University of Vermont
Department of Plant and Soil Science

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RAIN GARDENS FOR THE NORTH

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

If youíre like me, when you first heard of ďrain gardensĒ you thought of gardening in the rain, or some tropical rain forest.  Rain gardens actually are slightly sunken gardens, usually with native plants, whose purpose is to collect rain run-off.  These ďbioretentionĒ gardens are easy to create, fit into odd shapes in most landscapes, and are easily maintained.  While rain gardens are a destination for runoff, rain swales (longer than wide), are for water to travel through them.

Up to 70 percent of pollution in our above-ground waterways comes there from storm water.  About half this storm water, especially in urban areas, comes from landscapes and building roofs.  Collect even some of this in your yardó in a rain gardenó and youíll be helping the environment.  You donít need a large surface area for collecting rain, or a large garden, to make a difference.  Any size rain garden helps. Rain gardens can remove up to 90 percent of nutrients, and up to 80 percent of sediments, from rain runoff.  They allow 30 percent more water to soak into the ground, compared to a traditional lawn.

Since the idea of your rain garden is to hold large surges of water, then let it seep slowly into the soil instead of running off your property, place such a garden in low spots fed by a gentle slope, if possible.  Try not to place it where water already pools, as this indicates poor drainage.

It should be near drain spouts, but not too near buildings (at last 10 feet away) to avoid water seeping into basements.  If the rain garden needs to be 30 feet or more away from drain spouts or runoff area, you can direct it there either with a shallow swale or buried drainage pipe.

Beware of underground utilities if excavating.  But if digging, donít do so under trees or you may seriously damage tree roots.  In fact, donít place rain gardens under tree canopies where they will fill up with leaves.  Make sure they are 25 feet or more away from a septic system, or wellhead. 

Rain gardens donít need to be deep, only four to eight inches is sufficient.  Much deeper and the garden will take too long to drain.  Keep the bottoms flat to help disperse the water more evenly.  Shapes are usually curved, like an oval or kidney shape, rather than a formal rectangle. The usual size to deal with runoff from most homes is 100 to 300 square feet.

An ideal rain garden soil will have an infiltration rate of at least one-quarter inch per hour.  What this means is that if you dig a hole in your soil, and fill it with an inch of water, in four hours it should be goneósoaked into the soil.  After a heavy rain, water should be gone from a well-designed rain garden in one to two days.  The purpose is to have rain soak into the soil, not just stay there as it would in a pond.

Since the water from a rain garden should seep slowly into the surrounding soil, soil type is important.  You donít want clay soil that will merely hold the water.  If your soil is even light clay, you should replace it for your rain garden. Merely adding sand to a clay soil likely will not help. Sandy or gravely soils, with added compost, are ideal.  If replacing soil, you may need to do so to a depth of two feet.

Since this is a garden, and will remain dry much of the time when not raining, what about the plants?  These not only help beautify a rain garden, but also their roots create channels for water to infiltrate the soil.  They absorb water, giving it off through their leaves.

You may use non-native plants, but native ones are particularly adapted to local conditions and often have deep roots.  Youíll want plants that arenít too fussy, that will live both in dry conditions and during wet periods.  One of my favorite plants for such conditions is the Siberian iris.  Whatever plants you choose should be vigorous, to establish easily, and have healthy root systems.  Large plants will establish more quickly.  If you canít afford all large plants, perhaps use 20 to 25 percent mature size plants, the rest being smaller. 

In addition to looking for a large proportion of native plants, look for a diversity of ones with various interestsófoliage, blooms at various seasons both for an attractive design and to serve pollinators, and seedheads for birds and visual interest.  If you donít find actual species, cultivars (named cultivated varieties) of them often work as well.  Just make sure to choose ones with flowers as similar to the species as possible (no doubles for instance), to be most attractive to pollinators.  

Pollinators also will be looking for a group of the same plantóeither together or scattered throughout a garden bed. The latter also will add more visual unity to your bed design. To attract pollinators, use at least six plants of each species.

Consider sedges, rushes and ornamental grasses (such as switchgrass) to add with flowering perennials and shrubs.  One of my hardy favorites is Blue Mohawk rush, with its upright bluish leaves about two feet tall, and growth in both dry soil and standing water.

Some herbaceous perennials for northern rain gardens in shade might include Drummondís aster, Joy-Pye weed (not really a weed), wild geranium (Geranium maculatum), cardinal flower, ostrich fern, and wild blue phlox (Phlox divaricata).  Perennials for sunny rain gardens include the above plus New England aster, swamp milkweed (Asclepias incarnata), Queen-of-the-prairie (Filipendula rubra), Helenís flower (Helenium autumnale), daylily, Siberian iris, blazing star (Liatris), marsh marigold, great blue lobelia (Lobelia siphilitica), beebalm, summer phlox, goldenrods (one of the best perennials to provide insects for birds), spiderwort, ironweed (Vernonia), and Culverís root (Veronicastrum).

Shrubs might include redosier dogwood, elderberry, ninebark, winterberry, and viburnums such as highbush cranberry. 

Plants with invasive roots such as Ribbon Grass (sun) and sensitive fern (shade) may work well if there is no chance that pieces of such can wash into other waterways, escape into the wild, or take over other garden beds.  Invasive plants, which spread by seeds such as the perennial purple loosestrife or shrub honeysuckle, should not be used.

Once planted, keep plants well-watered, as you would any garden plants, if it doesnít rain sufficiently.  If water flows into your rain garden from one or two main points, you may want to place some rocks there to break the strong water flow.  This will prevent erosion of soil and mulch. 

Finally, donít just leave this rain garden and assume it needs no care.  It may need less care than other more intensely maintained beds, but still keep it weeded and plants divided as needed.  If a plant isnít to your liking, or isnít thriving, donít be afraid to move or replace it.  If more than rain flows into your garden, such as sediment and debris, remove it before drainage slows and plants are buried. 

Mulch (two to three inches) will help plants become established, and lessen weeds. Use a shredded mulch as it will bind together and not wash away in heavy rains as wood chips might.

Follow these basic tips and youíre ready to begin your rain garden, helping the environment both with water runoff and pollinators, and having an attractive garden too.  You can learn more about rain gardens, with links to further resources, from the Groundwater Foundation (www.groundwater.org/action/home/raingardens.html).

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