Ash borer expected to kill 17% of Mankato's trees

Mar. 2—MANKATO — The inevitable arrival of the tree-killing emerald ash borer is expected to take one in every six trees in Mankato in the next decade or so and leave hefty bills in the laps of homeowners with large backyard ash trees.

A report to the Mankato City Council Monday night also warned of $1.5 million in expenses just to deal with the estimated 2,500 ash trees on city land. And the report sets out proposed processes for identifying infested trees on private property and requiring their removal at the property-owner's expense, although options may be provided to pay the bill — which could top $1,000 for large, difficult-to-remove trees — over several years.

"It's going to have a big financial impact on all of our residents as well as the city of Mankato," said Ashley Steevens, the city forester and superintendent of parks.

An "Emerald Ash Borer Management Plan" was originally to be presented to the council a year ago before being delayed by a more pressing scourge — the COVID-19 pandemic. But with the invasive beetle closing in on Mankato from all directions, city staff said preparations for the ash borer can't wait much longer.

"With an estimated 17,400 ash trees on public and private properties combined in Mankato — including 2,500 in boulevards, parks and city managed properties — the city is at risk of losing approximately 17% of its existing tree population in the next 10 years," the report states.

The plan's multi-pronged approach was developed based on lessons learned in parts of the United States that have already been devastated by the ash borer, an Asian beetle first discovered in Michigan in 2002 and now confirmed in 35 states, including virtually every state east of the Rocky Mountains. As of Jan. 1, 25 counties in Minnesota have infected trees, including counties in all directions from Mankato — Steele, Martin, Scott and Brown.

If the plan is adopted by the council, preemptive removal of ash trees will begin this summer on city-owned boulevards and in municipal parks to reduce the number of dead trees needing to be removed when the infestation arrives in force in coming years. The insects are currently within 25 miles of Mankato in New Ulm.

A second part of the strategy is to begin chemically treating the most valuable city-owned ash trees once the ash borer is found within 15 miles of Mankato.

Finally, city ordinances will be amended to provide authority to force private ash trees to be removed at the expense of the property owner.

All three elements of the plan are aimed at avoiding a surge of large and dangerously brittle dead trees.

"It's mainly to eliminate a public nuisance or a hazard," Public Works Director Jeff Johnson said. "(The effects of the ash borers) can cause large branches to fall on people or homes or cars."

And if Mankato is suddenly inundated with dead trees, prices for removal are expected to escalate significantly — driving up costs for both the city and homeowners.

Under the plan, 200 ash trees will be removed every year starting this summer from boulevards, parks and other city-owned property. Targeted trees will include younger ash, poorly placed trees such as those under power lines, and trees along streets undergoing reconstruction.

The city intends to replace each tree, avoiding the mistakes made during the Dutch elm disease infestation of 50 years ago when most of the replacements for the dead elms were from just a few species of trees including ash. The plan calls for no more than 5% of the replacement trees to be of the same species, no more than 10% from the same genus and no more than 15% from the same taxonomic family — making the city's urban forest more resistant to future diseases or invasive pests.

The plan calls for chemical treatment of 300 trees to allow them to resist the emerald ash borer. The proposed treatment, which appears to be 95% effective when done properly, would cost $7,500 annually for 25 years or more.

"We're looking at the best of the best, the nicest trees we have," Steevens said of the lucky 300.

For private tree-owners, there's no proposal to use taxpayer dollars to help them pay for tree removal or chemical treatment. The city may set up a program where people can hire the same private contractor used by the city and be charged the same bulk rate.

The plan calls for a citywide tree survey of every street at least once a year — an inspection that will include private ash trees. Property-owners will be notified of infested trees and they will need to commit to a treatment program for specimens that can still be saved or to removal of doomed trees. If they fail to act, the city will have the tree removed and bill the owner.

But Steevens recognizes that some people will be unable to cover the entire cost, particularly if they own a large ash that can't be reached by bucket trucks: "The cost of tree removal goes up exponentially when we have limited access."

In those cases, the plan recommends allowing people to set up a payment plan or have the cost placed on their property tax bill. Already, $600,000 has been set aside for the looming emerald ash borer expenses, partly to allow the city to cash-flow the upfront removal costs for privately-owned trees and be repaid over time.

Under the plan, forced removal of dead ash won't occur on woodlots an acre or more in size, whether private or public, unless the trees are within 100 feet of places frequented by people, such as streets or trails.