Chicago has tried for more than a decade to slow the declining health of ash trees, some of the city’s oldest and most mature trees that provide communities benefits like shade and flood mitigation, as the trees have been overcome by a small but mighty green pest.

Starting in 2008, the city began inoculating the ash trees, which numbered about 96,000 at the time, against the invasive emerald ash borer beetle.

But by 2018, with almost half of the ash population dead and removed, the city decided to stop inoculations and let the remaining 50,000 ash trees die off.

Now with a flush of federal money in the city’s coffers, some activists and aldermen are pushing for the city to bring that inoculation program back, arguing it could save Chicago’s second most populous tree, a key part of the overall tree canopy.

That’s in part because of the type of trees ash are — featuring larger, wider leaves.

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“In terms of their leaf surface area, they are one of the larger leaves that we have,” said Daniella Pereira with the conservation and tree-planting group, Openlands. “So … we have to remember that if we lose an ash tree, we’re losing a significant amount of canopy.”

The years-long issue of whether to treat the trees or let them die to make way for new ones took center stage at a budget hearing this week for the Department of Streets and Sanitation, which oversees the city’s tree population.

Aldermen questioned whether this year’s budget should include a few million dollars to give the remaining trees another dose of chemicals so that they don’t all die off at once — causing a headache, potential safety hazards, and a hefty price tag for the cost of removing and replacing the trees.

Advocates say the cost of treating the city’s ash trees would be around $2.7 million. But the city’s Streets and Sanitation department officials say it would be more.

Commissioner Malcolm Whiteside, who leads the Bureau of Forestry within the department, says it would take at least $6 million, or about $120 per tree, to protect the remaining ash for just an additional three years.

Advocates of saving the trees point to the heftier cost of removing dead ones — Whiteside estimated it’s about $1,500 to remove and replace a single tree.

“I am struggling to understand why we wouldn’t put $6 million to save ourselves more money in the long run,” said Ald. Andre Vasquez, 40th Ward. “I’d love an answer to that logic because it doesn’t make sense.”

Department officials argue the inoculations only save the trees temporarily, meaning the city could end up having to pay for removal at a later date anyway. Whiteside said the existing 50,000 trees have gotten two or three treatments in the past decade to keep them alive for a while as the department works to remove dying ones.

“So we feel there’s enough chemicals in those trees to last the next ten years,” he said. “That’s why we were comfortable making that decision … to go ahead and stop doing that inoculation and go ahead and move on with the process.”

But advocates say the city’s estimates of how long the inoculations last are off. They say the treatment only last for three years, and that experts estimate that the best case scenario for an untreated tree with residual chemicals is a 6-year lifespan.

“The idea that these trees will be around 10 years from now is completely, completely misleading,” said John Friedmann, with the group North River Commission, which has been running a so-called “Save Your Ash” program that helps Northside residents fund their own ash tree inoculations.

Friedmann and Pereira also pushed back on the idea that chemical treatments won’t help save trees permanently, particularly those that don’t show signs of deterioration.

“It’s not true. You could save American elm trees, you could save ash trees, at least the big healthy ones that have no sign of decline,” Pereira said. “There are many, many elms — like all of the large trees in Grant Park — those are all American elms. Those get inoculated, the city puts a budget to it. That is how we keep these big, large, beautiful trees.”

Pereira said while Openlands doesn’t believe it’s necessary to inoculate ash trees that show clear signs of invasion, the group is urging the city to identify healthy ash trees that could be saved and potentially stand tall for decades through chemical treatment.

The push for funding to maintain and treat some of the city’s largest trees comes as the mayor’s office has set aside a whopping $46 million for planting new baby ones. The program has been described by advocates as one of the largest tree planting initiatives in the history of Chicago.

But at a budget hearing Tuesday, aldermen urged the department to invest more in maintenance of existing trees, with numerous aldermen complaining of years-long backlogs in tree-trimming requests.

Whiteside and Acting DSS Commissioner Cole Stallard assured aldermen that an additional 23 full-time employees, or 11 new tree-trimming crews, will help the historically understaffed department address these concerns.

Advocates like Pereira emphasized the importance of balancing new planting with ongoing maintenance work, like tree-trimming and, yes, ash tree inoculations.

“It’s really great that the mayor’s office is putting a lot of money toward tree planting,” she said. “But you can’t plant a very large tree. So to get the canopy that we’re losing from our ash decline is going to take, you know, decades.”

“One of the best ways to manage the forest would be to save any healthy ash tree you can,” Pereira said.

Mariah Woelfel covers Chicago city government at WBEZ. You can follow her on Twitter @MariahWoelfel.

This story has been corrected to note that the ash tree is the second most populous tree in Chicago.