Sam Gannon, a senior park ranger for the City of Denver, understands the appeal of stringing up a hammock and lounging underneath a tree. He has one hanging in his own backyard. But the trees that Gannon monitors in Denver’s parks remain off limits to attachments of any kind, including hammocks. That’s not to say that hammocking is banned in the parks: People are allowed to bring their own free-standing setup. As long as your “mock” isn’t touching a tree, hang away.

Having been a ranger for seven years and supervisor for two, Gannon has noticed an increasing popularity in hammocking. With the release of newer nylon hammocks that come with a set of straps, it makes it even easier for people to tie up virtually anywhere. While recreationists might consider hammocking to be a relatively non-threatening activity, the high volume of park usage means that attachment to trees is damaging, Gannon says.

“When people attach hammocks, slacklines, tents—I’ve seen trampoline attachments—when people attach these over time, it wears away the bark,” he says. “The layer underneath the bark helps bring water from the roots up to the tree. When that gets damaged, the tree dies.”

One hammock isn’t going to cause significant damage, but when the same trees work to support them over and over, that’s when it’s problematic. Some hammockers prioritize using wider tree-safe straps instead of rope or cord. Slackliners have placed blankets between the trees and their straps. While Gannon applauds these techniques, they don’t make hammocking legal.

“Your general public aren’t tree experts,” Gannon says. “A lot of times trees can have root rot or disease or underlying issues you can’t see on the surface, or they’re too small to support the structure [people are] trying to attach.”

Denver considers trees to be an invaluable asset, which is why the city has a forestry division and team of rangers dedicated to protecting them. In addition to providing shade and clean air to the city, some of Denver’s trees carry historic value. Still standing are some of the city’s first-planted trees, Gannon says.

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It’s common for Gannon’s team to ask people to remove their hammocks from trees every day. He says they don’t see safety concerns related to hammocks, so it remains a low priority, but they will address it while on patrol. Though people are often willing to comply, they have had to issue $100 fines, which can be higher for repeat offenders.

Beyond Denver, hammocking is prohibited in many Colorado state parks, including Boyd Lake, Eleven Mile, Golden Gate Canyon, Lory, North Sterling, Pearl Lake, Staunton, and Steamboat Lake. Jackson Lake allows hammocking at designated sites that provide posts. If you’re unsure whether or not you can hang a hammock, call ahead to specific ranger districts and state parks for more information.

Guidelines on hammocking in national parks also vary. Rocky Mountain National Park (RMNP) allows hammocking within the footprint of most campsites, as long as it doesn’t overhang or cause damage to vegetation below it. RMNP Public Affairs Officer Kyle Patterson suggests padding trees with towels prior to setting up your hammock as a way to mitigate damage, though as Gannon notes, that won’t prevent all damage.

As the weather heats up and you bring your hammock out of storage, do a little research before you tie it up between trees and take a snooze. Otherwise, a ranger like Gannon might be waking you up.