By: Julianne Mason, Ecological Management Supervisor, Forest Preserve District of Will County, IL

If you haven’t read Kaleb Baker’s recent post about his study that showed the effectiveness of basal bark treating amur honeysuckle, an invasive shrub, please do so now.  In the comment section of the post, one person noted that their previous basal bark herbicide treatments had been ineffective that had used diesel fuel as the oil carrier. The commenter wondered if the type of oil carrier used in the basal bark application may make a difference.  I came across my own case study recently suggesting that the type of oil carrier may make a huge difference in effectiveness of basal bark treatments.

As background, my agency (Forest Preserve District of Will County, Illinois) has been using basal bark treatments extensively to combat invasive shrubs for the past 8 years or so.  Our crew, contactors, and volunteers have done basal bark treatments to invasive shrubs covering over 10,000 acres during that time.  Our most common invasive shrubs have been bush honeysuckle (Lonicera tatarica, L. maackii, L. x bella), common buckthorn (Rhamnus cathartica), glossy buckthorn (Frangula alnus), multiflora rose (Rosa multiflora), and autumn olive (Elaeagnus umbellifera), but we target around 25 different species of invasive shrubs.   We typically use 15-20% triclopyr ester (e.g., Garlon 4) herbicide in bark oil. My general assessment of our treatments is that Kaleb’s reported effectiveness of >95% mortality of honeysuckles from basal bark treatments has been true for us across all invasive shrub species.  It has been consistently excellent, especially for non-rhizomatous species.  However, the shrub must receive a 360-degree application for almost all species, except multiflora rose and Japanese barberry.  For those two thorny invasives, all the canes must be sprayed at the base, but (thankfully) a 360-degree application is not necessary.

Proper 360-degree basal bark application (left) – this shrub will die. Missed individuals and ones only partly treated (right) with a basal bark application will not die.

Of note, it often takes bigger shrubs a long time to die from a basal bark treatment, especially big honeysuckles, autumn olive, and buckthorns.  They often leaf out and look perfectly happy the spring following a treatment, only to slowly succumb to death one year or so after treatment.  I generally find that by 12-18 months or so after treatment, most everything will have died if it had been properly treated.  Also of note, like Kaleb’s study, we generally use backpack sprayers to do the application.  I have not compared the effectiveness of basal bark applications done using paint rollers or brushes.

This past week, I came across a notable exception to our consistently effective basal bark treatments.  There are two hedgerows at a preserve near Braidwood, Illinois that were basal bark treated during the fall of 2022.  The treatment had been done thoroughly and well (i.e., proper 360 degrees).  The hedgerows were primarily composed of large honeysuckles, although there was a smattering of multiflora rose, autumn olive and other invasive shrub species too.  The honeysuckles and other invasive shrubs leafed out during the spring of 2023, which was not surprising or unusual following a fall basal bark treatment.  However, it was surprising to me that all of the basal-bark treated shrubs are still alive now, 18 months after treatment.  The treatment was completely ineffective. 

Unusually ineffective Fall 2022 basal bark application, viewed 18 months after treatment.

When I inquired into the unusually ineffective treatments in more detail, I found out that methylated seed oil (brand: Dyne-amic) had been used as the oil carrier for the basal bark treatments, not one of our usual bark oils.  This makes me think that we should be reporting and considering the type of oil used in basal bark treatments, especially given the seemingly inconsistent and differing results among different people and organizations.  To answer the commenter’s question more precisely, I have never used diesel or kerosene as the oil carrier for basal bark treatments.  However, my experience from the ineffective basal bark treatment of large honeysuckles when methylated seed oil was used as a carrier suggests that the type of oil carrier may have a huge impact on its effectiveness.

This list is not exhaustive, but it contains the brand names of bark oil that our crew and contractors have been using with reliably effective results. 

Not Exhaustive List of Bark Oils

• Bark Oil Blue LT
  • Bark Oil Red LT
• Impel Red Oil
• JLB Oil Plus Improved
• Premier Blue bark oil

Also, if the cost of bark oil is an issue or barrier, here’s a link to my previous post about cutting the oil with water for basal bark applications.  Using an oil-water emulsion for the basal bark herbicide application has been nearly as effective for us as using straight bark oil as the carrier.  And, it uses less than a quarter of the amount of oil. Some caveats: since it is water-based, it can’t be used below freezing temps. Also, we have observed that the application may not be as rain-fast as quickly. But, it definitely uses less oil product and therefore the materials are less costly when using oil-water emulsions for the basal bark treatments. My recommendation: ditch the methylated seed oil and other weird oil carriers. Use a proper bark oil, do a proper 360-degree application, and enjoy highly effective basal bark herbicide treatments to invasive shrubs.