Looks Like It's Starting to Rain! Why Not Make a Rain Garden?

by Thomas McGeehan, PSU Montgomery County Master Gardener

There is no better water for garden plants than rain.  Rain water is rich in nutrients and devoid of chemicals that municipal water must use to make our drinking water safe but can have an adverse effect on some plants.  it's a shame to waste such a resource.  In addition, run-off carries debris and pollutants into downstream creeks, rivers and lakes.  Even our bays and oceans are affected by excess run-off. 

Increasingly, gardeners have promoted rain gardens to help ameliorate the problem.  The resulting benefits are an increase in the amount of water that filters to the local aquifer,  a new bed  to beautify the garden and perhaps valuable habitat to attracts birds, butterflies and beneficial insects.  If your garden is plagued by standing water after a rainstorm, a well placed rain garden can turn an unsightly mud puddle into an esthetically pleasing garden of plants and hours of enjoyable butterfly and  bird watching. While standing water is an ideal breeding place for mosquitoes, a fast draining rain garden eliminates that threat.  What's more, the rain garden plants attract dragon flies that eat the mosquitoes that may come from neighboring sites.

A "Rain Garden" is simply a shallow basin in a yard that is planted with flowers, shrubs and grasses that are tolerant of or prefer damp conditions.  The basin collects runoff from roofs, intercepts ground water runoff and sump pump discharge. The substructure of the basin in designed to  let the water percolate naturally deep into the ground.

A little research is in order to ensure a successful rain garden project.  the main considerations are where to put the rain garden, how big and deep to make it, and what soil and amendments to use to maximize the infiltration of water into the deeper ground.  Where to put it is a matter of esthetics and engineering.  How large it needs to be depends on how much water it must collect.  What kind of substructure soil to use depends on the slope of the site and the ability of the main garden soil to drain the water completely.

Use basic design principles for placement of the garden.  Consider how it fits in with existing and future landscaping plans. Use a pleasing shape like a crescent, kidney or teardrop rather than a square or rectangle.  Pay attention to the views from inside the house as well as from strategic points from  the overall garden such as outdoor gathering places, play areas or a patio.  When choosing plants, consider color, fragrance and seasonal interest. 

Some practical criteria are - avoid under big trees where it will be difficult to dig and where additional water may damage the tree, the garden should be at least 10 feet away from the house so that infiltrating water doesn't seep into a basement, it should not go over a septic tank, avoid high traffic areas where existing soil can be compacted, avoid areas where water already ponds up as that shows that the soil is too easily saturated in that spot.  If the roof drains more to one side than the other, put the rain garden in line with the dominant downspout.

The size of the rain garden is relative to the amount of water that will drain to it.  The surface area can be almost any size.  However, less surface area means that the deeper (and usually more expensive) the basin must be.  Precise calculations are vital only if the goal is to contain 100% of the runoff.  Any reasonable size will serve to control some storm water and will provide garden space for some water loving plants.  An average residential garden would range from 100 to 300 square feet and be four to eight inches deep.  Let's say a house covers 2400 square feet.  One inch of rain (1/12 feet) on the house would deliver about 200 cu ft. of water to the rain garden.  A rain garden six inches deep and 300 square ft. of surface would contain 150 cu ft. of water.  So you either expand the rain garden, go deeper or accept the fact that 75% to 80% of the water will be contained.  That might be enough.   

On the other hand, feel free to make the rain garden bigger if you want to have more plants.  That's not usually a dilemma for gardeners.   To help the water infiltration go deeper, one could excavate deeper than six inches, fill the excavation with sand and compost which will increase the infiltration efficiency and  bring the basin up to six inches deep when dry.  You could also bury a perforated trash can, fill it 2/3 way with rock, wrap and cap it with a water permeable barrier and then fill it the rest of the way with a mix of sand and mulch and it becomes a container for plants.  Complete the rain garden by mounding a berm around the perimeter with well compacted clay soil to form the basin, prevent erosion and which then can be planted with plants that tolerate heavy clay.

Plants for Rain Gardens
Plant choices are a matter of personal preference.  That said, the plants must be appropriate for sun or shade depending on the location and be able to tolerate periods of standing water and damp conditions.  In addition, Southeast Pennsylvania can have periods of extended dryness.  Plants native to the area have evolved to tolerate these condition so it is best to include some natives in your plans.  On the other hand, gardening should be fun and occasionally exciting.  So why not experiment with some plants that are a bit out of the ordinary just because you can now that you have a rain garden.

Finally,  add some non-plant structure to the rain garden such as statuary, native stone or boulders, stepping stone trails or even a bench.  This gives the garden a well planned appearance and ensures multi-seasonal interest that both you and your neighbors will appreciate.

Our Plants in the rain garden proper

• Hakone chloa macro 'Naomi' (Hakone grass)
• Ilex verticulata 'Red Sprite (winterberry holly)
• Ilex verticulata "Jim Dandy'
• Filipendula ulmaria 'Aurea' (Meadowsweet or Martha Washington's Plume)
• Dryopteris erythrosora 'Brilliance'  (Autumn fern)
• Colocasia esculanta 'Gigantica' (Taro)
• Iris lousiana 'Black Gamecock'
• Iris fulva (copper Iris)

Our Plants at the higher perimeter

• Host spp
• Bletilla striata (Hardy Japanese orchid)

Other Plants to Consider:

Natives for the Northeast and Middle Atlantic States from a list by the Brooklyn Botanic Garden

• Asclepias incarnata (Swamp milkweed)
• Chelone glabra  (Turtlehead)
• Eupatorium maculatum (Joe-Pye weed)
• Lobelia cardinalis or syphlitica (red or blue lobelia)
• Monarda didyma (Oswego tea)
• Athyrium filix-femina (Lady fern)
• Osmunda cinnamomeaa (Cinnamon fern)
• Fothergillla gardenii (dwarf fothergilla)
• Viburnum dentatum (Arrowwood)
• Mertensia virginica (Virginia Blue Bells)