Is it thumbs up or thumbs down for tree-of-heaven (Ailanthus altissima), now standing out among the general greenery for the mops of yellowish or orangeish seeds capping its stout branches?
Thumbs down.
With compound leaves and coarse, chubby limbs, this tree could have been mistaken for staghorn sumac or black walnut earlier in the season, before the seeds became prominent. (The peanut-y aroma of crushed tree-of-heaven leaves or stems easily distinguishes this tree from the others any time of year.) And those seedheads are one of the problems with tree-of-heaven: It is extremely fecund, each tree potentially casting more than 300,000 seeds to the wind.
Each seed has wings that ensure it doesn’t drop to the ground before first hitch-hiking a ride on the slightest breeze.
• A WEED IN SO MANY WAYS: Another reason tree-of-heaven is snubbed as a weed: Cut it down and it won’t go away. New sprouts enthusiastically pop up from the cut stump, even after years of recutting. What’s more, the spreading roots send up sprouts that eventually can grow into full-size trees at some distance from the mother plants. “Full-size” for tree-of-heaven means 40 to 60 feet or more.
The plant’s short lifespan, rarely more than 50 years, does nothing to diminish its weediness. Those roots sprouts stand ready and waiting to replace any old top growth in decline.
That’s still not all: Tree-of-heaven also is among the fastest-growing trees. At 3 to 5 feet per year, it can quickly outstrip competitors, whether they are cultivated plants or weeds. It also tolerates adversity. This is “a tree that grows in Brooklyn,” thriving despite heat, cold, alkaline or acidic soil, wet or dry soil, even infertile soil and polluted air.
Given tree-of-heaven’s ability to seed and grow, it’s no wonder that it’s famous for popping up in cracks in pavement and along abandoned railroad tracks, as well as in the woods. The plant is native to China, but has been hopscotching around the eastern U.S. since 1784, when a Philadelphia gardener introduced it by way of England. In the 18th century, tree-of-heaven was often planted as a street tree in urban areas; that legacy survives, as weeds.
During the Gold Rush of the 1890s, Chinese immigrants brought the plant over and contributed to its spread on our West Coast.
• IT’S NOT ALL BAD: Despite tree-of-heaven’s weedy nature, we can’t write it off as just another weed; if nothing else, it has some practical uses. Those Chinese immigrants used it as a medicine and insect repellent. It also has been used to re-vegetate mine spoils, where soils are very acidic.
And tree-of-heaven has one more weedy trick up its bark that might prove useful. It produces a chemical that suppresses growth of nearby plants; it’s an in-house herbicide factory that gives the plant a leg up in the race skyward. (Tree-of-heaven is not alone with this ploy; other plants — including black walnut, sunflower and rye — also produce natural compounds that inhibit the growth of, or kill, other plants.) Advantages of this natural herbicide, if it could be used in gardening, are rapid biodegradation, low toxicity to non-target organisms, and production without factories or petrochemicals.
I’ll admit to a soft spot for tree-of-heaven because of a serene, stately, towering grove of them that I admired as I lounged in their shade a quarter-century ago in rural Virginia. Despite that experience and the tree’s other qualities, however, I wouldn’t suggest actually planting it. It achieves elegance only when a few trees are planted, and they are given plenty of space and time to grow. Even then, I would worry about the hundreds of thousands of seeds each would spread every year.
