Bioretention Sizing | PDFs | Links

Determine the final shape and placement of the garden.

Rain gardens have some construction similarities to a typical garden but other distinct features which require specific construction as part of the creation of the garden. Two main differences are the depth of excavation required and the focused delivery of water to the garden.  

The placement of the rain garden may be in a natural depression which already collects water or may be sited uphill from that depression to intercept the water so that it is infiltrated prior to reaching the current soggy depression.   In all cases, the rain garden will benefit from having a grassy strip in the area uphill from the garden to act as a sediment filter for the entering water. 

The diversion of water into the garden may be done through a reshaping of the earth adjacent to the garden, by lowering the finished ground elevation of the surface of the garden or by piping water directly to the garden. 

The surface of the bioretention portion of the garden should be 8-9” below the surrounding area.  Mulch is added to the surface of the garden (2-3”) so that the final elevation of the bioretention areas is 6” below the surrounding area.  Usually there is a slope that connects the bioretention area to the surrounding area; this can be populated with plants which are adapted to slope and well drained conditions.  If there is no slope, there may be a low curb or wall which defines the edges of the garden. 

Outline the garden shape on the ground 

The shape of the garden as drawn on paper should be transferred to the ground. This shape can be transferred using a hose, a rope, flour, non-toxic marking paint or utility flags.  Once it is on the ground, adjust it as necessary to meet the design idea. 

Always call to have your utilities marked before you start to dig. This can save both time and money.  If this was not done during the inventory and analysis phase of design do it now.  Also, most utility marking allows digging for a finite period of time; make sure that prior to digging, you have a current digging clearance. 

Start digging (after locating utilities) 

Depending on the scope of the excavation plan and the type of soils and  the budget, the digging will be by machine or hand.  If the garden area is currently a turf area, cut off the turf and either reuse it or compost it.  If the garden area is adjacent to a wooded area, the edge of the garden should not go more than a foot into the dripline of the existing trees so that the existing trees feeder root structures are not compromised. Excavated dirt should not be piled deeper than ½” on top of existing tree roots. 

When the desired depth and bottom profile is reached, stop. The material that is excavated can be used to build a berm on the downhill side of the rain garden to handle larger storm events and/or it can be hauled to other places on the site or removed, depending on the particular situation.   

Mix and Add soil amendments 

The ideal amendment is comprised of : 

Washed sharp sand (50% by volume)

Double shredded hardwood mulch  (15% by volume)

Topsoil (weed seed free) (30% by volume)

Peatmoss (5% by volume) 

Note: 20% shredded hardwood mulch may be substituted for the shredded hardwood mulch and peatmoss; the mulch adds structure and drainage to the garden.  Or, compost can be substituted for the peatmoss 

If using, install the gravel sump, pipes, inlets and outlets, and liner

The gravel sump should be comprised of 57 drainage stone, the pipes should be not greater in diamer than 4” and should be a perforated pipe within the garden.  Overflow inlets should be located in the deepest section of the garden. The liner should be specific for use in French drains.  Typical weedcloth is not the appropriate liner to use.  Ask at the store which liner fabric that is in stock which would be appropriate. 

Install the soil amendments 

Install the amendments into the garden in such a manner as to minimize any compaction.  Ideal installation would be one in which all the amendments were added and no feet or machinery went across the garden.   

Fertilizers 

Slow release fertilizers are recommended to be added at planting time.Typical desirable composition is 20-10-5. Put the fertilizer into the bottom of the individual planting pits for each plant.  

Install Woody Plants 

The old adage “dig a $10 hole for a $5 plant applies in a rain garden.   The shrub planting pits should be 8” wider than the width of the shrub rootball. Leave 1/8^th of the shrub rootball above the surface grade of the garden (plant them “high”).  Trees should have holes dug that are twice the width of the root ball and planted with 1/8^th of the rootball above the surface grade of the garden. All planting pits for the woody plants should be the equivalent of 7/8 of the depth of the rootball of the plant being installed. 

Apply mulch 

If you have a high erosion potential install erosion netting at this point and cover with mulch. A 2-3” layer of double ground shredded hardwood mulch should be added to the top of the rain garden after the surface of the rain garden has been raked smooth and level.  Do not walk on the garden surface or run machinery over it. 

Install Herbaceous Plants  

Install the herbaceous plant materials (annuals, perennials and grasses) through the mulch and apply a liquid fertilizer.  Herbaceous plants are installed so that their crowns (where the roots meet the top growth) are even with the top of the mulch. 

Water for 30-60 minutes after planting 

PDFs 

Minnesota Rain Garden Construction Spec

 

Links 

http://www.lid-stormwater.net/bioretention/bio_schedule.htm   

Rainscapes site with a variety of rain garden examples
http://www.montgomerycountymd.gov/Content/DEP/Rainscapes/gallery.htm

Miss Utility (Maryland, Delaware, Washington DC)
http://www.missutility.net/

National Call Before You Dig Website, Common Ground Alliance (CGA)
http://www.call811.com/

Bioretention Sizing
http://www.lid-stormwater.net/bioretention/bio_sizing.htm