay Small Watershed Grants Program.

Determine the direction of runoff and low spots where water collects, either by observing the flow of water during rainfall or by running hose water on the ground. If the low areas are away and downhill from the building foundation, this is a good place to locate rain gardens.

Many rain gardens are designed to contain runoff from roof downspouts. An extension of the downspout may be buried and discharged into the rain garden. For single-family homes, rain gardens are typically between 150 to 400 square feet. Ideally, all of the rainwater that falls on a piece of land would stay on-site and be captured by rain gardens and rain barrels.

To determine the optimal size for a rain garden for a single-family home, estimate the amount of impervious surface by multiplying the rooftop width by length to get square feet, then add the square feet of paved areas.
Determine which part of the roof, and the square footage, will drain to each downspout, that will then drain to the area sited for the rain garden.

Ideally, all downspouts and impervious surface runoff from driveways, etc., would be channeled to one or more appropriately sized rain gardens.

Follow these formulas, from Rain Gardens of West Michigan, an environmental education program:

For sandy soil, the rain garden should be 20-30 percent of the drain area. For example, if the roof and driveway measure 1,200 square feet and all the rain from them will be used, the rain garden should be 20-30 percent of that, or 240-360 square feet.

For clay soil, the rain garden should be about 60 percent of the drain area.

Rain gardens are part of a water-conservation market landscape designers and contractors can tap into, in addition to green roofs, rain barrels and permeable surfaces.