#girdling

The plant that I girdle is Ligustrum lucidum (common name: glossy privet).  It’s a shrubby tree that grows about 20-30ft tall here, and creates monoculture stands (read: it’s the only plant in the area).  Bugs don’t recognize it as food, and it tends to soak up available groundwater in such quantity that it kills our native trees during droughts due to additional stress. Where I live it’s considered invasive, though due to deregulation many types of ligustrum are used in landscaping.  Ligustrum originates from china, japan, and korea, and in these areas, it’s a fairly important plant (frequently used in nonwestern medicine, but it’s also been documented as a poison and an allergen, so….don’t try at home?).

Riparian areas–especially disturbed riparian areas–is where most escaped ligustrum is found. 

Ligustrum is notoriously difficult to remove. It’s got really large energy reserves in its root system, so if it’s cut down, the tree will re-sprout rapidly. There are ways to chemically poison it, but over vast areas that’s dangerous and very expensive.  Usually, the best method is to prevent it from entering the ecosystem in the first place– once it’s there, best efforts are really about controlling its spread rather than completely eliminating it.  I do think part of the issue is that it’s commonly used in landscaping, so there’s endless ‘stock’ revitalizing escaped populations.  Also, ligustrum leads the way for other invasive species to colonize more land, as it is extremely quick to mature and reproduce, thus choking out other native vegetation.  It’s a bit brutal to see. Anyway, it’s everywhere once you know how to spot it. 

One drop of good news: the seeds that enter soil tend to rot within the year if they don’t sprout– though, of course, individual trees may produce up to three million viable seeds. One thing they do is feed cedar waxwings, which I can’t really be upset by. However, outside of their original ranges, they don’t do much to support the new communities they’re in, and actively kill legacy trees.

Now, to your questions:

could you tell me a little more about your girdling method for ligustrum?

Sure! Very simple and satisfying. I’ve also included a video link– similar method to mine.

The goal of girdling is to disrupt energy flow from the leaves of the tree to the roots, causing the trees to starve and die. It usually take 6 months to 1 year to fully kill the tree, though I’ve seen the effects (leaf wilt, drooping) as early as 2 weeks after girdling.

Tools needed:

Paint scraper

Isopropyl alcohol in spray bottle, or soapy water

Rough sponge that’s fairly abrasive

Hedge clippers

Steps: 

Identify the tree. Look for glossy, dark green leaves with grey-green bark.  The edges of the leaves will light up when you hold them up against the sun.  The edges of the leaves are also smooth, not serrated, and they are opposite each other on the stem. Ligustrum also tends to have 2-8 mini “trunks” that support its branches. When flowering, its white, perfume-y, and the flower branches are sort of shaped like a triangle or a cone.

Their seeds are drupes, blue and similar to seeds from some species of juniper. Please don't eat them.

Clean the tree up– frequently there’s lots of growth around the base, with a bunch of tiny little branches that should be trimmed away. The hedge clippers are handy here.

Remove the outer layer of bark from one of the trunks with the paint scraper, all the way around. It should come up easily in the spring, but will resist strongly during the summer and winter.  You want to make sure that there’s at least a 5-6 inch gap between the top and bottom edges of bark. I usually go between 1 and four feet of space, when the bark is easy to peel up. This layer is about an eighth to a quarter of an inch thick, depending on the age of the tree.

Repeat this step to all of the trunks present.  Make sure that you remove the bark from all the way around each branch. This prevents it from scabbing over.

You’ll notice that the sapwood is slimy to the touch where the bark has been peeled away. This is a tissue layer responsible for generating more bark, and it must be scraped off, or else the trunk will scab over and recover.  Use the paint scraper to get rid of all of this material in a ring around the trunk, about 5-6 inches wide.  If you miss any, it will start drying a dark brown color on top of the lighter sapwood, and is easy to spot.

Make sure to remove all leaves and non-girdled branches that are below the cut / where the ring of bark was removed. This prevents sugars from being created by leaves that are still connected to the roots.

Finally, take your spray bottle of isopropyl and spritz each girdled band all the way around, and scrub it in with the abrasive sponge.

Congrats! It’s been girdled!

The tree will likely try to sprout new growth underneath the girdle bands over the next year. If you return and remove them before the stems become woody, the tree will die.

what age group is the ligustrum in when you treat ti that way and at what density in the stand?

I’ve girdled trees that are fully mature (30ft) as well as smaller saplings that are still part of the understory (8-10 ft).  Typically, I prioritize removing ligustrum within 20 ft of our legacy trees (oaks, pecans, hackberry that range between 50-200 yrs old). The project area’s pretty large, and has stands of trees with various percent ligustrum, probably between 10% ligustrum at the least affected areas and 85-95% ligustrum in most affected areas.  Most of my work has focused on areas with less than 50% ligustrum monoculture.  I’ve noticed that smaller trees tend to die faster when girdled, while more mature trees need to be checked frequently to prevent regrowth. This is probably due to large amounts of stored energy in the roots. 

 A lot of follow up is needed for these plants– about half of the ligustrum I’ve girdled so far has actually died, while the other half (while severely injured and on the way out) have needed new growth removed from their bases at least once, if not twice so far.  It’s a highly intensive process, and probably isn’t suited for large scale restoration projects, unless endless time and money is available. I just happen to be the right flavor of nuts to do this in my free time. You’ll need to check girdled trees at least once every three months for a year or so.

One thing that kills ligustrum pretty effectively are temperatures below 15 degrees F for a week or more, though larger trees will recover from the damage. Those kinds of temperatures clear out small 4-5 ft trees decently, and guarantee death for the 3ft bushes. If you figure out a way to selectively control the weather, let me know.

For trees below 4 feet, I use a tree puller to uproot them. It’s more efficient than girdling.  Also, snails really like climbing up ligustrum when it rains, so watch out not to smash them!

Thanks for the questions, feel free to reach out for more info!