This article is part 2 of 2 on Tree-of-heaven describing why Tree of Heaven is considered invasive and potential control methods. Please see part 1: How to ID Tree-of-heaven for identification tips.
While numerous exotic plants have little or no impact on the ecological systems into which they’ve been introduced, over two centuries of Tree-of-heaven’s presence in the United States’ has taught us it’s worth making a stink about – and not just because it stinks (both its crushed leaves and male flowers are known for their acrid odor). Some call it Chinese sumac or, you guessed it, ‘Stink Tree,’ but the botanical name for Tree-of-heaven is Ailanthus altissima, implying its preternatural ability to put on height rapidly – 6 to 10 ft in a single growing season.1 Of course, this is part of the problem.
According to Matthew Kassan of Virginia Tech (PhD Penn State), Tree-of-heaven originally spread from the gardens of 18th century Philadelphia botanists via new railroad systems extending beyond city limits. The disturbed soils along the tracks served as highways for this quick growing tree to take-off, and urban environments across the states have since supplied prime territory for Tree-of-heaven’s continued outbreak. Kassan claims, "It's the number one cause of native regeneration failure in clearcuts in Pennsylvania." Natural or urban environment, it doesn’t matter, the tree simply outcompetes.2
The Bureau of Environmental Services’ The Problem with Invasive Plants claims “invasive plants are a leading cause of declines in native plant and animal numbers, and are a factor in Endangered Species Act listings.” They go on to address the how invasive plants negatively impact water quality, tree cover, and even act as conduits for wild fires. They state the $120 billion dollars that the U.S. economy spends annually on invasive species control get passed on to consumers through many avenues, and reduces Oregon personal incomes by $83 million dollars per year.
Once established, Tree-of-heaven has its way of sticking around. Firstly, it’s allelopathic. Its roots release a toxin that prevents other plants from establishing nearby. The root structure itself concentrates near the surface, and sends vertical shoots that lead to clonal colonies, like the thicket (below) on Columbia Blvd near NE 63rd Ave, and the means by which it easily spread across Pennsylvania. The shoots have been found up to 50 feet away from the main trunk. Female trees are extremely fecund, producing about 350,000 viable seeds per year.
Once the trees are full grown, which doesn’t take long, they become a danger to the neighborhood. Their wood is brittle, making major limb failures almost inevitable in Portland’s stormy winters.
Quick disclaimer: please see the part 1: How to ID Tree-of-heaven before implementing any of the following control tactics.
As discussed, Tree-of-heaven is a mighty foe. Its complete eradication from urban areas has been unsuccessful to date, and largely unattempted. To do so will require coordination between tree care professionals, government agencies, and resident stewards, which one group in Portland fully intends on pursuing... Contact Tree-of-Heaven Eradication Now! (TEN!) for further information, and see how you can help the cause. But to get a better idea of what it’ll take, here are some considerations and tactics split into two divisions based on tree size.
Young Trees (<3” DBH)
With so many rather expensive herbicides on the market that fall under a Restricted-Use Pesticide appellation, choosing an appropriate product for your yard can be daunting if not impossible without an applicator license.
So here are two DIY options:
Adult Trees (>3" DBH)
The process changes for large trees since spraying or pulling the leaves would hardly make a dent.
have the arborists (or you if it's non-chemical) immediately treat the stump with one of the following options:
The stump and roots will react to the canopy’s removal, which cuts off the tree’s food source; i.e. foliage. Naturally, the remaining live material will produce many sprouts as the tree tries to compensate – hence the necessity to apply products effectively.
Each of these stump-treatment methods requires the agent be absorbed through the tree’s vascular tissue, particularly the phloem. Find this thin, darker ring just the inside of the bark, and outside of the cambium. Apply the agent to the freshly cut stump and don’t hold back. The agent will be transmitted down to the roots, where it has the best chance of killing off clonal suckers before they sprout.
Wait about a month for the agent to run its course in the roots before grinding the stump. Arrange for this stump-grinding with your arborist during the removal. Once this is done, stay vigilant – now the process becomes a project, albeit one that requires little effort. Keep an eye on the surrounding area for a good 3 years - and don't neglect your neighbors' role in this! They'll have to keep on top of the shoots as well. This tree has the complete invasive package, so root suckers will happen. When they do, you all be ready.
Extrapolating from data compiled across the nation, the Oregon Department of Agriculture has recognized the economic impact that Tree-of-heaven could have on urban areas like Portland. Although only listing this potential impact threat as 5 out of 10, repairs to sidewalks, house foundations, and property damage from limb failures are associated with significant costs at the individual level. The report notes, “Eastern Oregon [Tree-of-heaven] populations are expanding but currently are not located in areas where they create economic problems. Lateral rooting can push up pavement and sidewalks, and has been noted to ruin septic tank drain fields in its search for moisture.” Importantly, this report comes from 2012. Take a drive along I-5 in north Portland today, or I-84 east toward The Dalles, and it’s easy to see how quickly Tree-of-heaven can expand. The report continues, “On the west coast, tree of heaven creates problems in natural systems by forming large thickets via root suckering. Riparian areas are especially affected.” So just imagine the threat it imposes on the Sandy River or Johnson Creek watersheds -- an invasion of either would essentially render control attempts futile.
We see that critical to the eradication of Tree-of-heaven isn’t only the cooperation amongst government agencies and experts, but also homeowners and community groups – those people who are likely directly affected. It falls on each one of us at the local level to attack the problem before it’s a much larger, costlier issue. As we like to say in the tree world, make that 25 cent cut now and avoid the $2,500 cut down the line. Or in this case, just pull those sprouts when you see them!
... and Further Research
Some research is being conducted into two bio-controls with larger, wide-spreading implications for Tree-of-heaven. The first is a weevil (Eucryptorrhynchus brandti), and the second is a fungus (Verticillium nonalfalfae). Each of their applications is being studied intensely before use could be approved. Ensuring these bio-controls don’t unexpectedly affect non-target species is of utmost concern. If one or both prove effective, the whole eradication game could shift, but eyes on the local level will always be necessary. Be sure to contact TEN! for information on what to do when you see Tree-of-heaven in your neighborhood!