Field & Street 

My mistake was not noticing the young tree when it first sprouted behind the garbage cans. In a couple months it's grown into a robust young thing, insinuated in the slit between the asphalt of the alley and the foundation of my house. Now, with branches higher than my thighs and roots entwined with the house foundation, the sapling refuses to be yanked. When I tug hard, the upper branches tear away, leaving the stalk and the roots behind. I hold the leaves in my fists and disgustedly contemplate herbicide.

The tree that's taking root in this inhospitable place could be none other than the infamous tree of heaven, Ailanthus altissima, the plant home owners and urban foresters love to hate. If you live in Chicago and can identify only three kinds of trees by name, this is probably one. It's that pushy little devil that sprouts from cracks in sidewalks, out of rain gutters on houses, and in the forks of other trees. Native to the temperate region of China, ailanthus has gargantuan leaves crowded with up to 41 leaflets alternating their way up a central stem. The tree can grow to 100 feet under perfect conditions.

To call the species fast growing is as much an understatement as saying that light travels at a pretty good clip. Scientists have monitored specimens that have sprouted 12 feet in a single season. This can be a good trait if you're impatient, but it also makes the ailanthus a poor choice as a street tree. Speedy growers tend to be weaker than species like oaks, which grow ponderously, inch by inch, year to year. The fragility becomes a drawback when high winds break off heavy branches and drop them onto the hood of a car or the cranium of a pedestrian.

Ailanthus was introduced to the United States in the 18th century by horticulturalist William Hamilton, who planted them on his Philadelphia estate. Writer Edmund Newton credits the name "tree of heaven" to a French botanist who mistook it for a Molucca islands tree of the same name. Others credit the name to the tree's fantastic growth--its grasping for the heavens.

Frederick Law Olmsted created an ailanthus grove in Central Park in the late 1850s, and for a while trees of heaven were planted enthusiastically in eastern cities. But this love affair with the weirdly tropical-looking plant faded fast when its astonishing skills in procreation started making it an urban menace. The trees began to crop up everywhere and killing them off was no easy task. Female trees send out a million seeds each summer. And they don't take chances with only one method of reproduction; each tree also has the capacity to send out suckers from its roots that saturate the surrounding area with new clones. Cutting one down may result in the creation of fifteen more. By 1875 the District of Columbia was fining citizens who allowed trees of heaven to grow, and other cities halted their once-enthusiastic planting.

Openlands Project director Jerry Adelmann's third-story apartment in Lincoln Park has a deck overlooking the tops of three mature trees of heaven. On the day I visit in late July, the enormous leaves are a deep glossy green. Clusters of seed pods are a silky chartreuse tinged with pink. When I ignore the negative traits of the ailanthus and observe with fresh vision, I have to admit the effect is magnificent.

"They're always changing," Adelmann says, trying to get me to feel as fondly about them as he does. "The trees don't leaf out until late, but when they do their leaves are a pale yellow green. Gradually they become the color you see now. The pods have this pinkish, rusty cast now and darken later in the season. Sometimes the colors are almost psychedelic."

Even among some horticulturalists, ailanthus has its supporters. George Ware, a research associate at the Morton Arboretum in Lisle, has devoted much of his time to finding trees that grow well in cities. "I study the trees that volunteer along the Metra tracks, like Siberian elm, mulberry, and cottonwood. I keep using the word 'tough' to describe them because I can't find a better one," says Ware. "Tree of heaven is one of the toughest." Ware explains that most plants appreciate loamy, well-aerated soil that allows for oxygen uptake. In Chicago these plants are out of luck. City soil tends to be dense, hard-packed clay that has to endure bouts of being overly saturated with water in the spring and episodes of drought in the summer. In this setting, ailanthus manages to thrive. It has a deeper root system than other trees, which helps it withstand drought, and it has porous wood that increases air exchange.

"Ailanthus's favorite place for germination is in the crack of a building foundation," Ware says, without any prompting from me about my current nemesis. "The round seeds seem to drift into those spaces easily." It's an odd place to prefer, since building material turns the dirt alkaline and the house's overhang blocks the rain, making it drier than ordinary city soil.

When the tree of heaven first gained its powerful foothold in northeastern cities, biologists feared it would take over in the native forests as well. There seemed to be nothing to stop it. Except, as it turned out, the woods themselves. Exposed to the fertile conditions of a forest, ailanthus loses its competitive edge. Other trees--the oaks, maples, and ashes of the world--are so successful in this environment there's no room for trees of heaven. Studies have shown that when ailanthus seeds are placed in rich soil and in sidewalk cracks, the trees sprouting from cracks have a higher survival rate. In one case, a forester from Milwaukee grew ailanthus trees in a nursery and found that most died over the winter. He surmised they grew so lushly that they didn't properly harden off in preparation for winter.

The tree of heaven approaches the northernmost edge of its range in Chicago--and doesn't fare quite as well here as it does in warmer cities. Still, even in a more amenable climate, New York City's many thousands of ailanthus are plagued by a mysterious killer. In a New York Times article that ran in early June, entomologist Michael Birmingham says, "Whole stands are dying. The tree is so abundant on vacant lots and disturbed sites. For some folks, it's the only shade. If it died, they don't have a tree." Ware says the disease or fungus or whatever is causing this illness seems to be limited to New York. Chicago trees show no sign of it. For now, anyway, trees of heaven continue to have their place on earth.

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