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

Design Considerations
• Suggested structural elements of planters are
stone, concrete, brick, or pressure-treated wood.
Flow-through planters are completely contained
and, therefore, not designed to drain directly
into the ground. Pipes can also be designed to
transport water to an approved disposal point. It
is recommended that planter boxes have setback
distances of 10 feet from structures and five feet
from property lines, unless the planter height is less
than 30 inches or as required by local ordinances.
• The flow entrance/inflow must be designed
to prevent erosion in the planter box. Some
alternatives include gravel, splash blocks,
perforated pipe, and erosion control mats.
• A positive overflow system should be designed to
safely convey away excess runoff. The overflow
can be routed to the surface in a nonerosive manner
or to another stormwater system. Some alternatives
include domed risers, inlet structures, weirs, and
openings in the planter box wall.
• Planting soil should be capable of supporting a
healthy vegetative cover and should generally
be between 12 and 36 inches deep. Planting soil
should be approximately four inches deeper than
the bottom of the largest root ball.
• A subsurface gravel layer, if used, should be at
least six inches thick and constructed of clean
gravel with a significant void space for runoff
storage (typically 40 percent) and wrapped in
geotextile (filter) fabric.
• If used, underdrains are typically small diameter (4-
12 inches) perforated pipes in a clean gravel trench
wrapped in geotextile fabric (or in the gravel layer).
Underdrains should have a flow capacity capable of
draining the planter box system in approximately 12
hours. They can daylight to the surface or connect
to another stormwater system. A way to inspect and
clean underdrains should be provided (via cleanouts,
inlet, overflow structure, etc.)
• Native trees and shrubs may require irrigation
during dryer summer months to remain healthy.
Monitoring vegetation in planter boxes is critical
to the health of the plants, as they may need
supplemental watering, in addition to the water
received from storms.
• Many planter box styles and sizes are used
to improve site aesthetics and stormwater
management. Incorporating smaller planter boxes
over the site adds visual appeal and a greater
surface area.
Design variations:
° Contained boxes
Plants should be relatively self-sustaining, with
little need for fertilizers or pesticides. Irrigation
is optional, although plant viability should
be maintained. Trees are encouraged and will
receive added credit for the canopy that will
extend beyond the planter walls. Structural
elements of the planters should be stone,
concrete, brick, wood, or other durable material.
Treated wood that may leach out any toxic
chemicals should not be used.
° Infiltration
Allow captured runoff to drain out in three
to four hours after a storm event. The sand/
gravel area width, depth, and length are to be
determined by an engineer or a dry well may be
required for complete onsite infiltration. Planters
should be designed to receive less than 15,000
square-feet of impervious area runoff. Minimum
planter width is 30 inches; there is no minimum
length or required shape. The structural elements
of the planters should be stone, concrete, brick,
or pressure-treated wood. Treated wood that may
leach out any toxic chemicals should not be used.
° Flow-through
Allow captured runoff to drain out in three to
four hours after a storm event. Minimum planter
width is 18 inches; there is no minimum length
or required shape. Planters should be designed
to receive less than 15,000 square-feet of
impervious area runoff. Structural elements of
the planters should be stone, concrete, brick, or
pressure-treated wood. Treated wood that may
leach out any toxic chemicals should not be
used. The flow-through planter box is contained
and, thus, not designed to drain into the ground
near a building. Irrigation is optional, although
plant viability should be maintained.
LID Manual for Michigan – Chapter 7 Page 262