Type: Capital Project
MWMO Funding: Not to exceed $408,582
The City of Minneapolis installed a series of stormwater-absorbing tree trenches south of 37th Avenue NE along portions of Fillmore Street NE, Buchanan Street NE, and Lincoln St NE in the Hoyer Heights area as part of a street reconstruction project. This pilot project, supported by a not to exceed $408,582 grant from the MWMO and a $113,459 Minnesota Clean Water Fund grant through the Board of Water and Soil Resources (not to exceed $522,041 total for the project), was among the first in the city to capture and treat runoff directly from the street in a residential neighborhood using tree trenches, which is a type of green stormwater infrastructure.
The project was planned as part of the Northeast Stormwater Management Initiative, which seeks to improve water quality and flood resiliency in Columbia Park (including Columbia Golf Course) and the surrounding neighborhoods. Hoyer Heights was identified as an area that would benefit from flood mitigation measures, with stormwater modeling suggesting that street flooding would occur in a two-year storm or greater. Hoyer Heights also lies within a watershed that received very little water quality treatment prior to discharge to the Mississippi River. The MWMO and the city saw the opportunity for a project partnership, as the city had been planning to reconstruct the streets and replace the sanitary sewer lines in the Hoyer Heights area (part of the Waite Park neighborhood) in 2020. Three of the streets were wider than typical residential streets and were slated to be narrowed for traffic-calming purposes. This provided an ideal opportunity to install new green stormwater infrastructure in the expanded boulevard space. The city held three open-house meetings regarding the project, contacted the individual homeowners on the streets where the tree trenches were constructed, and applied their input in selecting planting schemes.
Curb cuts with sediment traps allow stormwater runoff to feed directly from the street into the tree trenches, which then absorb water and filter out pollutants. Stormwater is intercepted by a variety of deciduous trees, grasses, sedges, and flowering plants, and filters through approximately 4.5 feet of soil media installed in the tree trenches. At the bottom of the soil media, an 8-inch perforated PVC drain pipe, which is surrounded by stone to prevent clogging, collects excess runoff if there is more than the trees and plants can use. Excess runoff can also exit the tree trenches via overflow curb cuts. A total of 3.71 acres drain to the tree trenches. The deep-rooted plants selected for this project provide excellent food and habitat for pollinators and other wildlife. The project will also help the city and the MWMO evaluate the effectiveness of tree trenches in capturing and treating runoff, assess their maintenance needs, and determine their life-cycle costs. In urban settings, trees are often put in spaces that are too small or don’t receive enough water; this project will give the trees a better chance at reaching maturity and provide important co-benefits like shade, carbon storage, improved aesthetics, and energy absorption. Construction of the tree trenches was completed in the fall of 2020. It is estimated that the project will keep 562 pounds of sediment and 2.8 pounds of total phosphorus out of the Mississippi River each year, as well as reduce the total volume of annual runoff by nearly 305,000 gallons.
The plants and trees are under warranty care by the contractor until the fall of 2022. After the end of the contractor’s warranty care period, the property owners will be responsible for regular maintenance, such as watering during drought conditions, pruning or cutting back dead plant material, removing unwanted plant litter (typically no more than once or twice per season), and removing trash or debris that has blown or fallen into the tree trench. MWMO staff will help conduct education and outreach to help residents learn how to monitor and care for the tree trenches. Minneapolis Public Works will inspect the tree trenches on average once per year and remove large accumulations of sediment, inspect underdrains, replace plants, and perform any necessary repairs.
Resources for Hoyer Heights Residents
The resources below are intended to help residents care for their tree trenches*.
*Note: Please do not begin weeding your tree trenches until October 2022. A contractor is responsible for weeding the tree trenches and ensuring the survival of the installed plants through the end of September 2022.