Rain Gardening

Posted by Joe Bauer

Rain falls on the ground, gets soaked up and then travels through the ground to a river, dumps into a large body of water, evaporates, then turns into rain and the cycle continues.  This is the classic water cycle we all learned in school.  Problem is, in developed areas a major step of this cycle is skipped.  Take a walk down a city street and look around at all the roofs, concrete, and streets.  Water can’t get to the ground, it instead gets routed to drains which then go directly to the river.  This means two things; 1) water gushes into the river much faster than it used to, and 2) the water carries all the pollutants with it directly to the river. 

Water traveling through the water table feeds a river at a steady, gradual rate.  If we route water directly to the river via pipes and sewers then that steady water flow turns into abrupt floods of water during even mild rain events followed by long dry spells between rain events.  Water levels oscillate between high and low making them become unstable with their banks looking like cliffs and trees falling over into the river. 

The ground and plant life on the surface act as filters for the water before it arrives to the river.  It reduces pollutants and turbidity.  The sewer lines provide a direct line to the river without this filter.  For example, runoff from a yard brings with it the phosphorus from fertilizer.  Concentrated phosphorus in a river creates algae blooms which alter the amount of sun and oxygen the water in the river gets, which kills plants and fish.

A rain garden is an attempt at putting that missing piece of the water cycle back.  Instead of diverting rainwater runoff from a roof to a sewer drain, you divert it to a rain garden where it soaks into the ground.  At its most basic a rain garden is a shallow basin with native plants planted in it. 

Site selection:  You want to find a spot that’s ten to fifteen feet away from your foundation.  You can test the absorption rate of the ground by digging an 18 inch hole in the ground.  Fill it with water once- after that’s soaked up fill it up again and if it soaks up within 24 hours then you don’t need to amend the site.  If it doesn’t soak up within 24 hours then you’ll want to mix sand and compost in. 

Site preparation:  First, figure out how big your rain garden should be.  This formula is what the City of Ann Arbor recommends:  Take the square footage of your roof, divide it by the number of downspouts, then take one fifth of that- the resulting number is the square footage your garden should be.  So, if you have a 1,000 sqft. roof with 2 downspouts then you need a 100 sqft. garden.  Mark out the border of your rain garden and then start digging.  The center of the garden should be 3 inches below the border of the garden.  To get the water to the garden you can extend your downspout, or dig a little trench to the rain garden. 

Plants:  Native plants are among the best plants for this job.  They have deep long roots that can help water find a path down to the water table.  The deep roots also help the mature plants survive periods of no rain.  You’ll need to pick plants that are accustomed to becoming temporarily flooded.  A few good candidates in Michigan are swamp milkweed, new england aster, blue flag iris, and big blue stem grass.  In natural settings most of these kinds of plants are found at the edges of ponds and streams. 

http://www.raingardens.org/Index.php

http://www.ewashtenaw.org/government/drain_commissioner/dc_webWaterQuality/rain_gardens/tour/raingardentour.html