Rich Harrison is a Registered Landscape Architect with Metro Blooms who has worked on water quality issues for more than 20 years. His degrees in urban regional planning and environmental sciences has led to a commitment to reverse the impact of humans. He aspires to help cities redevelop and grow without destroying the natural systems around them, to mitigate the built world and take steps to improve the environment we’ve unbalanced. With Metro Blooms — and thanks to grant monies that help defray costs for residents — he designs systems for on-site treatment of stormwater, particularly raingardens and alley projects to reduce the pollution that fills our lakes.
Q: Why is stormwater an issue?
In Minneapolis, 85% of our rainstorms consist of of 1 inch or less of rainfall in a 24-hour period. Before our landscape became filled with impervious surfaces — sidewalks, alleyways, roofs — the rain would fall onto the forest that existed here, soak into the earth surface area, cling to organic material, evaporate.
Now, however, what happens to that rain? We tend to hear about “point source pollution” — factories and treatment plants that send contaminated fluids through pipes. What we tend to be less aware of is the everyday occurrence of “non-point source pollution.” The carbon emissions from our vehicles and factories enters the atmosphere and comes down in polluted rain that flows into our water stream. This combines with road salt, oil, fertilizers, herbicides, pet waste and other pollutants as it runs through our backyards, roofs and roadways into city storm sewers that feed directly into our lakes and rivers.
Raingardens have been designed as a cost-effective way to reduce the effects of this pollution in our water, as well as to beautify our neighborhoods, provide food and habitat for our winged friends, while making yard maintenance easier for all of us.
Q: How does a raingarden work?
Our lawns tend to be referred to by some landscape architects as “green concrete.” In other words, the compact nature of irrigated lawns leads to soil that doesn’t absorb much water. The Kentucky bluegrass roots (above right) go only 2-3 inches deep, which is why they need so much maintenance. Fertilize… water… mow… then do it all over again, year after year, to maintain that status quo American dream of the perfect patch of green lawn that says “I’ve made it.”
On the other hand, if you grow something like the native Little Bluestem, which has roots that can grow as deep as nine inches, a much deeper “mining” system is created that absorbs water deeper into the soil, filters it, and naturally reduces the runoff of pollutants reaching our watershed.
We recommend using native plants for raingardens because they have adapted to our harsh winters, storms and droughts — leading to roots and leaves that are well-suited for self-sustaining growth and filtering.
Q: How do you create a raingarden system?
1) Test your current soil infiltration where you naturally get roof runoff. This involves creating a coffee-can-like hole — 9 inches wide, 9 inches deep — that you fill with water and let drain. After the water empties and the soil is saturated, fill it again. Attach a popsicle stick at the level of water and time it for an hour. How much water drains into the saturated soil in 60 minutes? Multiple that number by 24. That tells you roughly how much water your soil will absorb in a day. If it takes six inches in 24 hours, that means you can have a baseline of garden six inches below the surface.
Why 24 hours? You want to avoid standing water mainly because we don’t want to create a mosquito breeding ground. Mosquitos incubate in about 72 hours; we like to be safe and make sure your raingarden drains within 24 hours.
2) Measure the depth and width of your raingarden carefully. There is a science and mathematics to this, but it’s not difficult. You want the basin of your raingarden to fit into your landscape. Its size should be large enough to absorb roughly the equivalent of your roof runoff from a 1-inch /24-hr storm. A simple equation is to take the area draining to the raingarden (A) divided by the depth of the raingarden (D) = square feet of raingarden (basin + side slopes).
In other words, a 20×10-foot roof (200 sq. ft.) divided by 6 inch depth = a garden of roughly 34 square feet.
It is critical that the raingarden basin bottom is flat. You want infiltration to be even and efficient or you might clog the whole system through erosion and sedimentation at the low end. Use a carpenter’s level or line level to test it. In a typical basin bottom, this might be six inches deep and three feet wide.
Your side slopes should be created at a horizontal-to-vertical 3:1 ratio. In other words, if your horizontal flat basin is six inches deep, your side slope will slant at an angle for about 18 inches before it reaches the surface. In total, you will have created a garden that is six feet wide at the surface, tapering down into a three-foot basin bottom that is six inches deep. We recommend keeping the basin bottom at least ten feet away from your foundation (or equal distance to the depth of your foundation).
It is also important that the depth is measured from the overflow point, which should always flow away from your house. And the ground should be higher on the inlet side, closest to the house. So, you need to find the most logical place in your yard where water is flowing away from the house at a natural angle and slope — and then you enhance that with the water absorbing garden.
3. Stay away from digging near the root structure of existing trees. You want to use caution digging within the trees dripline — where the canopy of leaves extend. That is generally the reach of your root system and the closer to the trunk, the more prevalent and extensive the root system become. You can trim roots, but avoid cutting roots larger than the size of your wrist. Work around them as you create your raingarden. We try to create the basins in the best spot — it’s not always going to be uniform.
4. Dig — after calling Gopher One (651-454-0002 or 800-252-1166). It’s a free service. Even if you don’t think you’ll hit a buried utility power line in your yard, you might hit a power source to your garage. You can easily avoid a disastrous situation.
5. Amend the soils. Commercial specifications call for 18″ of a 1/3 compost, 2/3 sand mix. This is below the basin bottom. The point of amending the soils is to create the best growing environment for your new plants so that they will quickly send down their roots. Compost provides nutrients and retains moisture. At a minimum you want to add 1″ of organic compost to the soil and mix it in the depth of your shovel! A good rule of thumb is a 40 pound bag per 10 square feet of raingarden.
6. Add mulch before planting. Use about three inches in depth. Use shredded hardwood mulch — not chipped, which is commonly found — because it is more fibrous and creates an interlocking surface that keeps moisture in, instead of floating away. It also works well to suppress weeds. Spreading mulch first also distributes your weight and prevents soil compaction.
7. Plant natives, with certain water-absorbing plants at the basin that can handle wet feet as well as drought conditions once they are established. Plants along the sloping sides should be good in part-wet and part-dry conditions. And plants at the top level should like dry conditions. BlueThumb.org has a good plant selector tool. [See this good Northern Gardening blog about recommended plants.] Four-inch plants will work fine — no need to get the more expensive one-gallon pots. It takes 2-3 years for roots to get established. You’ll want to be sure they get an inch of water each week until they are established. Then the plants take care of themselves.
- MPLS Green Series: The Why of Minneapolis’ Climate Action Plan
- Metro Blooms — workshops, consulting, design
- Blue Thumb — includes contractors to help, plant lists
- Grants for raingardens and permeable pavers
- Minneapolis loans and rebates for energy efficiency
- Raingarden plants recommended by University of Minnesota resource
- You’ll want to be sure the native plants you use are free of pesticides — or it defeats the purpose. Good local nurseries include Glacial Ridge… Prairie Moon… Mother Earth Gardens.
- Weeds to learn to love
- Why local food and gardens are vital
- Giving Tree Gardens: The importance of urban centers