Avoiding problems with your rain garden

In my last post, I mentioned finding many google topics involving problems with rain gardens.  It's not surprising that it took a few years for these situations to creep up as rain gardens were not put to use until about ten years ago.  There has been a learning curve and it will continue.

Three main issues contribute to a rain garden's functionality problems.  They are:

- Poor design
-Poor maintenance
-Poor initial contruction

We'll talk about the poor design aspect today.

The depth, width and length of your rain garden must create a depression large enough to handle the volume of water you would expect from a normal rain event.   To determine these measurements,  you  must know the rate at which water will perk or pass through your yards soil type.  Even though you will be excavating a depression that you'll fill with a special rain garden soil mix, the underlying and surrounding soil are going to influence how quickly water passes on down through subsequent layers of sub soil and into soil areas surrounding your rain garden. 

If we assume you have a sandy loam soil, water will pass through that soil in a matter of hours given a normal rain event.  This allows you to dig a deeper depression with less surface area (width & length).  Keep in mind we want the standing water gone within 24 to 48 hours maximum.  You should be taking into consideration what square footage of impervious surface you are draining to this rain garden.  Unless your house is quite small, it is doubtful that you'll be mitigating the rain water from more than a portion, perhaps one side of a roof.  Every run off solution makes a small contribution to the overall problem so don't be dissappointed that you can't contain all your storm water.

If we assume that you have a heavy clay soil, water may take many days to pass through sub soil layers if it drains at all.  In this case, the portion of impervious surface you can drain to your garden will be smaller and your rain garden will be much shallower and have a larger surface area.  You'll probably want to increase the sand percentage of your soil mix.  You won't encounter mosquito problems from standing water if all the water is actually down within the sand, compost, soil mix.  A combination of evaporation and plant root water uptake will eliminate the rain water that was contained in your rain garden eventually.

Size isn't the only design issue that can cause problems.  If the edge of the rain garden slopes off too steeply, you may not be retaining the rain water but merely slowing it down as it moves downhill.  Make sure that the edge of the garden is high enough to prevent the water from running in and then right back out.  You want to keep that water in the rain garden.

If you are just now considering the use of a rain garden in your yard, careful research will help to avoid  making mistakes in your overall design.  It's easier and less costly to do it right than to do it over.

We'll save the discussions on maintenance and installation problems for future posts.