By Gregory DL Morris
Invasive wisteria is a “severe threat,” according to the North Carolina Native Plant Society. It grows fast and has no natural controls. As a result, wisteria is not just taking over wooded areas, it is actively killing trees. Within just a few seasons vines will start to smother a tree, blocking sun and stealing water and nutrients. In particularly malevolent cases, a powerful vine will coil tightly up the trunk, cutting into the bark and killing the tree.
This is not just an ecological threat. There have been numerous instances of weakened or dead trees falling and blocking roads and paths or tearing down power lines, causing blackouts and creating electrocution hazards to people and animals. Removal of fallen trees is complex and expensive.
The specific invasives are Chinese wisteria (W. sinensis), and Japanese wisteria (W. floribunda), but that is a distinction without a difference. A 2007 study of the invasives spreading around Charleston, South Carolina and Tallahassee, Florida found that 82% of the vines in those areas were hybrids of the two Asian species.
There is also an indigenous species, (W. frutescens). All are woody vines that climb quickly and bear large fragrant bunches of lavender flowers. Since the middle 1800s, gardeners in the U.S. have favored the Asian varieties because the flowers bloom before the leaves sprout, making for a more dramatic display.
In fits and starts over the last few years, and with growing urgency this year, homeowners have been collaborating to remove wisteria from their yards and from surrounding public spaces, especially along Battle and Bolin creeks in Chapel Hill. Community volunteers have accelerated their efforts to remove wisteria, because nothing else will (see guide, below).
“Rarely have I seen any herbivory on it,” said Johnny Randall, director of conservation at the North Carolina Botanical Garden in Chapel Hill. “That is one of the main reasons that invasives are such a problem. We see that over and over. Not even the deer eat them. There does not seem to be any significant level of predation by mammals, insects or pathogens.”
Alarmingly, some gardening guides and websites actually tout wisteria because deer won’t eat it.
That is, in part, because wisteria is toxic. According to National Capital Poison Control, “wisteria seed pods and seeds are considered the most toxic parts of the plant, but all parts contain the harmful chemicals lectin and wisterin, which can cause a burning sensation in the mouth, stomach pain, vomiting and diarrhea if swallowed. These symptoms can last for up to two days.”
Another diabolical aspect of wisteria is its quick reaction to being attacked. To the despair of volunteers who have dedicated long and sweaty weekends to whacking the wicked weed, cutting the big trunk vines as is recommended stimulates the root system to sprout elsewhere. Like a real-life horror-movie monster, wisteria won’t stay dead.
Randall explained that “root sprouting if the main vine is cut is a common phenomenon in plants. The growth regulators in the plant respond to the damage by saying, in effect, ‘let’s get growing!’”
The root system is really the heart of most plants. What humans see above ground — the majestic trunk and crown of a tree, the sprawl of wisteria vines and leaves or a little mushroom — is mostly just the plants’ organs for making food or reproducing.
Wisteria does both prodigiously. “Once it establishes a fecundating root system, wisteria can really take off,” said Randall. “There is momentum in the rate of growth. Wisteria mostly relies on vegetative propagation, either stems running along the surface, called stolons or runners, or just under the surface, called rhizomes.”
Wisteria seeds occur in hanging pods. They are disks about half an inch in diameter, looking a bit like dull pennies or flattish coffee beans. They are too big to be spread by the wind, but travel well down water courses. That explains how the vines get established along creeks, growing low and shrubby for years unnoticed until that tipping point is reached. Then the runners and rhizomes invade nearby woods and gardens.
The North Carolina Botanical Garden is trying to reverse that process in its nature preserves. “When I see how much work it is to remove wisteria, I lament over how people think it’s so pretty,” Randall said. “Yes, but it’s not worth the pain. People like the flowers, but the week or two of pretty flowers is not worth the hell it causes the rest of the year.”
That hell looks like “a wall of wisteria, a wisteria desert,” said Sarah McGill, a physician and volunteer organizer in Chapel Hill. Her Road-to-Damascus moment was “several years ago I noticed some vines coming into my yard and wrapping around the trees. One vine was already bending one of the small trees, starting to kill it. The vines were running along the ground, splitting, sending up shoots, and diving back into the ground. Then I saw that the Battle Trail was covered in wisteria, it was hard to see anything else.”
McGill got her family to help, “but we realized we needed to mobilize our neighbors, even the whole community,” she said. “Homeowners put in this stuff because it looks nice, not knowing how harmful it is. It seems pretty for a little while, but the trees are dying. After the trees are gone it will just be a wisteria desert.”
The North Carolina Forest Service agrees. In an advisory ‘leaflet’ for the public it states frankly: “As enticing as it is to plant exotic wisteria, it would be best for the environment if these plants were removed from commercial trade altogether. Our native W. frutescens is every bit as stunning in the spring with no detrimental environmental effects. Unfortunately, to date it has been much more difficult to find in commercial trade.”
An infestation of any appreciable size will usually require a combination of cut stump and foliar herbicide treatments, the Forest Service states. “Where vines have grown into the tree canopy, cut each stem as close to the ground as possible, cut again a little higher up and remove the cut pieces. Treat the freshly cut surface of the rooted stem with a 50% solution of triclopyr.
“Groundcovers of exotic wisteria can be treated with a foliar solution of 2% triclopyr plus a 0.5% non-ionic surfactant to thoroughly wet all of the leaves. Exotic wisteria is best treated from the spring to fall when air temperatures are above 65 degrees Fahrenheit. Repeated treatments will likely be necessary for complete control,” the Forest Service recommends.
The North Carolina Department of Agriculture and Consumer Services (NCDA&CS) has adopted the federal Noxious Weed list, but “wisteria is not listed due to its wide distribution in the U.S.” said Phil Wilson, director of the Plant Industry Division of NCDA&CS. “Regulatory measures would not be effective. With such wide distribution, the education of the public about the invasive qualities of the pest is important to help them with control measures and the eradication on their properties.”
Last Roundup for Glyphosate
A 2010 North Carolina Forest Service bulletin about wisteria listed glyphosate in addition to triclopyr as a foliar spray, but not as a stump treatment. Since then, glyphosate, the active ingredient in Roundup, has been linked to human health problems. Recent studies have also linked it to colony-collapse disorder in honeybees because it kills bees’ natural gut bacteria, making them more susceptible to disease. Glyphosate also kills more types of plants than does triclopyr, thus posing risk of harm to benign plants when sprayed to kill wisteria. After losing several class-action suits over Roundup, the herbicide’s maker, Bayer, announced early in August that it would remove glyphosate from the product’s formulation by 2023.
Guide to Whacking Wisteria
Patience and Fortitude are not just the names of the lions in front of the New York Public Library, they are the essential weapons in fighting entrenched wisteria. As reported, the vast root system will keep throwing up new shoots. It is a battle of attrition, so plan accordingly:
1) Protect Yourself, Feet First
This is copperhead country. Snakes can hardly be blamed for defending themselves if people go tromping through their home. Wear sturdy hiking boots or wellingtons, and thick socks. Sweep the ground cover ahead with a stick or long tool. Be extra careful stepping over logs or rocks. Don’t reach into thickets. Snakes are generally not aggressive unless they feel cornered or attacked.
2) Head to Toe
Unlike poison ivy, wisteria is only toxic by ingestion. That said, poison ivy is often intertwined with wisteria, as can be vines with thorns. Twigs, dirt, and occasionally insects will fall out of overhead leaves, especially as the vines are pulled and cut. Wear a broad-brimmed hat, long sleeves, and sturdy garden or work gloves. If it is too hot for long sleeves, make a pair of forearm covers by cutting toe and heel out of old socks and wearing them under the gloves.
3) Bring and drink lots of water
4) Arm Yourself. The basic kit is:
5) Get to Work: triage
Step 1: Critical vines are the big old ones spiraling and strangling tree trunks. Saw through as close to the ground as possible, being careful not to cut into the bark of the tree. Then saw again at shoulder height and unwind the vine. That creates the essential air gap to prevent new shoots from climbing dead vines.
Step 2: Cut as many vines as possible with the clippers and hori hori knife at ground level, then pull down and cut as much of the hanging stems as can be done can by hand, working about eye level. What has been cut can be left on the ground. Collect any seed pods, seal them in double plastic bags, then throw them in the garbage.
Step 3: Once the bulk of the wisteria leaves and vines have been removed, cut remaining thicker vines with loppers or saws if necessary. Cut these at shoulder height first for best tension in the vine, then cut them cleanly off at ground level for the air gap.
Finally, always be alert to falling debris and to where you are stepping; it is easy to trip over roots and vines. Keep tools in a clear space away from the immediate work area. Many a wisteria warrior has put down clippers to pick up a saw and not been able to find the clippers again.