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Plant spacing in a rain garden is dependent on
a number of variables. The templates are designed assuming
that perennials with be either Qt. or #1 size, shrubs will
be no larger than a #3 and that trees will have no greater
caliper than 1” at time of planting.
Many plants will tolerate the conditions of a
rain garden better if they are installed relatively small and
have the time to adapt as they grow. This is particularly
crucial with woody plants; mature specimens are not
appropriate for installation unless they have been growing
in similar conditions prior to installation.
Although the spacing in the templates varies
due to the particular plants selected, a rule of thumb for
perennials is 1’ on center (o.c.) for a quart and 18” o.c.
for a #1 (aka 1 gallon). Shrubs that are in a #1 or #2
should be spaced 3-4’ o.c. depending on foliage spread and
if # 3 shrubs, they should be spaced according to typical
planting distance recommendations for any installation. In
all cases, plant the shrub material so that 1/8 of the root
ball is above the soil planting medium so that when the 2”
of mulch is installed around the plants, the rootball is not
covered by much mulch.
Trees are typically spaced at a 15 – 20’ o.c.
interval to facilitate their health as they grow. This means
that when using trees, the materials under the trees will
either need to be tolerant of very sunny conditions. Ideally
they will be both sun and shade tolerant or will need to be
replaced a few years after installation with shade tolerant
species.
Sizing
the Rain
Garden
Several online guides are available for determining the
appropriate size for the rain garden. The size is determined
by volume of stormwater and by the soil types in which the
rain garden is situated and by the amount of impervious
cover that exists on a lot. Use the calculators on these
websites to determine the appropriate volume of storage and
footprint (SF) of your rain garden.
Sizing Websites:
http://www.lid-stormwater.net/bioretention/bio_sizing.htm
http://www.dof.virginia.gov/mgt/resources/pub-Rain-Garden-Tech-Guide_03.pdf
http://www.dof.virginia.gov/mgt/resources/pub-Rain-Garden-Tech-Guide_05.pdf
http://www.dof.virginia.gov/wq/index.shtml
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