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

oxes, etc. should be installed in accordance
with the guidance of the manufacturers or design
engineer.
4. Infiltration planter boxes should be excavated
in such a manner as to avoid compaction of the
subbase. Structures may be set on a layer of clean,
lightly compacted gravel (such as AASHTO #57).
5. Infiltration planter boxes should be underlain by a
layer of permeable nonwoven-geotextile.
6. Place underlying gravel/stone in minimum six-inch
lifts and lightly compact. Place underdrain pipes in
gravel.
7. Wrap and secure nonwoven geotextile to prevent
gravel/stone from clogging with sediments.
8. Install planting soil per the recommendations of the
landscape architect. Do not compact.
9. Install native vegetation (trees, shrubs, etc.) per the
recommendations of the landscape architect.
Perforated pipe used for inflow/distribution in a stormwater
planter box
Source: www.wsud.org
Maintenance
Planter boxes are relatively high maintenance, as
is the case with any containerized garden. Property
owners should be especially prepared for maintaining
the vegetation itself, which will vary depending upon
planting. In many cases, planter boxes may need additional
watering during extremely dry periods. Selection
of planter box construction material is also important
(e.g., masonry construction is easier to maintain than
wood construction).
Generally speaking, stormwater facilities need an
adequate amount of space for proper maintenance. The
minimum required width for maintenance is typically
eight feet and the maximum slope is 10 percent. If
structural surfaces need to support maintenance vehicles,
access routes should be constructed of gravel or
other permeable paving surface.
Winter Considerations
Michigan’s winter temperatures can go below freezing
for four or five months every year and surface filtration
may not take place in the winter. Winterizing
becomes an important issue in plant species selection,
especially for larger hardy or nearly hardy species
intended to winter over. In these cases, planter boxes
must be designed and dimensioned so that plantings are
adequately protected.
Depending on the composition of the planting soil, it
may hold water, freeze, and become impervious on the
surface. Design options that allow directly for subsurface
discharge into the underlying infiltration bed, if applicable,
during cold weather may overcome this condition,
but at the possible expense of surface filtration.
Cost
Costs for planter boxes are quite modest. However,
based on unit cost of cubic foot or gallons of runoff
being managed, costs tend to be rather high. Because of
the extreme variability of design and construction, costs
will range based on the goals of the designer. Smaller
boxes with smaller-scale vegetation will be less expensive
than larger boxes with more mature vegetation.
LID Manual for Michigan – Chapter 7 Page 264