I first noticed it on a trail cut above the Columbia River Gorge, just east of Cascade Locks. A familiar texture — pinnate leaves fanning out in long, feathery arches, each leaflet ending in a tiny glandular notch. From a distance, it could pass for a sumac, or even a young black walnut. But the smell gave it away: a pungent, faintly acrid musk that clung to my hands long after I brushed a leaf. That smell is Ailanthus altissima. The Tree of Heaven.
It shouldn't be there. And yet it was everywhere.
An Ornamental with a Long Memory
Ailanthus altissima is native to central China. It arrived in North America in 1784, first planted in Philadelphia as an ornamental — prized for its rapid growth and tropical silhouette. By the 1800s it was a staple of urban planting schemes, celebrated for thriving where nothing else would: cracked pavement, rubble lots, the edge of railroad cuts. William Ashburner Stiles called it "the most accommodating tree in the world."
That accommodation is the problem.
Ailanthus grows up to 6 feet per year. A single female tree can produce more than 300,000 winged seeds annually, each a samara that spirals downwind for hundreds of metres. Cut it and the stump resprouts. Poison it and the roots send up new shoots from nodes metres away. It tolerates drought, compacted soil, industrial pollution, and deep shade. It has, in the language of invasion biology, a very wide niche breadth.
What's Happening in the Pacific Northwest
Ailanthus has been established on the East Coast for nearly two centuries, but the Pacific Northwest has largely been spared — until recently. Warmer, drier summers driven by climate change are extending the thermal envelope into Oregon and Washington, allowing Ailanthus populations to move north and west from established colonies in California and the Columbia Basin.
Survey data from Oregon Department of Agriculture tracking stations show rapid range expansion since 2015. Documented infestations now extend from the Columbia River Gorge — where the rain shadow creates the warm, rocky conditions Ailanthus loves — up into the Willamette Valley and east along Interstate 84. Scattered populations have been confirmed in the Puget Sound lowlands and on the drier eastern slopes of the Cascades.
"Ailanthus doesn't just occupy habitat. It restructures it. The allelopathic compounds it releases suppress germination of native species, effectively rewriting the seed bank of the soil beneath it." — Dr. Sarah Reichard, University of Washington, Center for Urban Horticulture
That restructuring is what distinguishes Ailanthus from a merely vigorous weed. It's allelopathic: the roots and bark exude ailanthone, a compound that inhibits germination and growth of many plant species. A stand of mature Ailanthus doesn't just shade out competitors — it chemically suppresses the seed bank, making recovery extremely slow even after removal.
What It Displaces
The Pacific Northwest forest understory that Ailanthus threatens is one of the most ecologically rich in North America. Western red cedar, bigleaf maple, and Douglas fir canopies shelter an intricate community: sword fern, trillium, oxalis, bleeding heart, and the mossy nurse logs that nurse the next generation of conifers. This understory is habitat — feeding, nesting, and overwintering territory for dozens of bird species, pollinators, and small mammals.
When Ailanthus moves in, that diversity collapses. Mature stands of it resemble green monocultures: a dense, closed canopy of compound leaves with almost nothing growing beneath. Native grasses, forbs, and shrubs are gone. The structural complexity that wildlife depends on is erased.
Allelopathic soil chemistry suppresses native seed germination · Rapid canopy closure eliminates understory light · Low wildlife value compared to native species (few insects can process its chemistry) · Resprouting roots make removal a multi-year commitment · Samaras disperse widely along waterways, roads, and disturbed edges
Why It's So Hard to Stop
The biology of Ailanthus is practically designed to defeat control efforts. Cut it down and the stump immediately sends up multiple vigorous shoots — often growing faster than the original trunk. The root system remains alive and can persist for years, sending up new stems from nodes far from the cut. Mechanical removal without chemical treatment simply stimulates regrowth.
Herbicide is more effective but requires precision. Foliar spraying in riparian areas — where Ailanthus loves to grow — risks off-target damage to native plants. Basal bark treatment with triclopyr is generally the most reliable approach, applied in late summer when the tree is moving sugars toward roots. Even with correct treatment, follow-up inspections over three to five years are necessary to catch resprouts before they re-establish.
The seed rain complicates everything. A treated stand can be recolonized within two years if there are productive trees upwind or upstream. Without coordinated landscape-scale control, individual removal efforts are often overwhelmed by reinvasion.
What You Can Do
Early detection is the highest-leverage intervention available. Ailanthus is easiest to identify and control when stands are small. In the Pacific Northwest, it most commonly establishes on:
- Roadsides and highway corridors, especially where I-84 passes through the Columbia River Gorge
- Disturbed edges of urban parks and riparian buffers
- Old homesites, abandoned lots, and fence lines
- Gravel bars and rocky south-facing slopes in the rain shadow
If you find it, report it. Oregon and Washington both maintain invasive species reporting portals (Oregon Invasive Species Council's iMap Invasives; Washington State's WA Invasives). Your sighting adds to the distribution data that state agencies use to prioritize control work.
Looking Closer
I photograph wildlife and flora in places that are still largely intact — and the pressure of watching those places change is part of what drives me to keep going back. The trail above Cascade Locks where I found that first Ailanthus stand? I've watched it for three seasons now. The patch has grown. The trilliums that used to come up along the cut bank in April have been absent for two years.
Conservation is often described as a matter of large interventions: protected areas, legislation, funding. And those things matter enormously. But the actual work is intensely local — one stand at a time, one report at a time, one landowner persuaded to let a crew onto their back slope with triclopyr and brushhooks. It is slow, unglamorous, and easily undone.
The camera teaches you to pay close attention to small things. It turns out that's exactly what conservation asks of you, too. The slope that looks fine from the road may be losing its understory to a tree that smells like a wet peanut. Worth stopping to look.