by Tom Hixson, UC Master Gardener of Napa County
Most Napa Valley residents recognize the plant known as tree of heaven when they see it sprouting wild in fields, taking over their gardens or defiantly establishing itself in sidewalk cracks. But despite its purported celestial provenance, this tree poses a significant threat to our landscapes and to agriculture.
Tree of heaven (Ailanthus altissima) is a medium-sized tree first exported from China to Europe in the mid-18th century and to the eastern United States in the 1780s. It was later introduced to California during the Gold Rush by Chinese miners who believed it had medicinal properties. But initial enthusiasm for the tree was based mostly on its ability to flourish virtually anywhere, regardless of drought, extreme heat or excess rainfall.
Because it grows so quickly (up to 90 feet) and can easily be pruned into a handsome tree, it promised to become an attractive addition to landscape plantings. Appreciation for this tree declined, however, once gardeners became aware of its invasive nature and its foul odor, sometimes described as akin to rotting peanut butter.
Despite these drawbacks, tree of heaven was used extensively as a street tree during much of the 19th century and achieved top billing in Betty Smith's 1943 novel A Tree Grows in Brooklyn. It has proliferated throughout the United States ever since.
In addition to having an unpleasant scent and aggressive growth habit, tree of heaven is the preferred host plant of several harmful insects, including the dreaded spotted lanternfly (Lycorma delicatula). This large plant hopper feeds on (and largely destroys) more than 100 different plant species, including grapes, stone fruits and hardwoods.
Currently, the spotted lanternfly has been identified in 18 states on the East Coast. It has the potential to spread rapidly, leaving severe damage in its wake. Tree of heaven is also the preferred host plant for another exotic and destructive pest, the brown marmorated stink bug, already found in much of California.
If you've ever gardened near an established tree of heaven, you will understand why its invasiveness is so concerning. It drops tens of thousands of its devilish seeds, which are carried in samaras, 1- to 2-inch pods cleverly engineered for maximum dispersal. On a windy day, the samaras scatter widely, then bed down for the winter under fallen leaves. In the spring, they are resurrected, sending vigorous new shoots to the surface.
Fortunately, at this early stage, the seedlings can be easily vanquished by pulling them up by the roots. If not eradicated in the first few months, however, the vigorous young plants will develop tenacious root systems and become more difficult to remove. The entire root system must be dug up or the plant will return in no time. It's easier to uproot when the soil is damp.
What can you do to limit the spread of tree of heaven? First, learn to identify it. Its crushed leaves emit a strong and disagreeable odor, and by late summer, it trademark seed pod clusters appear.
Besides germinating from seed, the mature tree can reproduce through runners. Seedlings from runners can appear up to 30 feet away from the parent tree and need to be dug up and removed. Regularly removing these runner shoots will eventually discourage new growth, but the gardener must remain vigilant. This tree never relaxes its efforts to spread.
The most important thing you can do to manage this tree (besides removing the adult altogether, which is not always feasible) is to remove the samaras when they drop in the fall. A simple vacuum attachment to a leaf blower does an excellent job of picking them up.
Don't put these seeds in a municipal compost bin. They will flourish elsewhere unless killed by rigorous hot composting. Instead, place them in a bag, seal it and send it to the landfill. It's worth the effort to collect the samaras in the fall as you'll greatly minimize sprouts the following spring.
California Master Gardeners are collaborating on a program to identify tree of heaven throughout the state with the goal of reducing its presence. Three hundred Master Gardeners have been trained to identify this tree and report its location, age and size. Researchers and public works officials use this data to generate maps and prioritize trees for removal, but it will take years for this massive undertaking to have an impact.
Given that awareness of the problem is a key part of managing it, we home gardeners can do our part by teaching ourselves to identify tree of heaven. Then we can utilize available techniques to control its spread in our own neighborhoods.
Library Talk: Join UC Master Gardeners of Napa County for a Napa Library talk on “Growing Peppers” on Thursday, January 2, from 7 pm to 8 pm via Zoom. Let's parlez peppers—hot, sweet and mild. Learn the basics of starting peppers from seed, plus growing tips and usage ideas for your eventual harvest. Click here to register!
Help Desk: The Master Gardener Help Desk is available to answer your garden questions on Mondays and Fridays from 10 am until 1 pm at the University of California Cooperative Extension Office, 1710 Soscol Avenue, Suite 4, Napa. Or send your questions to mastergardeners@countyofnapa.org. Include your name, address, phone number and a brief description
References:
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=60006 (tree of heaven removal)
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=59944 (scout and report info)
https://www.facebook.com/UCCEAmadorMG/photos/as-if-the-invasive-ailanthus-altissima-tree-of-heaven-infestation-throughout
https://ipm.cahnr.uconn.edu/invasive-species/invasive_plants_tree-of-heaven/#:~:text=Tree%2Dof%2Dheaven%20is%20not,(June%2Dearly%20July)
https://ipm.ucanr.edu/PMG/PESTNOTES/pn74142.html
https://en.wikipedia.org/wiki/Ailanthus
