Low Impact Development Manual for Michigan - OSEH - University ...

subsurface infiltration bed is used to supplement
surface storage where appropriate.
4. Planting soil depth should generally be between
18 and 48 inches where only herbaceous plant
species will be used. If trees and woody shrubs
will be used, soil media depth may be increased,
depending on plant species. Native soils can be used
as planting soil or modified to be suitable on many
sites. Small, backyard rain gardens can generally use
existing soils without a specialized depth. Planting
soil should be approximately four inches deeper than
the bottom of the largest root ball.
5. Planting soil should be capable of supporting a
healthy vegetative cover. Soils should be amended
with a composted organic material. A recommended
range of a soil mixture is 20-40 percent organic
material (compost), 30-50 percent sand, and 20-30
percent topsoil, although any soil with sufficient
drainage may be used for bioretention.
Soils should also have a pH of between 5.5 and 6.5
(better pollutant adsorption and microbial activity),
a clay content less than 10 percent (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
Selecting proper plants
Source: City of Troy
substances and unwanted plant material, and have
a 5-10 percent organic matter content. Additional
organic matter can be added to the soil to increase
water holding capacity.
If brought from off site, sand should be clean,
coarse, and conform to ASTM C-33 (Standard
Specification for Concrete Aggregates).
If the void space within an amended soil mix will
be used in calculating runoff volume capacity in
the system, tests should be conducted on the soil’s
porosity to determine the available storage capacity.
6. Proper plant selection is essential for bioretention
areas to be effective. Typically, native floodplain
or wet meadow plant species are best suited to the
variable environmental conditions encountered in
a bioretention area. Suggested species may include
Cardinal Flower (Lobelia cardinalis), Blue Lobelia
(Lobelia siphilitica), New England Aster (Aster
novae-angliae), and Brown Fox Sedge (Carex
vulpinoidea) (See recommended Plant List in
Appendix C for a detailed list).
In most cases, seed is not the preferred method
for establishing plants in a bioretention area. The
fluctuating water levels make it difficult for the
seed to readily establish, while the random nature
of seeding produces a look previous experience
indicates is unacceptably “wild.” Therefore, it is
strongly recommended that live plant material in
plug or gallon-potted form be used and installed
on 1-2 foot centers for a more formal appearance.
Shrubs and trees are also recommended to be
included in a bioretention area.
A landscape architect can be used to design a native
planting layout. Additional resources for planting
layouts are Rain Gardens for West Michigan (www.
raingardens.org), Washtenaw County Free Designs,
Wild Ones Natural Landscapers, and MDEQ
Landscaping for Water Quality booklets.
LID Manual for Michigan – Chapter 7 Page 141