The people who almost brought you advertisements on shade trees are at it again.

The city of Miami wants to sell commercial space on its parking pay stations and give businesses the freedom to festoon their windows with mini-LED billboards.

At a time when Metromover cars, baseball uniforms, restaurant menus and even newspaper buildings are plastered over with advertising, this might seem like a tiny turn of the screw.

Not to a group of citizen watchdogs, who have declared they will not stand by and let ads clutter the natural landscape.

“We need to stop this, and we need to stop this now,” activist Nathan Kurland told the Miami City Commission last week.

Advertising has become a high-stakes issue at Miami City Hall. That’s partly because outdoor advertising companies have become important contributors to political campaigns, and as such, a powerful lobby.

Anti-billboard groups like Scenic Miami have also raised their profile and proved their ability to derail legislation.

It isn’t clear who has the upper hand.

Last year, the commission approved an ordinance allowing blinking billboards on three city-owned facilities: the Gusman Center for the Performing Arts, the Miami Children’s Museum and the James L. Knight Center. The measure was billed as a way to create at least $430,000 a year in new revenue for the city.

Two weeks later, commissioners were supposed to consider a proposal to allow advertising on contemplated bicycle-parking stations and parking pay stations. But the legislation was so broadly written that it would have opened the door to ads on all city-owned fixtures, including telephone poles and shade trees. Citizens called the proposal “outrageous,” and it was yanked from the agenda before receiving a formal hearing.

A new proposal to allow ads on parking pay stations surfaced last week.

Like last year’s bill, the new version was loosely written, and could have been interpreted to permit ads on a variety of public surfaces.

It was enough to sound the alarm bells among community activists.

In a matter of hours, they fired off more than 3,000 emails opposing the legislation. The email blast caught the attention of top city leaders and prompted Miami Parking Authority chief Art Noriega to rewrite the bill. The new language limited the advertising to “parking meters or structures situated on property within or adjoining the public right-of-way.”

At a commission meeting last week, Noreiga said he had only intended to allow advertising on the pay stations. He showed a rendering of a sample station featuring several small ads and a walking map of the neighborhood.

“This is about more than just the advertisements,” Noreiga said. “There would be a public-service aspect, too.”

He said it was too soon to say how much revenue the ads could raise for the parking authority.

But activists like Peter Ehrlich were not satisfied.

Ehrlich argued that the legislation was still ambiguous. “You can easily interpret ‘structures’ to mean any physical building that’s owned by the parking authority,” he said.

After Noriega promised to tweak the language further, commissioners voted 3-1 to give preliminary approval to the bill.

“I think you can get this right,” Commission Chairman Marc Sarnoff said to Noriega. “No liquor advertisements. No smoking advertisements. No sex stores.”