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 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
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.
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