The federal Clean Water Act imposes requirements on stormwater discharges from specific municipal, construction and industrial activities. Minnesota is helping communities use trees and other green infrastructure to help address these requirements while mitigating the high cost of installing only pipes.
The state of Minnesota has just developed a first-of-its kind formula and credit system that could change the way communities think about trees and stormwater management—and potentially help them save thousands of dollars by investing in trees rather than pipes. The Minnesota Pollution Control Agency created the credit system as part of a new chapter on trees in their stormwater manual, which also provides recommendations on how to ensure that trees thrive and provide the maximum ecological benefits for planners, developers, landscape architects, and builders.
“To our knowledge, Minnesota is one of the first states, if not the first, to add a chapter on trees to its manual, as well as add analysis on the stormwater benefits of tree and soil systems,” said Nathalie Shanstrom, a landscape architect with the Kestrel Design Group who led the efforts to develop the credit system. “While trees have always provided stormwater benefits, they are just recently starting to be recognized by regulators as viable stormwater control measures. Cities, states and homeowners are taking notice.”
The new credit system is part of Minnesota’s Minimal Impact Design Standards (MIDS), which focuses on treating rain where it falls to minimize negative impacts from stormwater runoff and to preserve natural resources. The credit is based on a formula that establishes criteria and methods to measure the benefits of evapotranspiration, which is the combination of water evaporating from the soil and transpiration from the plants growing in the soil.
An additional focus of the Minnesota stormwater manual is the importance of planting trees properly with adequate soil volumes, even in urban areas. The manual recommends soil requirements of two cubic feet of soil for a square foot of canopy area—the minimum for a healthy tree.
Tree size is crucial for maximizing stormwater benefits. For trees that are planted and maintained correctly and provided with adequate soil volume, the state is therefore giving credits based on the projected mature canopy size. If a tree is planted with less soil than it needs, the credit is reduced.
"Trees are the oldest form of green infrastructure in cities, but the urban forest is now broken,” said Peter Macdonagh, of Kestrel Design Group. “Planting trees in appropriate quantities of good soil and using stormwater and its nutrients to irrigate is beneficial to the urban forest and reduces city taxes by tens of millions of dollars. Minnesota's visionary rule incentivizing correct tree-planting to manage urban stormwater will clean our lakes, rivers and oceans to be safely swimmable and fishable."
Cities like Minneapolis, which do not have the space for open planters due to concrete and pavement, have already begun to invest in innovative solutions to attain the greatest stormwater credit possible. Through the use of products like the Silva Cell, which creates an underground framework that provides soil access to support long-term tree growth, the city is planting achieving the proper soil volume.
To learn more about the new Minnesota stormwater credit, click here.
