Designing rain gardens (bio-retention areas)

              10
plant type. Ornamental grasses and evergreens are well
suited for spring planting. Deciduous trees and shrubs
are best planted in the fall to early winter. Smaller
herbaceous species can be planted in spring or fall.
Planting the rain garden often dictates the timing of
construction. It is very important that the watershed
draining into the rain garden be stable before planting.
As with any garden, rain gardens must be maintained.
Make sure you know the horticultural needs of the plants
you choose. Once the plants are in the ground they
cannot be entirely ignored and be expected to survive. It
is important to inspect rain gardens seasonally or after
substantial rainfall—particularly early on. Small mainte-nance
needs include removing trash and other unwanted
debris from the garden and replacing mulch. The more a
rain garden is treated as a garden, the more apt it is to be
attractive and flourish.
Latin name Common name N.C. habitat Size/form
Acer negundo Box elder Across N.C. Small tree
Acer rubrum Red maple Across N.C. Medium tree
Aronia arbutifolia Red chokeberry Across N.C. Medium shrub
Cercis canadensis Redbud Across N.C. Large shrub
Clethra alnifolia Sweet pepperbush Coastal plain, piedmont Medium shrub
Cornus sericea ssp. stolonifera Red osier dogwood Piedmont, mountains Medium-small shrub
Cyrilla racemiflora Ti-ti Coastal plain Large shrub (semi-evergreen)
Diospyros virginiana Persimmon Piedmont, mountains Small-medium tree
Euonymus americana Strawberry bush Across N.C. Small shrub
Fraxinus pennsylvanica Green ash Piedmont, coastal plain Medium tree
Hypericum frondosum St. John’s wort Piedmont, coastal plain Ground cover/herbaceous
Ilex vomitoria Dwarf yaupon Coastal plain Small shrub (evergreen)
Juniperus virginiana Grey owl red cedar Across N.C. Shrub (evergreen)
Magnolia virginiana Sweetbay (magnolia) Coastal plain Small tree (evergreen)
Myrica cerifera Wax myrtle Across N.C. Large shrub (evergreen)
Pinus palustris Longleaf pine Coastal plain, piedmont Tall tree (evergreen)
Pinus taeda Loblolly pine Piedmont, coastal plain Medium tree (evergreen)
Quercus pagoda Cherrybark oak Piedmont, coastal plain Large tree
Sambucus canadensis American elderberry Across N.C. Medium shrub
Scuttellaria integrifolia Scull cap Across N.C. Ground cover
Table 7. Listing of shrubs and trees potentially used in rain gardens/bio-retention areas
Costs
Costs of rain gardens are affected by four primary
factors: 1) the type of in-situ soil (sand or clay), 2) the
topography (flat or hilly), 3) the types of vegetation
selected, and 4) the required surface area. The first two
factors can make rain garden construction much less
expensive in the coastal plain and sandhills than in the
piedmont and mountains (though some mountain soils,
particularly in abandoned flood plains, tend to be quite
sandy).
The type of soil dictates the costs of excavating,
hauling, installing pipes, and importing new soil and
rocks. With sandy soils, most excavation and hauling
costs are minimized; there are often no pipe and soil
importation costs. However, if the rain garden site is
located in an area where heavy equipment has passed
several times (such as a townhouse construction site), it
may be important to install underdrain pipes even in