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

• The plants within the perimeter planter boxes are
designed to accept stormwater runoff from adjacent
impervious areas. Plants and vegetation absorb
most of the water volume. Overflow gradually
drains to the surface, which slows the peak rates.
• Review the materials list in Appendix D for
recommended planter box specifications.
• Landscaping requirements
The following quantities are recommended per 100
square feet of planter box area:
° Four large shrubs/small trees in three-gallon
containers or equivalent.
° Six shrubs/large grass-like plants in one-gallon
containers or equivalent
° Ground cover plants (perennials/annuals) one
per 12 inches on center, triangular spacing.
Minimum container: four-inch pot. Spacing may
vary according to plant type.
• Plantings can include rushes, reeds, sedges, iris,
dogwood, currants, and numerous other shrubs,
trees, and herbs/grasses (Appendix C).
• Container planting requires that plants be supplied
with nutrients that they would otherwise receive
from being part of an ecosystem. Since they are cut
off from these processes, they must be cared for
accordingly.
• Tree planting in planters is encouraged where
practical. Tree planting is also encouraged near
planters.
• Generally, plants requiring moist-wet conditions
are preferred for flow-through planters.
Stormwater Functions
and Calculations
Volume reduction
If a planter box is designed to infiltrate, the volume
reduction is a function of the area of the filter and the
infiltration rate. There is generally less volume reduction
for planter boxes that are not designed to infiltrate.
Infiltration Volume* = Bottom Area (sf) x Infiltration
Rate (in/hr) x Drawdown time** (hr)
*For filters with infiltration only
** Not to exceed 3-4 hours
Peak rate mitigation
Planter boxes generally provide little, if any, peak rate
reduction. However, if the planter box is designed to
infiltrate, then a modest level of peak rate attenuation
can be expected (see Chapter 9, LID Stormwater Calculations
and Methodology, for more information on peak
rate mitigation).
Water Quality Improvement
Planter boxes are considered a moderate stormwater
treatment practice with the primary pollutant removal
mechanism being filtration and settling. Less significant
processes can include evaporation, infiltration (if
applicable), transpiration, biological and microbiological
uptake, and soil adsorption. The extent to which
planter boxes remove pollutants in runoff is primarily
a function of their design, configuration, plant species/
density, and soil type.
For planter boxes that are also designed to infiltrate, see
the water quality summary in the Subsurface Infiltration
Bed section, or in the other infiltration BMP sections.
For manufactured planters, see the manufacturer’s information,
as well as findings from independent studies.
Also see Chapter 9, LID Stormwater Calculations and
Methodology, which addresses the pollutant removal
effectiveness of this BMP.
Construction Guidelines
Constructing or retrofitting planter boxes varies in difficulty
at each site. Boxes may be ideal for inclusion
in patio or walkway design and integrate easily with
roof downspouts. In most cases, a landscape architect
is essential, especially if the more complex infiltration
and flow through variation is being constructed, and as
the size/scale of the planter box grows larger.
1. Areas for planter boxes, especially the infiltration
type, should be clearly marked before any site work
begins to avoid soil disturbance and compaction
during construction.
2. Planter boxes should generally be installed after
the site is stabilized. Excessive sediment generated
during construction can clog the planter and prevent
or reduce the anticipated post-construction water
quality benefits. Stabilize all contributing areas
before runoff enters the filters.
3. Structures such as inlet boxes, reinforced concrete
LID Manual for Michigan – Chapter 7 Page 263