This guide is for residential use only. For regulated stormwater BMPs in St. Louis County, please refer to the MSD BMP Toolbox. In other areas, check your municipal regulations.

Now let’s start planning! To make it as simple as possible, our Rain Garden Guide will walk you through the basics from location to soil type to long-term maintenance.

STEP 3: BUILD

Mark & Dig

With shovel and design in hand, outline your rain garden with a watering hose, chalk, or eco-friendly spray paint. A tiller is helpful for breaking up the soil to the proper depth in the basin area of the rain garden. If you don’t own one, you can rent. Other typical tools include a tarp for excavated soil, and a level, stake and string if building a berm.

  • Edge out the perimeter with a shovel and then remove soil from the interior of your garden to a uniform depth (usually 6-8 inches). Think BOWL-SHAPED—gently sloping sides with a level flat bottom— and you’ve got it.
  • If building on a slope, cut and fill, using the excavated dirt to build the berm and packing down each 2 inch layer as you go. The berm should raise the “rim” of the garden to be level with the high side. You can eyeball it or use a long board or stakes and string with a level.
  • Return the rest of the soil to the rain garden, add any amendments, and mix thoroughly with the shovel or a tiller.

Note: The berm is the ONLY area that should be compacted. If you cannot reach all areas of your garden from the edges, consider strategically placing flat rocks or pavers to allow you access for weeding and other maintenance without compacting the rest of the garden area.

If part of the plan, now is also the time to construct rock channels or grass swales or install perforated drainpipe and a pop-up drain.

 

Soil Amending

We encourage using native plants adapted to native soil. But even so, if the soil composition is high in the amount of clay or sand, some amending may be useful. Sandy soils lack essential nutrients and water-holding capacity, and too heavy a clay soil may not allow for proper drainage.

  • To increase a sandy soil’s ability to hold moisture and store nutrients, use well decomposed organic amendments like composts, peat, or aged manures.
  • To increase porosity and permeability of clay soils and improve aeration and drainage, add fibrous amendments like peat, wood chips, tree bark or straw. Be careful if adding sand and choose only large, coarse sand (also called builder’s sand).
  • For both sandy and clay soils, cover garden surface with 2-3 inches of organic matter and till in thoroughly

 

A great source of inexpensive organic matter is to make your own compost which can then be an ongoing supply of nutrients for your rain garden and other landscaped areas.

Test the pH of your soil.

If the pH is high – above 8.0 – the soil is alkaline. Add an acidifying mixture such as peat, sphagnum peat moss, elemental sulfur or fertilizers containing ammonium.

If pH is low – below 6.0 – the soil is acidic. Apply lime.

Planting

Check your plant layout according to your design template and make any changes you want. When you’re satisfied, follow plant specific instructions when provided, but in general, holes should be dug about 2x the size of the root ball and deep enough to accommodate loose roots when the plant crown is at ground level.

  • Gently loosen roots if wound around soil ball and place in hole. Add soil to fill and gently tamp down.
  • When planting is complete, add 2-3” of undyed wood mulch or rock to hold in soil moisture, protect against erosion and weeds. Avoid pine bark, which is too lightweight.
  • Water well to help plants settle in.
  • Stepping stones provide access to interior of garden to avoid compacting the soil anywhere else.