Minneapolis has planted hundreds of trees in the past few years in an effort to green up downtown, but many aren't surviving past their first year.

City staff have been trying to figure out why, and they think they might have found the culprit: salt.

Soil tests show that salinity levels in some of the planting spots are much higher than what’s ideal for trees to thrive, said Ben Shardlow, director of urban design for the Minneapolis Downtown Council and the Downtown Improvement District.

"I don't think there is such a thing as tree autopsies, so we never know exactly for sure why a tree hasn't done well,” Shardlow said. “But in a lot of spots, it’s been pretty normal for a tree to have to be replaced every year or two, again and again and again … It’s not the tree’s fault. It’s something to do with the ground that it’s growing in.”

Salt is used liberally in downtown Minneapolis to keep sidewalks and parking lots clear of ice. After the ice melts, the extra salt left behind piles up or gets pushed to the side — sometimes directly into the places where the trees are trying to grow.

Ben Shardlow, director of urban design for the Downtown Improvement District in Minneapolis. Christine T. Nguyen | MPR News

For three years in a row, a tree census showed only half of the trees the city planted had survived, Shardlow said.

City staff began watering the trees during the critical period just after they’ve been planted.

They were able to raise the survival rate to 75 percent last year, but they want to get it higher.

Focus on salt

The Downtown Improvement District, which works to make downtown cleaner, greener, safer and more attractive, teamed up with the Mississippi Watershed Management Organization, which focuses on the health of the nearby river, to encourage property managers to use less salt around their buildings.

"The message is not pointing fingers and saying that somebody is the bad guy for using salt,” Shardlow said. “We know that we need it. I think it's just a matter of making sure that we collectively aren't using more than we need to."

The two organizations hosted a smart salting training event in December for property managers. The properties they represented included large retail and office buildings, the Minneapolis Convention Center and the Minneapolis Central Library.

Property managers were already all too familiar with some of the negative effects of salt use — how it sticks to people's shoes and gets tracked into buildings, where it damages carpets and floors, said Abby Moore, training and community learning specialist for the watershed organization.

“There’s incentive for people to learn how to use salt more wisely, so that they can avoid some of those costs to repair things that are damaged by salt,” she said.

Salt used to melt ice accumulates on a sidewalk in downtown Minneapolis. Overuse of salt to clear ice from sidewalks could be contributing to the decline of newly planted urban trees. Christine T. Nguyen | MPR News

But Moore said people aren't as aware of how the excess salt dumped on sidewalks and parking lots ends up in the environment, where it permanently pollutes the groundwater, lakes and rivers.

"So, what we really try to work on is this whole idea that more salt isn't always better, and it’s actually rarely better — that salt actually is really effective, and a little bit goes a long way,” she said.

About 50 Minnesota waterways are considered impaired due to chloride pollution, mainly from road salt and water softeners. Excess levels of chloride are toxic to fish and aquatic life.

What’s unique about chloride as compared to other pollutants is that it can’t be removed from water in an efficient or practical way, said Stephanie Johnson, director of surface water and sewers for Minneapolis’ public works department.

“So basically, we have to put less chloride into the environment to keep it out of our water bodies,” she said. “That’s really where these education efforts are imperative, because a lot of the chloride that enters our environment during the wintertime is put down by average citizens.”

Protecting the investment

Shardlow said until about 15 years ago, trees and landscaping weren’t a high priority in downtown Minneapolis, and it shows.

“We don't have many trees that are older than about 15 years,” he said.

But over the past decade or so, hundreds of new trees have been planted throughout downtown. Minneapolis’ Downtown 2025 plan adopted in 2011 included a goal of increasing the tree canopy. Street projects such as the recent reconstruction of Nicollet Mall provide an opportunity to add trees, landscaping and the underground infrastructure to support them.

Fullscreen Slide Previous Slide 3 of 3 Snow accumulates in a planting bed with trees in downtown Minneapolis. Snow cleared from sidewalks and streets often get pushed into planting beds during the winter. Christine T. Nguyen | MPR News 1 of 3 Ben Shardlow, director of urban design for the Downtown Improvement District, holds a tree tag on an American linden tree in downtown Minneapolis. People can send a text to the number on the tag to receive information about the tree and the city's urban trees. Christine T. Nguyen | MPR News 2 of 3 A pedestrian walks past river birch trees planted in downtown Minneapolis. Christine T. Nguyen | MPR News Next Slide

Urban trees offer many benefits, including slowing the movement of stormwater and helping to filter out pollutants. They clean the air and make it healthier to breathe.

Trees also keep pedestrians cool and reduce energy costs for nearby buildings. Studies show that an urban tree canopy increases property values and helps attract visitors, Shardlow said.

"We want people to be downtown in the height of summer enjoying themselves. We want the streets to be full of life,” he said. “And if they're beastly hot, they can't be so shade it is really important."

That’s why it’s important to protect the investment the city and businesses have made in greening the downtown landscape, Shardlow said.

"Nobody likes wasting money replacing landscaping when we don't have to,” he said.

The Mississippi Watershed Management Organizations plans to hold more smart salting training sessions later this year.