Ash borer 2

About 100 ash trees, the second most common tree along Mankato streets, have been marked for removal this fall and winter.

Ash borer 1

Justin Lundborg, natural resources specialist for the city of Mankato, is overseeing the planning to preemptively remove nearly 90% of the ash trees on public land.

The blue “X” of death has been spray-painted on about 100 trees along Mankato boulevards, and those trees — many perfectly healthy — will experience the chain saw starting in October and continuing through the fall and winter.

Another 35 mature trees will be felled in North Mankato.

But that’s only the initial stage of a plan to drop thousands of ash trees from boulevards and parks in the two cities in the decade ahead. Tens of thousands more ash will eventually be dying on private property and in ravines and woodlots when the Minnesota River valley faces the fate of forested areas elsewhere in the United States where the voracious emerald ash borer has already altered the landscape.

Mankato is looking to double the number of condemned ash trees in future years, according to Justin Lundborg, a natural resources specialist for the city who oversees the management of the trees in the right-of-way of streets and in municipal parks.

“Typically, our goal is going to be about 200 trees annually,” Lundborg said. “And we’re always out there looking for grant opportunities to boost that number. ... We’re anticipating 8-10 years to take care of the preemptive removal.”

Advance work

Some residents will undoubtedly be miffed to see thriving, attractive, shade-bestowing large trees toppled, hauled away and ground up. To understand why it’s being done, people need only to travel as far as St. Paul.

The emerald ash borer was first discovered in Minnesota’s capital city in 2009, but the invasive insect is believed to have arrived in St. Paul as early as 2004. The beetle, which binges on ash trees during its larval stage, is thought to have made its American debut in Michigan in the 1990s, traveling on wood products from East Asia, and has since spread to 35 states.

Ash borer damage 3 (copy)

The small emerald ash borers are a huge threat to ash trees as they feed and burrow under the tree bark, eventually killing the tree.

St. Paul has been removing around 3,000 dead, dying or doomed ash trees annually in recent years. Even at that pace, crews are falling behind. They decided to delay removing stumps and planting of replacement trees and discontinued virtually all of their other tree work simply to try to keep up with the dying ash trees. An ash tree killed by EAB isn’t just an eyesore, because the trees quickly become brittle after death and drop large limbs — a public safety issue.

“Now 17 years in, EAB in St. Paul has reached a critical stage,” the city’s website states.

This summer, $18 million in bonds were sold through the St. Paul Port Authority to finance additional ash removal, along with replacement plantings.

The EAB has not been confirmed in Mankato yet. St. Clair, 11 miles away, remains the closest town where the beetle has been found, although EAB-infested ash trees have been confirmed in every direction around Mankato-North Mankato, including Freeborn County, Martin County, Brown County, Sibley, Scott County and Rice County.

When the beetles arrive, it’s only a matter of time until the ash trees start to die. A joint study by the U.S. Forest Service and Ohio State University monitored 457 plots with ash trees in Michigan, Ohio and Pennsylvania and found that within six years of an EAB infestation, more than 99% of the trees were dead. Approximately 1 of every 1,000 ash trees survived, although the researchers aren’t sure if those will succumb later or if they have some sort of resistance.

U.S. Forest Service and Ohio State University EAB chart

A joint study of the emerald ash borer infestation in Ohio, Michigan and Pennsylvania found that virtually all ash trees die within about six years.

The beetle’s preferred diet is an unfortunate one for Minnesota, which is estimated to have about 1 billion ash trees — more than any other state. That prevalence is true in area cities, as well.

Coming affliction

Mankato has an estimated 17,400 ash trees on public and private properties, including 2,500 on city-owned boulevards, in parks and on other landscaped municipal property. That’s more than one of every seven city-managed trees, surpassed only by maples. In St. Peter, ash represent one of every five public trees.

The percentages are similar in North Mankato, and that city began treating some high-value ash around Caswell Park with anti-EAB insecticide in 2019. But the treatment has to continue indefinitely to protect a tree, so for cost reasons North Mankato plans to preemptively remove the vast majority of its public ash trees.

“We know at some point EAB is going to be in Nicollet County and it’s going to be a potential issue for us,” said Public Works Director Nate Host. “I think everybody fears getting inundated.”

Rather than scrambling, like St. Paul, to keep up with dying trees after the infestation begins, the local cities are working ahead.

“Keep getting ‘em down, keep getting ‘em down, keep getting ‘em down, because we know what’s potentially around the corner,” Host said.

Asked about his use of the word “potentially,” whether it’s an indication that he holds onto some optimism that his city will be spared, Host pauses and then concedes it looks pretty hopeless: “I like to think positively, but I think Nicollet County’s kind of surrounded right now.”

EAB in Minnesota

Much of southern Minnesota is in quarantine because of the verified arrival of the emerald ash borer within a county. The green areas are places with general infestation.

Felling trees

North Mankato’s target of about 35 ash removals annually was set before the St. Clair EAB discovery in March. The pace will need to be ramped up, although the city has slightly exceeded its initial yearly target in removing about 40 annually the past three years — inflated partly by reconstruction of a street that happened to have a large number of ash.

As in North Mankato, Mankato’s preemptive removal of ash trees is focusing first on ones that were planted beneath utility lines and don’t have room to grow properly, as well as trees with roots that are heaving up sidewalks or curbs.

“That is approximately 100 ash trees that we’ll be taking down this fall,” Lundborg said. “They are all marked and they are throughout town.”

Ash borer 3

Justin Lundborg poses with a series of ash trees in Tourtellotte Park that will be among the first of about 2,000 citywide that will eventually be felled. Many of the ones targeted initially are poorly placed below utility lines.

The $92,000 of work is being financed by a $65,000 Department of Natural Resources grant plus local funds.

Winter is the appropriate time for ash tree removal because the trees need to be transported to the compost site north of Mankato or, in North Mankato’s case, a designated location at the municipal yard-waste composting site.

If the wood was being moved during the summer and there happened to be ash borers on board, they could be getting a free ride to an uninfected neighborhood. It’s the same reason people are prohibited from moving firewood from an EAB-infected county to a non-infected one.

“If you move that firewood from point A to point B, you may be moving that beetle,” Lundborg said.

Oct. 1 to Nov. 1 is the best time for removing the trees because the beetles aren’t active.

Ash alternatives

Next spring, replacement trees will be planted in Mankato — smaller species such as Japanese tree lilac, spring snow crabapple, serviceberry and redbud. The trees don’t grow as tall, making them appropriate choices below utility lines. And they also tend to be attractive.

“They’re all going to have a flowering bloom at some point that is quite attractive,” he said.

But a higher goal is diversity. The clusters of ash trees along many streets came about because ash were planted as replacements for elm trees wiped out by Dutch elm disease a half-century ago. This time around, no neighborhood will be left reliant on a single species that might become the next victim of a future tree-killing plague. If a neighborhood already has a lot of lilacs, for example, city officials will pick one of the other choices for the ash replacement trees.

“They are all quality trees that do well in our area,” Lundborg said. “And then we’re diversifying our neighborhoods.”

That 100-tree goal for ash removal this winter, and replacement planting next spring, is just the start.

“Typically, our goal is going to be about 200 trees annually. And we’re always out there looking for grant opportunities to boost that number. ... We’re anticipating 8-10 years to take care of the pre-emptive removal.”

Nature’s course

By no means will that mean all of Mankato’s ash trees will have been turned into wood chips a decade from now. For every ash in a park or along a boulevard, countless more are in woodlots, ravines, nature areas and private yards. If the infestation hits the region as it has elsewhere, the vast majority will be dead trees standing.

Deceased ash in wilder pieces of city-owned land will be left to break apart and decompose unless there’s a public safety threat. A large tree on a woodlot’s border or one looming over a public trail might need to be taken down before it collapses on a hiker or someone on a neighboring property.

“Things like that we’ll have to take care of,” Lundborg said.

As for the appearance of the river valley’s wooded hillsides, Lundborg doesn’t necessarily foresee an end to lush scenery. Some spots, where the ash trees are dominant, might look a little dead even in the midst of summer. But elsewhere, surviving tree species will work to grab the sunlight and soil moisture previously gobbled up by the ash trees. Existing trees will spread their branches. New trees will grow up around the dead ash’s trunk.

The lucky few

Some ash trees will be saved. In Mankato, treatments will begin next year during the growing season for ash trees.

“June 1 to about Sept. 1 is the ideal time frame,” Lundborg said. “That’s when the tree is actively growing, when it’s moving nutrients up and down the fastest.”

The city will be using the trunk-injection method, which introduces the insecticide through a series of holes in the trunk right at ground level and lets the tree transfer the chemical throughout its branches to the EAB larvae feasting beneath the bark.

“It’s basically like an IV. The treatment is put directly into the tree’s vascular system,” he said. “That is what we recommend because it’s more pollinator-friendly.”

The alternative soil-drench method can be done by anyone using chemicals purchased at a home-improvement store or garden center. But that method not only can harm bees and other beneficial insects, it loses its effectiveness with larger trees — those with truck diameters of 10-15 inches or larger. And it must be applied every single year.

The trunk injection method requires a licensed applicator but needs to be done only every other year.

Mankato officials haven’t decided how many will be treated in 2022, but the current goal is to ultimately protect 300 trees.

“We’ll probably do half the first year and half the second year. That balances costs out.”

The treatments will be an ongoing expense until the trees succumb to weather or other factors causing their decline. The every-two-years regimen might be relaxed, however, once the ash borers finish their initial years of feasting in Mankato’s ash-rich environment and the larvae find little to eat at the buffet.

“It is believed that after the peak infestation wave comes through, instead of treating every two years we’ll be able to treat every five years because there will be fewer EAB around.”

The initial slate of condemned trees has been chosen in Mankato but the precise trees to be saved haven’t been marked, although they’ve all been evaluated and prime candidates for treatment have been identified. The ash trees getting a reprieve will be those in good condition and well-placed in a location where there’s plenty of room to grow.

City Ash Map Detail (copy)

In 2019, Mankato city officials inventoried all of the ash trees planted on boulevards and in city parks. (Some trees marked on this map may have since been removed.) To see the number of ash in other Mankato neighborhoods, check the larger version of this map at

The survey showed that not a huge number of trees meet that standard.

“Around 15-20% of our trees. We’re talking about 450 trees.”

Streets with a higher density of ash trees are likely to see a few of the best ones prioritized for treatment. On a street with a nice variety of other trees and just one or two ash, the ash can be sacrificed without leaving a barren neighborhood.

“That 300 could be bumped up if we find certain neighborhoods need more just to sustain the quality of the canopy,” Lundborg said.

So far in North Mankato, treatment has been focused on the Caswell Park softball complex, where virtually every large tree is an ash.

“We’re trying to save as many as we can up there just because they provide shade for so many spectators,” Host said.

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