By Kaleb Baker
Amur honeysuckle (Lonicera maackii) is an invasive shrub that flourishes along forest edges and in open woodlands. Amur honeysuckle shades out native flora with its early leaf-out and prolonged leaf retention, and when left uncontrolled, can produce a near monoculture, threatening biodiversity.
Land stewards everywhere have implemented a variety of different eradication methods, including hand pulling, cut-and-treat with herbicide, foliar-applied herbicide from backpacks or helicopters, basal bark herbicide treatments, and prescribed fire. Continuous treatments and monitoring are needed to eradicate Amur honeysuckle, making the cost, effort, time specificity (amount of nontarget damage), and applicability (when and where a method can work) important factors to consider.
This study explored how effective basal bark treatments and prescribed fire are at controlling honeysuckle, the amount of nontarget damage from those treatments, and the subsequent vegetative recovery. Basal bark and fire are commonly used control methods. The basal bark herbicide was made with 2.5 gallons of Garlon 4 Ultra mixed into 12.5 gallons of basal oil making 15 gallons of 16.67% Garlon 4 Ultra, equivalent to 10% triclopyr ester because Garlon 4 Ultra is 60.45% triclopyr. Basal bark treatments involved spraying the 10% solution of triclopyr around each plant’s base from a backpack, which was both quick and easy. The nozzle was pointed mostly downward (not horizontally), spraying with moderate pressure using a narrow cone pattern.
In this study we included 800 individually marked Amur honeysuckle (Lonicera maackii) at 5 different sites within Nachusa Grasslands and Franklin Creek State Natural Area (Ogle and Lee counties in northwest Illinois). In order to see if the season of application affected honeysuckle mortality or the extent of damage to non-target flora, five basal bark treatments were applied in fall 2017 (11/26 – 12/17), winter 2018 (1/28), early spring 2018 (3/10-3/11), late spring 2018 (5/4-5/5), and a control with no application. (Application caveat: there was no snow during any application). Prescribed fire was administered to half of each of the 5 sites in spring 2018. Honeysuckle mortality was checked in the early fall of 2018 to allow the honeysuckle time to fully die.
All basal bark applications were equally effective at killing Amur honeysuckle (statistically indifferent), regardless of treatment timing. There was an insignificant variation in how effective the treatments were with the late spring treatment only being 96.7% effective. The combined mortality rate of herbicide treatments was 98.4% across all four herbicide treatment seasons, compared to a 2.5% mortality with no basal bark treatment. Mortality was not dependent upon honeysuckle size (height: <1m to 3+m tall, number of stems: 1-12+ stems, or root collar size: 10-150+mm). Prescribed fire did not impact mortality positively or negatively.
However, if the plants had been checked earlier in the year, we would have concluded basal bark treatments were ineffective. A subset of individuals across treatments was opportunistically checked on 5/13/2018 to see if they were leafing out. Of the 317 honeysuckle checked, 75% of the treated honeysuckle showed signs of leaf out.
| Control | Fall | Winter | Early Spring | Late Spring | Grand Total | |
| Leafing Out | 64 | 43 | 46 | 47 | 54 | 254 |
| No Leaf Out | 0 | 21 | 16 | 16 | 10 | 63 |
| Grand Total | 64 | 64 | 62 | 63 | 64 | 317 |
| % Leafing Out | 100% | 67% | 74% | 75% | 84% | 80% |
A 1m2 quadrat was placed around 200 Amur honeysuckle to measure nontarget damage to the plant community in spring 2018. The “ring-of-death” equated to about a 10-inch radius on average. The size of the nontarget damage did not differ based on fire treatment, basal bark treatment season, or any measured size of honeysuckle.
Photo 3. The amount of nontarget damage (aka “ring-of-death”) on year one (2018)
The tendency to revegetate is further exemplified when looking exclusively at native species as shown in Figure 5, meaning the treated areas aren’t being recolonized just by nonnative species.
However, the communities were simplified in the area surrounding treated honeysuckle. The species richness was impacted by the herbicide treatments. Averaging across all four years, the early spring treatment had 1.4 fewer species than the control and the other herbicide treatments had 1.9-2.3 fewer species than the untreated honeysuckle (Figure 6). Though, there did not appear to be any species or group of species that were particularly susceptible.
Figure 6. Each box shows the richness of native (red) and non-native (blue) species from different herbicide treatments from 2018-2021 from 80 quadrats. The bold center bars are the median. The hinges are the middle 50% of the data, and the whiskers include data from the 1st to 3rd quartiles.
Note (mostly speculative): Some managers prefer using a paint roller instead of a backpack sprayer to minimize nontarget damage. This could limit nontarget damage but could also limit target species mortality because less herbicide is applied to the target. Other managers speculate that rainfall soon after application may increase nontarget damage preferring to not basal bark unless there will be 48 before precipitation.
Key take-aways:
- Basal bark treatments were highly effective at killing bush honeysuckle (98.4%).
- While prescribed fire was not effective at killing bush honeysuckle, it did not inhibit the basal bark treatments so fire should continue to be implemented to maintain fire-adapted communities.
- Basal bark treatments did impact the local vegetative communities (~10 inch radius on average).
- The “ring-of-death” recovers in time but is slightly simplified.
- There was some evidence that early spring basal bark treatments had slightly less nontarget damage (~0.5 higher species richness).
For managers, basal bark treatments are another tool in the toolbox with advantages and disadvantages. Below is my current mental framework for some common treatment options and considerations.
| Considerations | Cut-and-treat | Basal Bark | Foliar | Forestry Mow |
| Tool Cost | Low | High | Medium | High |
| Effort | High | Medium | Low | Low |
| Time per Area | High | Low/Medium (stem density dependent) | Low | Low |
| Specificity | High | Medium | Low | Low |
| Timing | Nearly anytime | Not summer | Not dormant season | Avoid growing season |
| Applicability | Can be used most places | Should not be used in wet or super sensitive areas | Should not be used in sensitive areas | Should not be used in wet or sensitive areas |
A peer reviewed paper detailing this study will be published this year. Below is a similar post as Kaleb’s. On the Blogsite you can search for related posts, and you can get our weekly posting sent conveniently to your email.
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Hey guys,
We’ve been using a very similar approach without any success. The one difference in our treatment is the basal oil. We’ve been using diesel fuel as the carrier. What basal oil have you been using?
Herbicide distributors sell basal oil from a couple brands, usually with a dye mixed in. I don’t recall which brand of basal oil was used. To my knowledge, the different brands are equivalent products. We’ve found red dye is more visible than blue dye which makes us less likely to miss individuals or individual stems. It can be purchased in a few sizes including 2.5 gal jugs, 15 gal barrels short filled to 12.5 gal, and 275 gal cubes.
One thing some folks do is apply basal bark in the dormant season and then the plants leave out, even flower, and the observer concludes the treatment did not work. We found you have to wait several months of growing season to see the results. Or it could be diesel does not work as a carrier. And make sure you were using Garlon 4, not Garlon 3A.
Hi Aric and Kaleb, First: great study! Second: I think Aric is on to something. I found some evidence just this week that the type of oil carrier definitely affects the effectiveness of basal bark treatments. I’ll write up a follow-up post on it this coming week. – Juli Mason
Non-target damage has been observed with basal application of triclopyr ester followed by rainfall within a couple days, and even mid-winter cut stump over snow where presumably there was some drip onto snow (manifested in what would have been completely dormant Hylodesmum). Those things have happened. Are there other factors at play (soil sandy vs. not, sap flow or not, applicator skill)? Maybe.
I would also note that at least the quadrat shown is a more opportunistic flora, so it would be expected to be resilient to a wide range of disturbances relative to a higher quality groundlayer, so it’s probably a reasonable setting for a treatment like this so long as there aren’t little, shade-surpressed members of what should be the more conservative flora and the matrix flora dying slow deaths beneath the shrub and tall herbaceous canopies. If there are, this approach would be ill-advised, but the response variables would probably show the same patterns…and the fraction of people out applying herbicide that reliably detect those things is low enough that I have come to promote greater caution across the board. Native cover, native primary production, and native richness can all increase with degradation of groundlayer vegetation and vice versa. Those variables really provide insufficient information on their own to draw any conclusions.
This is a nice study. I really like the fact that you looked at the effects of the “ring-of-death” 4 years after. I was also surprised that you got such good results from your applications in the spring. Thank you for posting this!
GREAT article!!!!!
I live in a 7 ac never forested woodland in N IL. The fringes are invaded by much Amur Honeysuckle. What method do you recommend for an average homeowner without EPA license and a sprayer/lopper/Stihl cutter?
For the average homeowner, I would recommend cut-and-treat methods to start with because it is simple, cheap, effective for many species, and the herbicide is readily available. Cut a few stems, treat the stumps with glyphosate concentrate. You don’t need a license to apply glyphosate or triclopyr ester on your own property. U of I Extension has a handy management guide where you can find more information: https://extension.illinois.edu/sites/default/files/management_of_invasive_plants_and_pests_of_illinois.pdf
Great article thanks.
Suzanne
Great work, Kaleb! The care and thought you took for your study is impressive.
I’m hope I’m not coming off as critical, this isn’t a big deal, and I may be wrong, but I think herbicide label percentages are all in weight/weight, not as weight/volume. At least according to some herbicide SDS documents I’ve read and the U of Nebraska extension #G1955. The only concentration that is not wt/wt, and is wt/vol are the pounds/gallon or gm/liter values they give usually as acid equivalent (a.e.). If I am correct, that would make your final concentration of triclopyr (a.e.) in basal oil equal to 8.04% w/v rather than 10%.
G4U a.e. is 4 lb/gal x 453.6 g/lb/3785 mL/gal x 100% ~ 47.94% w/v a.e.
A more precise value can be had by using the density of 1.11 found in the G4U SDS and back calculating from the 60.45% w/w “active ingredient” value and using the molecular weights (mwt) of triclopyr acid, 256.5 and triclopyr butoxyethyl ester mwt of 356.6. This gives a value of triclopyr a.e. 48.26% w/v. Or you can simply multiply time the label acid equivalent triclopyr 43.46% w/w x 1.11 (density) = 48.24% w/v. I assume the reason these % w/v aren’t exactly the same is due to small significant figure rounding errors.
Would anyone please correct me if I’m mistaken?
Great work, Kaleb! The care and thought you took for your study is impressive
I’m not trying to be critical, and this isn’t a big deal, and I may be wrong, but I think herbicide label percentages are all in weight/weight, not as weight/volume. At least according to some herbicide SDS documents I’ve read and the U of Nebraska extension #G1955. The only concentrations that is not wt/wt, and is wt/vol are the pounds/gallon or gm/liter values they give usually as acid equivalent (a.e.). If I am correct, that would make your final concentration of triclopyr (a.e.) in basal oil equal to 8.04% w/v rather than 10%.
G4U a.e. is 4 lb/gal x (453.6 g/lb)/(3785 mL/gal) x 100% ~ 47.94% w/v a.e.
A more precise value can be had by using the density of 1.11 found in the G4U SDS and back calculating from the 60.45% w/w “active ingredient” value and using the molecular weights (mwt) of triclopyr acid, 256.5 and triclopyr butoxyethyl ester mwt of 356.6. This gives a value of triclopyr a.e. 48.26% w/v. Or you can simply multiply time the label acid equivalent triclopyr 43.46% w/w x 1.11 (density) = 48.24% w/v.
48.24%w/v x 16.667% G4U in oil /100% = 8.04% w/v a.e. in oil.
I assume the reason these % w/v aren’t exactly the same is due to small significant figure rounding errors.
Would anyone please correct me if I’m mistaken?
Herbicide applicators don’t need to consider ai (active ingredient) or ae (acid equivalent). They can focus on the product label, which gives application rates in product volume (quarts or fluid ounces) per acre for herbaceous spraying or concentrations in volume of product per volume of oil carrier for cut stump or basal bark of woody plants. The latter can be shortened to % product in oil.
If you are communicating results to others, you can also add ai/ae concentrations, but that can cause confusion. Kaleb specified ai & Nathan is correct in his calculation for ae. Some say that ae is the best way to compare herbicides since the parent acid is what kills the plant. However, ai, which includes salts, amines or esters, can also affect herbicide effectiveness as well as surfactants, which are part of the “inert (or other) ingredients”. So effectiveness is determined by the total product ingredients & not just ai or ae. Instead of comparing herbicides based on the numbers, a better way is to establish treatment & control plots at your site, using your equipment, your plant growth stage & your application technique.
The parent acid of an herbicide is what kills the plant. The herbicide manufacturer can alter the chemical properties to form salts, amines or esters, which can provide benefits like better leaf penetration, water solubility or product stability. Active ingredient includes the weight of the parent acid as well as the salts, amines or esters. Acid equivalent is the weight of the product that can be converted back to the parent acid. Different formulations of a given herbicide can have the same ae concentration but different ai concentrations.
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