Taking lessons from elm losses, Minneapolis prepares for ash borer

Ash borer sign
This sign has been posted on the Ash tree with the trap on it. There are over 100 of the purple traps hanging from Ash trees in Minneapolis. They are covered with a sticky film designed to capture the emerald ash borer.
MPR Photo/Brandt Williams

Minneapolis city forestry officials are preparing for the arrival of an unwanted guest -- the emerald ash borer. The tree-killing bug was recently found in St. Paul, leading some to believe it's already in Minneapolis, but just hasn't been detected yet.

The invasive insect could wreak havoc on the more than 200,000 ash trees in the city. It's not the first time Minneapolis has faced the loss of thousands of trees.

Since the 1970s, cities across the country have lost millions of elm trees. In Minneapolis each year, city foresters continue to cut down thousands of elms infected with Dutch elm disease.

City forestry director Ralph Sievert said residents will soon see more sky above streets shaded by ash trees as well.

"It's inevitable, that eventually ash trees are going to die," Sievert said.

Ralph Sievert
Ralph Sievert is the Director of Forestry for the Minneapolis Park and Recreation Board. He's holding a branch from a young Ash to point out its distinctive compound leaf structure.
MPR Photo/Brandt Williams

Ash trees, mostly the green ash, make up more than 20 percent of trees in the city. They were a popular choice to replace the thousands of elm trees which succumbed to Dutch elm disease in the 1970s. Sievert said, at the time, elms made up 80 to 90 percent of boulevard trees. He said the idea was to create a uniformed canopy for each block.

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"In our case we had a master plan done that proscribed a certain type of tree for each block," he said. "So the idea was we were going to be able to get diversity throughout the city, but block by block, as opposed to necessarily tree by tree."

One of the advantages of concentrating a single species on one block is financial. If, say a block full of 50 elm trees dies off, city workers can focus their resources on that block instead of going out to 50 different sites. Of course, the plan is not so great for a person who lives on a block losing all its trees. Sievert said the city will still largely follow the same plan. For instance, if a linden located on a designated linden block is blown down in a storm, they will plant another linden in its place.

But Sievert said the city has become more flexible with the plan.

"When residents will call and say, 'I'm on a hackberry block, my elm disappeared so I want a new tree' - a lot of times they're happy with getting a hackberry and works well for us logistically," Sievert said. "However, if there's a request for something different, then we work [with] the folks on what they have in mind."

Topping a diseased elm
City crews topping a diseased elm tree. Once the top branches are off, another crew will come in and cut the rest of the tree down. City officials say they still cut down several thousand elm trees each year.
MPR Photo/Brandt Williams

The menu of replacement trees also includes river birch, Ohio buckeye, Japanese lilac, flowering crab tree and hybrid autumn blaze maple. But even tried and true tree species can fail when they're planted in a boulevard. The strip of land between the street and sidewalk is not great for trees because it gets exposed to road salt and often contains poor soil. Even some maple species are showing they can't handle those stresses, and are now showing signs of what foresters call maple decline.

Gary Johnson is a professor of urban and community forestry at the University of Minnesota. He said many cities throughout the country use a similar block-to-block master plan for trees. But Johnson said cities like Minneapolis and St. Paul have learned a valuable lesson from the Dutch elm epidemic.

"Insects and diseases tend to like the taste of one type of tree," Johnson said. "So the more diverse your urban forest is, the urban landscape, the more protected it is from severe damage from one problem."

But Johnson said diversifying tree populations may be harder for cities with less money. He said it's often cheaper to buy a lot of one type of tree.

Ash borer trap
An emerald ash borer trap near the University of Minnesota campus.
MPR Photo/Tom Weber

Johnson said there's some promising research being done on pesticides that can help save trees. But scientists have some concerns that the chemicals can harm non-target insects and birds. Still others say the amount of pesticide is too small to cause collateral harm.

Ralph Sievert said Minneapolis is not using pesticides on ash trees yet, but other cities are. He said it appears the pesticides, which are applied every few years, can slow down the spread of the disease. That can also buy a city time and prevent the need to cut down thousands of trees at once. For the Minneapolis forestry department, the savings could be crucial. Sievert said he is 10 workers short and doesn't have the money to hire them.