The Tree of Heaven at 21st and Broadway provides much-needed shade. Google Maps
The Tree of Heaven's arrival in Denver is something of a mystery. Topjabot via Wikimedia.org
The Tree of Heaven is considered a noxious weed in New Mexico. H. Zell via Wikimedia.org
A threatened tree will send up countless shoots. Eric Peterson
For many Denverites, the Tree of Heaven defies its moniker. This invasive plant is more apt to make a property owner say a deity's name in vain than praise its seemingly immortal roots.
Walk along almost any stretch of Denver's urban core, and you're likely to spot the Tree of Heaven -- Ailanthus altissima to botanists, "ghetto palm" or "tree of hell" to others. It pushes through cracks in parking lots and sidewalks, pries apart wood decks in quest of sunlight, ekes out an existence in alleys, and yes, occasionally earns grudging appreciation for providing a canopy of shade where no other tree could possibly grow.
It can also cause serious damage -- to foundations, clay sewer pipes, even other plants, not only by crowding them out, but by producing chemicals that inhibit the growth and germination of plants around it.
The Tree of Heaven might be the fastest-growing tree in North America, according to the U.S. Forest Service, capable of growing more than six feet per year during its first four years. More notable is what the tree produces underground. Its root system is capable of sending up shoots or "suckers" of new plants over a vast area, and cutting down a tree only makes matters worse; the severed tree and its roots shift into a sort of survival mode, sending up a barrage of new plants, as if to keep alive the Tree of Heaven lineage.
Indigenous to China and one of that country's oldest existing plants, the Tree of Heaven was brought to the United States in 1784 by a Philadelphia gardener and gained rapid popularity along the East Coast for its fast growth, adaptability to seemingly all soil types, handsome appearance and imperviousness to pollution. It is believed to have been introduced to California in the 1850s by Chinese immigrants. The Tree of Heaven was used as a metaphor for survival amid adversity and deprivation in Betty's Smith's acclaimed 1943 book, A Tree Grows in Brooklyn.
How the tree made its way to Denver is open to speculation. Denver was a treeless prairie when it was settled, so every species aside from those along waterways had to be brought here at some point, either purposely or from seeds that "hitchhiked" here.
"It's very invasive, obviously," says City Naturalist Kelly Uhing, though she points out that the Tree of Heaven is not on the state's noxious weed list like it is in New Mexico, mainly because it is not the threat in open spaces and natural areas that it is in some urban settings. The Colorado Noxious Weed Act has been in place since 1991 and requires private and public landowners to control species that are designated as noxious weeds.
"It's an interesting tree. I don't want to advocate for it one way or another," says Pam Regensberg of the Kathryn Kalmbach Herbarium at Denver Botanic Gardens where her work includes documenting the flora of Colorado. "In nature, there's this checks-and-balances system at play. Once certain plants or animals are moved from their indigenous environments they can easily and quickly exploit the areas that do not have those same checks and balances. And that's what's happening here. In China, it doesn't act like this. It's primarily just here in the urban areas."
A tree with an afterlife
Uhing and other area plant authorities have some advice for controlling the Tree of Heaven, though all agree eradication is an unrealistic aim. One suggestion for at least controlling the tree is to cut it down close to the ground and immediately -- within 30 seconds -- apply full-strength herbicide.
Robert Cox, a horticulture agent with Colorado State University Extension, recommends a herbicide product such as Brush-B-Gon that contains the active ingredient triclopyr. "But," he warns, "it has to be used repeatedly and every time you see new suckers coming up." Otherwise, he says, "It will just laugh at you. "
Cox also suggests, as a way of controlling growth without setting off the tree's sucker-producing alarm, slashing the sides of the trunk with an ax or knife and treating the sliced surfaces with triclopyr. "That'll hurt it," he says, "but it'll come back from that. You have to be so persistent with it."
Uhing is another who emphasizes quick application of a full-strength herbicide after a tree is cut down, before a layer of cells known as cambium is able to form over the stump.
"You want the herbicide to get into the root system," she says. "If you wait too long -- even a minute -- you've risked not being able to penetrate into that. That's the key to controlling Tree of Heaven -- controlling the root system. Because two-thirds of the tree is underground compared to above ground. What you see is not what you get."
City Forester Rob Davis says the Tree of Heaven's fast, furious and widespread propagation can cause conflict in residential areas "when you've got a neighbor who's not taking care of his yard, or if a neighbor cuts one down and next thing you know you've got 5,000 new sprouts coming up in your yard. It can be kind of a nuisance. I understand all that."
And yet, Davis has found that even in Denver there's a rightful place here and there for this mostly despised tree. One such place is a little triangle downtown where Broadway converges with Champa and 21st streets.
"There's a giant Tree of Heaven on that little parcel," Davis says, explaining that the tree was spared rather than removed sometime back during a construction project. "It's big enough that you couldn't wrap your arms around the trunk. Normally a Tree of Haven would be something we'd say, 'Let's get it out of there.' But if you look at an aerial view of the area, it's like the only canopy that's shading any part of that hot spot in Denver. So that was a rare case where I found myself protecting what is considered more of a noxious weed, basically."