Minnesota has seen no shortage of storms this summer, and if your weekend plans were rained out, you’re not alone. Some areas saw over six inches of rain in just one day last weekend. All that water has to go somewhere, making rain gardens more important than ever. So, what exactly are they, and how do they help us manage this abundant rain?
A rain garden is a depressed area of soil, typically lush with native plantings, which absorbs stormwater to remove water pollutants and reduce flooding. You’ll find them installed near impermeable surfaces such as parking lots or rooftops that cannot absorb runoff on their own.
Without intervention, stormwater runoff drags pollutants and eroded soil into our waterways. Rain gardens slow it down, mimic the natural landscape to filter it through deep plant roots and soil, and let it soak into the ground. In doing so, they help reduce flooding, improve water quality, and bring a bit of nature back into the built environment.
For example, back in 2020 MWMO partnered with the City of Minneapolis to install a series of stormwater-absorbing tree trenches in the flood-prone Hoyer Heights neighborhood. Think of them as rain gardens with a twist: not just flowers and grasses, but trees, and a more extensive network beneath the surface.
When it rains, curb cuts along the street act like little gateways, channeling stormwater into the trenches. From there, the water moves slowly through layers of trees, grasses, sedges, and native flowers, dropping its dirt and debris as it filters down through 4.5 feet of soil. What the soil can’t soak up continues down to a storm drain beneath the trench, but now cleaner than when it arrived.
Monitoring the effectiveness of the tree trenches isn’t as simple as taking a look under the hood. Water samples can only be collected during active rainfall, a narrow window. To track how well the system is working, samples need to be taken in two places: : the street before water enters the system, and the storm drain after water filters through the soil.
Before we ever collected real-world data at Hoyer Heights, we did a stress test, flooding the trenches with a firetruck to make sure everything could handle a downpour. Once we knew the system was sound, we started gathering rain samples during actual storms in 2022 and again in 2024.
Those samples told us a lot. We saw a clear drop in particulate pollutants like dirt, sand and debris. At the same time, we got valuable information about the behavior of dissolved pollutants, which may be leaching from organic material in the soil. It’s exactly the kind of thing we hope to learn from monitoring: what’s working, what needs fine-tuning, and how we can make the next project even better.
Like any system that works with nature, green infrastructure projects need to be monitored and adjusted over time — because nature doesn’t always follow a blueprint. The data we’ve collected so far from Hoyer Heights helps us start asking the right questions: Where are those dissolved pollutants coming from? What can we tweak to make the system even better?
There are a few likely suspects — maybe a bit too much compost in the soil mix, or natural things like fallen leaves or pet waste making their way in during storms. That’s exactly why we monitor: to better understand what’s happening underground and use that knowledge to improve future designs.
This led the City to cap the storm drains underneath the Hoyer Heights tree trenches and all of its rain garden projects that included them. Capping the drains prevents dissolved pollutants from entering our waterways by forcing more water to soak deeper into the soil, where it can be naturally filtered before reaching groundwater.
At the same time, we’re continuing to look ahead. Another promising strategy is being explored through a filtering media study we’ve supported at the University of Minnesota. The research suggests that mixing iron filings into the garden soil could help trap pollutants by binding to dissolved phosphorus — one of the key nutrients we’re keeping an eye on.
Not only have the tree trenches reduced pollution and flooding, but the native plantings within are visually pleasing and serve as valuable habitat for wildlife. From the river health perspective, its doing the work. By continuing to test and refine these kinds of solutions, we’re not just reacting, we’re anticipating. This is what it looks like to be prepared: to stay curious, stay collaborative, and keep building green infrastructure that gets better with every storm.
