A stormwater crediting system that gives Minnesota property owners and municipalities the opportunity to save thousands of dollars by installing trees for stormwater management has recently been put in place by the state's Pollution Control Agency.

As part of a new chapter in their stormwater manual, the Minnesota Pollution Control Agency developed a formula and credit system that they hope will transform the way communities think about stormwater management - leading them to replace pipes with trees – and earn thousands of dollars in tax credits
in the process.

The new credit system is part of Minnesota's Minimal Impact Design Standards, 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.

"The state encourages low impact development practices where they are appropriate," stated Mike Trojan of the Minnesota Pollution Control Agency. "Trees are potentially an important LID practice, and we want to ensure that the trees provide the maximum level of stormwater benefits."

Since tree size is said to be crucial for maximizing these benefits, the new chapter also provides recommendations on how to ensure that trees thrive; pointing out the importance of planting trees properly with adequate soil volumes, even in urban areas. The manual recommends two cubic feet of soil for a square foot of canopy area as a minimum for a healthy tree.

The state is basing the tax credits on the projected mature canopy size of a tree, taking into account if it was planted and maintained correctly and provided with adequate soil volume. 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, who led the efforts to develop the credit system. "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."

To attain the greatest stormwater credit possible, cities like Minneapolis that do not have the space for
open planters due to concrete and pavement, have begun to invest in innovative solutions such as the Silva Cell by DeepRoot, which creates an underground framework that provides proper soil volume to support
long-term tree growth.

"Depending on the size and design of the system, thousands of gallons of runoff can be prevented from entering, overwhelming or polluting sewers, creeks and adjacent watersheds," stated Graham Ray, CEO of DeepRoot.

As states look to alternative solutions to manage stormwater effectively, Minnesota and its new credit system will be a good testing ground of what can be done through formulas and technologies that help incentivize sustainable, tree-filled communities.