STRUCTURAL MEASURES: CONSTRUCTION, OPERATION, AND MAINTENANCE - Low Impact Development and Sustainable Stormwater Management - page 177

2. Planting soil should be capable of supporting a healthy vegetative cover.
Soils may need to be amended with a composted organic material. A recom-
mended range of planting soil components is a mixture of 50 to 80% organic
material (compost), 20 to 40% sand, and 0 to 10% topsoil, although any soil with
sufficient drainage may be considered for a rain garden. Planting soil should be
approximately 4 in. deeper than the bottom of the largest root ball.
3. Existing soils should also have a pH between 5.5 and 6.5 (better pollutant
adsorption and microbial activity), a clay content of less than 10% (a small
amount of clay is beneficial to adsorb pollutants and retain water, although no clay
is necessary if pollutant loadings are not an issue), be free of toxic substances and
unwanted plant material, and have a 5 to 10% organic matter content. Additional
organic matter can be added to the soil to increase the water-holding capacity.
If the void space within an amended soil mix will be used in the calculation of
runoff volume capacity in the system, tests should be conducted on the soil's
porosity to determine the storage capacity available.
4. To be effective, proper plant selection is essential for bioremediation areas.
Typically, generalist plant species native to the area are best suited to the variable
environmental conditions encountered in a rain garden, as they need to withstand
a wide range of soil and moisture regimes. Recommended plant lists of suitable
species are available in all parts of the country. When designing the planting, it is
important that plant species are located according to their tolerance of inundation
and prolonged saturated soils; less tolerant species should be located at the higher
elevations of a rain garden. Trees, shrubs, and herbaceous perennials all may be
used in a rain garden; they should be selected with other functions in mind
(e.g., shade, screening versus clear views, color) in addition to suitability for
bioremediation. For rain gardens that will have an underdrain, it is also important
to select species that do not have invasive roots, which have a tendency to clog
perforated drainage pipes. A landscape architect should help with plant selection
and rain garden design.
5. Planting time should occur during fall to take advantage of seasonal rains
in most regions. In most cases, seed is not the preferred method for plant estab-
lishment in a rain garden. The fluctuating water levels make it difficult for the
seed to establish readily, and the random nature of seeding may result in an unde-
sirable plant layout for some situations. Instead, it is strongly recommended that
containerized live plants be utilized: plugs or 1-gallon container for herbaceous
plants, 1- to 5-gallon container for shrubs, and 5-gallon container to 24-in. box
for trees. Plant spacing depends on mature plant size and the desired density of
plant cover.
6. A maximum of 2 to 3 in. of shredded hardwood mulch (aged at least
6 months to 1 year), leaf compost, or comparable product should be applied
uniformly immediately after planting to prevent erosion, enhance metal removal,
and aid plant establishment. Wood chips should be avoided, as they tend to float
during inundation periods. Mulch or compost should not exceed 3 in. in depth so