Here in the high desert, gardeners and nature lovers learn to regard almost anything that grows well on its own with respect and admiration. But there are exceptions. When a non-native plant is so adaptable that it spreads with abandon, sucking up nutrients and water and crowding out less robust species, it threatens native habitats and is a nuisance in gardens: it becomes “invasive.”
Santa Fe has its share of invasive trees, notoriously Siberian elm, but also Russian olive, salt cedar (or tamarisk), and the ultra-aggressive tree of heaven, Ailanthus altissima. Like many invasive species, ailanthus was introduced as an ornamental before its less desirable features became apparent. Sometimes called “stink tree,” this native of China was brought to the United States via Europe in 1784 and soon escaped cultivation. It looks a lot like our native sumac, but its flowers and leaves emit a fetid odor, unlike those belonging to the Rhus genus.
Tree of heaven is fast-growing, quickly shading out other plants. Though an individual specimen is relatively short-lived, ailanthus is very difficult to control because it spreads not only by seeds but also by root sprouts, clones of the mother tree that prolong its life indefinitely. It’s seen in weedy colonies along roadsides and in Santa Fe neighborhoods.
Ailanthus' tolerance of extremely poor soils and total neglect has earned it the nickname “ghetto tree” in places like Detroit, where it’s often the only thing flourishing in the ravaged urban landscape. Depending on the situation, that toughness can be virtue. In Betty Smith’s classic novel A Tree Grows in Brooklyn, a tenacious tree of heaven growing in concrete serves as the cen-tral metaphor for survival in adverse circumstances.
I have an ambivalent relationship with the tree of heaven on our property. On the one hand, it requires no irrigation and shades our west-facing driveway, keeping our cars cooler in summer. It also adds carefree greenery to the street scene. On the other hand, it sends shoots far and wide, through juniper bushes and under other trees. If I didn’t stay on top of pulling these shoots, we’d soon be in a tree-of-heaven jungle.
I asked Bob Pennington of Agua Fria Nursery what he would do if he were in charge of Santa Fe’s urban forest. He said he would attempt to eradicate the species. The only way to do that is by cutting down the trees and immediately poisoning the stumps; the best time to do so is in early fall, when plants are drawing in nutrients before going dormant. Even then new shoots may appear in spring.
The people at Plants of the Southwest offered a different perspective. They suggested pruning the trees for form and cutting back the shoots. They also pointed out that if all the tree of heaven and Siberian elm trees disappeared, Santa Fe would lose a huge portion of its cooling green canopy.
I sometimes regard our tree of heaven with a murderous eye. But in a desert climate that’s getting hotter, we may need to start making peace with the weeds that can tough it out.
Sarah Baldwin is a freelance editor, a writer, and a gardener.