With the arrival of spring, plants are blooming — in our homes, our parks, and even in the middle of some Milwaukee streets. WUWM listener Barbara Finley wondered about those peculiar gardens, and sent us this question for Bubbler Talk:
"In the last couple years while driving along Milwaukee streets I’ve noticed some sunken gardens in the median strips. I was wondering why the gardens were being built below-grade and what was being planted in them?"
These sunken gardens in street medians are technically called bioswales and they have a greater purpose, beyond beautification. Kurt Sprangers with the Milwaukee Department of Public Works (DPW) explains:
“They are a stormwater management device. So the goal of the device is to collect water, hold it back, treat it and then either release it back to the sewer system or to allow it to infiltrate.”
Sprangers is in charge of environmental engineering for DPW. These bioswales, also called bioretention basins or rain gardens, were first installed in the City of Milwaukee in 2008. Since then, the city has built over 330 bioswales.
You can identify them by looking for a depressed — or sunken — area in a street median that is planted with woody and herbaceous vegetation, and a layer of mulch at the top. As rain seeps through the surface, it flows through about 18 inches of engineered soil made of sand and compost that provides drainage and nutrients for the plants.
“And then below that further is a what we call a stone storage layer. So it's usually 18 to 24 inches of clear wash stone so it doesn't have any fines, it doesn't have any dust, and that acts as a storage area. So all the water that passes through the whole system, it's allowed to settle in the storage zone," Sprangers adds.
If rain is heavy enough to fill up the storage basin, the water drains back into the sewer system through an under drain at the top of the bioswale. Sprangers notes that the water that's returned to the sewer system is returned slower and cleaner.
You can see the importance of rain gardens when there’s a heavy storm, and paved streets become flooded. Porous surfaces that soak up the rain help prevent flooding and overflow of our sewage systems.
Bioswales are primarily for stormwater management, but they also add more green space and beauty to Milwaukee streets. Sprangers says DPW tries to add as many flowering plants as it can — and all of the plants in bioswales are perennials. This can include a variety of coneflowers, roses, Dogwood shrubs, grasses, daylilies, catmint and more.
The plantings are a mix of natives, cultivars and nonnatives. "We do try and incorporate natives when we can," notes Sprangers. "Unfortunately, it is a very harsh environment, you know, it's in the middle of say a concrete road which is getting a lot of sun all summer long. If we're not getting a lot of rain it can be very dry environment and then of course in winter it's also very salty environment — you're getting salt spray or salty water that's just running into the bioswales. So for that reason we do mix up the types of plants that we use just to allow them to, you know, stay alive and thrive essentially."
The funding for these bioswales comes from both the City of Milwaukee and the Milwaukee Metropolitan Sewerage District according to Sprangers. DPW does all the design, contracting and maintenance work.
“We do work with our forestry group to plan the plantings for each bioswale. So each one is unique, each one is different,” he notes.
Sprangers says that the DPW considers April to November bioswale maintenance season. In spring they cleanup any debris left over from the winter and perform inspections with the contractor to evaluate what else needs to be done — from adding more soil to repairing or reshaping the garden bed.
“And then later in the season when you start getting into May and June, have we seen any plant die off, do we need to replenish plants? So part of our maintenance contract is doing a plant inspection with the contractor walking basically every site and then seeing what needs to be replaced and once you get through those early months then it's really just a matter of monthly maintenance,” Sprangers explains.
Monthly maintenance includes checking on the bioswales to see if any plants have died off and pruning back grasses or bushes to make sure it’s not blocking a driver’s vision of traffic. Then, in the fall there’s a final pruning to knock the plantings down and remove debris before winter comes.
Bioswales are just one part of the City of Milwaukee’s larger Green Infrastructure Plan, which was introduced in 2019. Sprangers says according to the data, the plantings are doing their job.
“One of the metrics within the plan is a goal of gallons of stormwater captured, and that actually is looking quite good. I believe an 80% level of the 36,000,000 gallons by 2030,” he says.
Sprangers notes that bioswales will continue to be a priority as the city takes on new projects, including a host of street improvements.
"That effort really just has become more and more important, you know, in a sense maybe getting more aggressive with identifying locations where we can cite bioswales and permeable pavers as part of paving projects," he says. "So you'll see some of the traffic calming projects that are going on, we're looking to see you know hey how can we green those up a little bit so it's not just a plain concrete island?”
So like Bubbler Talk listener Barbara Finley who spotted the bioswales and asked us about them — keep an eye out! Notice each unique garden and you just might see more of them cropping up soon.
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