Comparison of Invasive Shrub Honeysuckle Eradication Tactics for Amateurs: Stump Treatment versus Regrowth Spraying of Lonicera maackii

R E S E A R C H A R T I C L E

Comparison of Invasive Shrub HoneysuckleEradication Tactics for Amateurs: Stump Treatmentversus Regrowth Spraying of Lonicera maackiiKurt E. Schulz,1,2 Jessica Wright,1 and Sabrina Ashbaker1

Abstract

Elimination of Asiatic shrub honeysuckles (Lonicera spp.)from preserves and conservation areas in eastern NorthAmerica is difficult because bird dispersal reintroducesseeds from shrubs in the neighborhood. To reduce thisproblem, honeysuckle control must be instituted on abroad scale and involve public participation. Many tech-niques for honeysuckle control are beyond the capabilitiesand inclinations of volunteers and local landowners. Ina replicated study, we evaluated two suitable techniquesand applied them in spring, early summer, late summer,fall, and winter 2009. These were stem cutting followed bypainting with 18% glyphosate, and stem cutting followedby spraying of regrown shoots with 1% glyphosate about40 days later. We regarded the spraying of regrown shoots

as more practical for neophytes. Overall, cutting followedby stump treatment is more effective, killing 75–85% ofindividuals in spring and early summer, and >90% laterin the year. Cutting and spraying regrowth was most effec-tive in spring (56% killed), and poorer thereafter (20–40%killed). The result for spring was much lower than previ-ously observed. Death rates for the cutting and regrowthspraying treatment were not affected by shrub size, butthe amount of regrowth after spraying responded stronglyto size. Cutting and regrowth spraying may be suitablein situations where reducing the competitive effects andreproduction of individuals is sufficient, or the resourcesto treat stumps with concentrated glyphosate are limited.

Key words: Asiatic honeysuckle, forest restoration,glyphosate, invasive species, Lonicera x bella.

Introduction

Not too long ago it was perceived that invasive plants could bemanaged by exterminating them from the places they were notwanted. Experience has taught us that extermination is only anelement of a long-term management strategy. Invasions haveimportant causes rooted in interactions between the invaders’biology, landscape configuration, landscape history, and dis-perser populations (Dietz & Edwards 2006). For some species,extermination is only temporary because the circumstancesthat allowed invasion usually remain, guaranteeing recoloniza-tion (TNC 2005; Love & Anderson 2009; Booth et al. 2010;Loeb et al. 2010; Kettenring & Adams 2011).

For Asiatic shrub honeysuckles (Lonicera spp.) in east-ern North America, a fundamental link in the cycle of col-onization—extermination—recolonization is the broad distri-bution of seed producers throughout the landscape. To breakthe link, control activities must extend beyond local naturecenters, parks, and preserves to managing invasives in res-idential lots and ruderal areas which serve as refuges and

1 Department of Biological Sciences, Southern Illinois University Edwardsville,Edwardsville, IL 62026-1651, U.S.A.2 Address correspondence to K. E. Schulz, email [email protected]

© 2012 Society for Ecological Restorationdoi: 10.1111/j.1526-100X.2012.00866.x

dispersal corridors. Appropriate extermination techniques forgeneral use by the public need to be identified and broadlyimplemented.

Techniques to kill invasive plants face obvious tradeoffsbetween efficacy, ecological damage, cost, worker hazards,and regulatory requirements. Techniques that involve licensing(certain herbicides), permits (fire), specialized equipment (e.g.“honeysuckle poppers” [MHHP 2011] and herbicide lances[Hartman & McCarthy 2004]), or unusual materials (concen-trated herbicides) are inappropriate for private individuals ornaïve volunteers. For professionals hoping to foster programsagainst invasives the problem is further complicated by thedearth of rigorous studies comparing simple techniques. Manypractitioners rely on experience, word-of-mouth information,“gray literature” case studies, and agency publications. Thisstudy was conducted to compare two extermination techniquesfor Asiatic shrub honeysuckles and test seemingly undocu-mented claims concerning one technique.

Four taxa of Asiatic shrub honeysuckle occur in east-ern North America. Amur honeysuckle (Lonicera maackii )appears to dominate in a broad band along the Ohio River.The “bella” hybrid complex (L. x bella and its parentalspecies L. morrowii and L. tatarica) (Barnes & Cottam 1974)is seemingly more common northward toward the Great Lakes(Schulz, author’s personal observation). It is clear that shrubhoneysuckles cause undesired ecological changes: preemption

Restoration Ecology 1

Honeysuckle Eradication Tactics for Amateurs

of forest edges and light gaps, competitive exclusion of for-est herbs and tree seedlings (Woods 1993, Gould & Gorchov2000; Collier et al. 2002), greater predation on tree seedsunder shrubs (Meiners 2007), reductions in the growth andfecundity of understory herbs (Miller & Gorchov 2004), degra-dation of wildlife habitat (Ingold & Craycraft 1983; White& Stiles 1992; Schmidt & Whelan 1999), and creation ofshrubby understory habitat benefitting parasites of mammals(Allen et al. 2010).

Various herbicides, mechanical techniques, and fire havebeen employed against honeysuckle with mixed success(Batcher & Stiles 2000). The most widespread and seem-ingly benign approach was first reported by Kline (1981),who cut tartarian honeysuckle (L. tatarica) stems at the baseand applied 17% glyphosate to the stumps, killing 89% of265 individuals in a southern Wisconsin oak savanna restora-tion. McDonnell et al. (2005) killed 100% of 34 L. maackii insoutheastern Illinois using the same method. Subsequent liter-ature has endorsed this technique, emphasizing the importanceof painting stumps immediately after cutting (MCC 2007;IPAW 2011). It is now applied to all taxa of invasive shrubhoneysuckle. On the basis of the frequency with which thetechnique is mentioned, it is apparently reliable, although avery limited number of published studies actually cite rates ofdeath as opposed to cover reduction or other metrics. Someworkers have suggested it is less effective as plants are leafingout than at other times of the year (MCC 2007), but to ourknowledge no comparative mortality data have been published.

Schulz et al. (2009) suggested a different approach whichcoordinated cutting with glyphosate spraying of regrownshoots. Shrubs are cut at the base just as spring foliagedevelopment is completed, shoots are permitted to regrowfor ca. 40 days, and the new shoots are sprayed with ca.1% glyphosate. The rationale behind this is that spring shootgrowth is largely supported by stored carbohydrates. Cuttingremoves shoots (invested carbohydrates) and remaining storedcarbohydrates in stems before they can be replaced (Richberg2005; Love & Anderson 2009). This weakens the shrub andlimits the quantity of regrowth. Regrowth provides a metabol-ically active, compact target treatable with a smaller absolutequantity of herbicide. Moreover, the herbicide involved is eas-ily available to amateurs premixed in a pump bottle. In threestudies (two in Schulz et al. 2009 and one in Schulz & Choud-hury 2009) the rate of death using this technique was slightlylower, but not statistically different from Kline’s (1981)result.

This study was designed to compare Kline’s (1981) “cutand paint” (CP) approach with Schulz et al.’s (2009) “cut,regrow, and spray” (CRS) approach applied to L. maackii.Our objectives were the following: (1) Compare the techniquesunder identical conditions rather than relying on limitedpublished data; (2) Evaluate the CRS technique in other timesof the year. The CRS technique is often applied on an ad hocbasis when honeysuckles are cut, left untreated, and regrowthis sprayed with whatever herbicide is at hand; (3) Provideestimates of the efficiency of the traditional CP approach atdifferent times of the year. We hypothesized that the CP and

CRS techniques would be comparable in spring and that theCRS technique would be less effective in summer and fallwhen shrubs had time to recover carbohydrates invested inspring growth.

Methods

Study Site

The study was conducted in poor quality 50-year-old sec-ondary forest stands on the on the Southern Illinois Univer-sity Edwardsville Campus (Madison County, IL, −90.000 W,38.800 N), about 30 km northeast of St. Louis, MO. Regionalclimate features an average 209-day growing season and0.95 m rainfall equivalent per year (NWSFO 2010), of which0.55 m (58%) falls April–September. Soils are moderatelyacid Menfro Silt loam, containing ca. 22% clay in the surfacehorizons (Leeper 2004). The dominant trees are Ulmus rubra,Fraxinus pennsylvanica, Prunus serotina, Acer negundo, andLiquidambar styraciflua. Quercus spp., Carya cordiformis,Asimina triloba, and Acer saccharum occur sporadically. Tree(stem dbh ≥10 cm) basal area ranged 22.0–31.3 m2/ha; treedensity ranged 500–1000 stems/ha; sapling (stem dbh =2.5–9.9 cm) density ranged 463–1400 stems/ha. The abun-dance of U. rubra and A. negundo, two disturbance species,and the presence of several exotic species (L. styraciflua,Lonicera spp., Rosa multiflora) suggest histories of degrada-tion through human activities.

Field Methods

The experiment was conducted using a randomized completeblock design, with five different forest stands used as blocks.The canopy diameter (mean of major and minor axes) of eachshrub in the study was measured before treatment. All shrubswere of the species Lonicera maackii. Within each block afactorial design was employed, consisting of two treatmentscrossed across four dates. The first treatment, CP, consistedof cutting shrubs at 15–30 cm with loppers, then imme-diately painting the cut stumps with 20% (v/v) glyphosate(Ortho® Weed and Grass Killer, Marysville, OH, U.S.A., 41%isopropylamine salt of glyphosate. Commercially availableRoundup® herbicide [Monsanto, Inc., St. Louis, MO, U.S.A.]has been reformulated to Roundup® Plus, which containsDiquat [1,1-Ethylene 2,2-dipyridylium dibromide], a more per-sistent herbicide and less suitable for natural areas.). The sec-ond treatment, CRS, consisted of cutting shrubs as before,allowing a period of 40–45 days for regrowth, and spray-ing the regrowth per label directions with 0.9% glyphosate(v/v). Cutting treatments were applied in late spring of 2009at the end of Lonicera leaf flush (May 9–15), early summer(June 25–29), late summer (September 3–4, 8–9), and fall(October 9, 12–13, 20–21). An additional rendition of the CPtreatment was applied in December (12–14), but this couldnot be repeated in the CRS treatment because shoot regrowthcould not occur before spring. Ten individuals received treat-ment in each block × treatment × date combination. Herbicide

2 Restoration Ecology


applications were confined to dry days when no rain wasexpected for 24 h. This necessarily caused gaps of a few daysin the treatment schedule.

Treatment results were assessed in June of 2010. Shrubswere first scored dead or alive based on the presence of greenleaf or inner bark tissue. Dry shoot mass was used to assess therelative harm inflicted by treatments that did not kill shrubs.All new shoots were collected from living shrubs in June of2010, dried at 65◦C, and weighed to the nearest 0.01 g.

Statistical Analysis

Shrub canopy diameter distributions were compared acrosstreatment dates by one-way analysis of variance (ANOVA)and chi-square contingency tables (50 cm classes) to ensureno systematic size bias was present. These procedures detectdifferences in mean canopy diameter (ANOVA) and the fre-quency distributions of canopy diameters treated at each timeof year. To assess killing effectiveness across categorical inde-pendent variables, the proportion of dead shrubs in each date× block × treatment combination was calculated, and arcsinesquare root-transformed to meet the assumptions of ANOVA(Steel & Torrie 1980). The CP treatment left too few sur-vivors to evaluate the effect of size on killing effectivenessand regrowth after treatment. The effect of shrub size onkilling effectiveness for the CRS treatment was evaluated usinglogistic regression (Menard 2010), a technique that estimatesprobabilities (here, death rate) based on categorical or con-tinuously scaled predictor variables. Block, the interaction ofdate × canopy diameter, and treatment date were forced intothe equation and then canopy diameter was added, testing log-likelihood ratios using a G-test after each addition. The effectof shrub size on the mass of regrowth the following springwas evaluated using an analysis of covariance model (Steel& Torrie 1980) of the form: log10(dry weight) = constant +log10(canopy diameter) + date + date × log10(canopy diame-ter). Dry weights and canopy diameters were log-transformedafter examining the data to linearize the relationship betweenthe two variables as required by the underlying theory.

Results

The size distribution of shrubs receiving each treatment wascomparable at each treatment date (overall range 60–500 cmcanopy diameter). Mean shrub canopy diameter (log10 trans-form) did not differ in two-way ANOVA (date: F[3,405] =0.2272, p = 0.08775; treatment F[1,405] = 0.2613, p =0.6905; interaction F[1,405] = 0.3263, p = 0.8064). Hetero-geneity chi-square tests of a date × treatment × size classmodel showed no heterogeneity across dates (X2 = 12.94,df = 12, p = 0.3737) or treatments (X2 = 15.61, df = 20,p = 0.7404). High significance levels in all tests (p >> 0.05)suggest low type II error rates (i.e. that true differences in sizedistribution were not present).

Overall, the two treatments had differing death rates anddifferent seasonal patterns of effectiveness (Fig. 1). The CP

Figure 1. Mortality rates for honeysuckle treatments (CP, CRS). Valuesare least-square means, and least-square means ± SEs back-transformedfrom arcsine square root transformations.

technique produced significantly higher and more consistentdeath rates than the CRS technique throughout the year(Table 1). About 75–80% of CP shrubs died after treatment inMay and late June, and 90–100% of CP shrubs died after latertreatments (Fig. 1). Maximum death rates for CRS treatmentswere in May (56%) and August (41%), while late June andmid-October treatments were much less effective (ca. 20%).

Death rates for the CP treatment in May versus theremainder of the growing season (December excluded) didnot differ by linear contrasts (Steel & Torrie 1980) (80.9 vs.88.7%; F[1,20] = 1.8139, p = 0.1931). Contrasts comparingthe December CP treatment to the growing season revealed asignificantly higher death rate late in the year (99.6 vs. 86.8%;F[1,20] = 6.8936, p = 0.0162).

Logistic regression to assess size effects on death rates in theCRS treatment showed significant responses with the additionof the date x canopy diameter interaction (Table 2). Subse-quent addition of treatment date also showed a significanteffect (Table 2). Addition of canopy diameter to the modelincorporating blocks, the interaction, and date did not sig-nificantly improve the model (Table 2), suggesting canopydiameter of itself does not have an effect. For comparison,the same set of models was calculated using log10(canopydiameter) instead of canopy diameter directly (not shown).Both sets produced concordant hypothesis tests, but the modelusing untransformed canopy diameter was slightly better basedon the Akaike (1974) Information Criterion (AIC = 219.5 vs.220.5).

The dry mass of new canopy growth in CRS shrubsincreased significantly with canopy diameter, but showedno significant response to date or the interaction of dateand canopy diameter (Table 3). A univariate regression oflog10(dry mass) on canopy diameter shows an ascendingcurvilinear response (Fig. 2). The quantity of new growthapproximately doubles as canopy diameter increases from 100to 160 cm; it increases 11-fold over the range of 100 to300 cm canopy diameter. If the results of the original model(canopy diameter, date, and their interaction) are plotted,shrubs >200 cm treated in June recover tremendously morebiomass, while shrubs treated in May recover the least (Fig. 2).



Table 1. ANOVA of transformed (arcsine square root) mortality rates in response to date and treatment.

Source Sums of Squares df Mean Square F p

Blocks 0.5291 4 0.1323 5.46 0.0022Date 0.5730 3 0.1910 7.89 0.0006Treatment 3.8130 1 3.8130 157.46 <0.0001Date × treatment 0.5040 3 0.1680 6.94 0.0012Residual 0.6780 28 0.0242

Table 2. Logistic regression testing the effect of shrub size on probability of death after treatment by the CRS technique. LL, log-likelihood ratio; AIC,Akaike (1974) Information Criterion.

Model Reduced Model LL df Sequential LL Sequential df p AIC

Constant (b) only −121.6401Blocks + b b −118.3639 4 6.552 4 0.1615 246.7Blocks + date × size + b Blocks + b −111.7303 7 13.267 3 0.0041 239.5Blocks + interaction + date

+ bBlocks + interaction + b −97.7947 10 27.871 3 <0.0001 217.6

Blocks + interaction + date+ size + b

Blocks + interaction + date + b −97.7362 11 0.117 1 0.7323 219.5

Table 3. Analysis of covariance model testing the effect of shrub size (canopy diameter) on dry weight of next seasons shoot regrowth (log10 transform).

Source Sums of Squares df Mean Square F p

Shrub canopy diameter 7.6626 1 7.6626 26.32 <0.0001Date 1.6255 3 0.5418 1.86 0.1403Date × shrub canopy diameter 1.2435 3 0.4145 1.42 0.2396Residual 32.9022 113 0.2912

Figure 2. Regression lines predicting shoot regrowth after cut and spraytreatment as a function of initial shrub canopy diameter. Lines labeledby month illustrate effect of a possible date x canopy diameterinteraction. Simple regression generalizes across months[log10(dwt) = −0.2094 + 0.0052 × canopy diameter, r2 = 0.17,F[1,119] = 24.80, p < 0.0001]. Lines are plotted in the original units afterback-transformation.

Discussion

The CP technique was unquestionably more effective killinghoneysuckle in this experiment. Throughout the year 75–100%of CP shrubs were killed. In the best season (May), only 56%of CRS shrubs were killed, with the next best less than half this

in two other months. From August to December CP death rateswere above 90%. Although it is periodically mentioned that theCP treatment is less effective in spring (MCC 2007), we couldnot statistically support this. Nonetheless, we observed reducedmean death rates in spring and early summer (May and June),the period in which woody plant growth is typically greatest.For the CP treatment, there is significantly higher effectivenessin winter (December). Although there may be practical reasonswhy treating shrubs in winter is desirable (better visibility,absence of non-target herbaceous species, more comfortabletemperatures), CP treatment applied any time after midsummeris a worthwhile effort.

In light of previous studies, the CRS technique producedunexpectedly poor results, killing only 56% of shrubs, evenwhen applied soon after leaf flush. This outcome is not aresult of insufficient herbicide concentration. Schulz et al.(2009) observed 75 and 85% death rates in two studies usingRoundup (Monsanto Inc.) at 2× recommended concentrationfor foliar application, while Schulz and Choudhury (2009)killed a similar proportion (80%) with 1× concentration. Thecentral difference may be that this study applied Ortho Weedand Grass Killer as contrasted with Roundup used previously.Notably, the Ortho product performed consistently well whenapplied to stumps, which suggests that leaf wetting agents orother additives differ between the products. Here, we avoidedthe current form of Roundup available to home users (RoundupPlus) because it contains Diquat herbicide as well.



The CRS technique was expanded beyond early springimplementation because it is often a fallback in settingswhere information or expertise is lacking. Quite commonlyhoneysuckles are acknowledged as undesirable and cut downwithout further treatment, only to resprout in short order. Ourlogic was that if spraying replacement shoots proved effective,a simple technique would be available to parties more inclinedto purchase/use more dilute premixed glyphosate.

Because the CRS technique yielded incomplete control, itwas important to rigorously compare shrub size distributionsacross dates and treatments to make sure herbicide responseswere not biased by sampling shrubs from natural populations.Although somewhat different sized shrubs still might beselected from each site, the design controls this bias byaccounting for site-specific size differences as block effects.

As a technique to kill shrubs CRS is marginally effective,and strongly subject to seasonal effects, as indicated by thesignificant date × canopy diameter and date effects in logisticregression. With respect to killing shrubs, size has no cleareffect. This contrasts with strong size effects on the quantityof post-treatment regrowth. Shrubs produce exponentiallymore regrowth with increasing size. An interesting sidelightof this issue is suggested in Fig. 2, which illustrates theadditional effects of season and the canopy diameter by seasoninteraction. The negligible quantity of regrowth after Maytreatment indicates substantial harm to the shrubs. In contrast,delaying treatment until June results in tremendous regrowthas compared to May. While the terms in the model that depictseasonal effects are not statistically significant, further researchto address this phenomenon might be worthwhile.

Few published studies provide the mortality data neededto evaluate these treatments against other approaches. Ouroutcome for the CP technique is concordant with the findingsof Kline (1981) and McDonnell et al. (2005), who alsoused it. Rathfon and Ruble (2007) applied various herbicidecombinations to honeysuckle as aerial sprays and achievedsimilar levels of success. Unfortunately, their approach goeswell beyond what might be contemplated for preserves andparklands with the assistance of largely untrained persons.Love and Anderson (2009), comparing a variety of techniques,applied the CP treatment to Lonicera morrowii in springand fall. They observed much larger reductions in shrubcover, stem density, and shrub density in the fall treatment.It should also be noted that the efficacy of the two treatmentswith glyphosate should be evaluated across other species. Forexample, we applied the CP technique to Ailanthus altissima,a world wide invader, only to discover that invasion wasaccelerated by the production of hundreds of root sprouts. Incontrast, Schalau (2006) points out that it is effective againstAlnus, Fraxinus, Populus, Ulmus, and Eleagnus.

The main objective of this study was to compare the CP andCRS techniques as viable tools for killing Asiatic honeysuckle.As a reliable approach the CP technique is superior, providedother aspects of the situation are not problematic. If it ispossible to educate the public at large, have them obtain theproper herbicide, and engage them in the focused labor neededto treat stems properly, this technique is excellent. Absent

any particular expertise, the CRS technique can potentiallykill a large fraction of honeysuckle provided it is executed inspring. Moreover, the CRS treatment applied in spring doesdiminish regrowth (Schulz et al. 2009). Despite its flaws, CRSeffectively reduces honeysuckle canopy size and the capacityof treated shrubs to shade desirable vegetation. It is of interestwhether the CRS approach can accelerate the decline of shadedindividuals that receive repeated cutting treatments (Luken &Mattimiro 1991).

Implications for Practice

• Invasive Asiatic shrub honeysuckles typically recolonizesites cleared of honeysuckle because birds disperse seedsfrom remaining shrubs in the surrounding neighborhood.

• Reduction of the flow of seeds into a cleared sitemight be accomplished by clearing honeysuckle onsurrounding private property and ruderal areas. Involvinglocal residents in this process would be necessary andprovide additional manpower. Involving citizens in localcampaigns to remove honeysuckle depends on havingtechniques suitable for neophytes.

• In a replicated study, we compared the effectiveness ofstem cutting followed by application of 20% glyphosateto the cut surface, and stem cutting followed by a 40-day regrowth period, then spraying regrowth with 0.9%glyphosate. Applications were made in spring, early andlate summer, fall and winter.

• Stem cutting followed by stump treatment is very reliable(>90% kill, somewhat less in spring and early summer).Cutting and regrowth spraying is not as reliable. Theapproach kills many honeysuckle (56%) in early spring(after leaf flush), but is much less effective later inthe year. Larger honeysuckles tend to recover muchmore biomass after the spraying treatment than smallindividuals.

• Although stem cutting and stump treatment are preferred,cutting and spraying regrowth has the benefit of killingmany shrubs and reducing their size.

• Cutting and regrowth spraying uses implements andherbicide concentrations familiar to homeowners. Thismay result in greater citizen participation and bettercontrol.

Acknowledgments

This work was supported by Illinois Wildlife PreservationFund Grant #09-021W to K.E.S. and internship support toJ.W. from the Vice Chancellor for Administration, SouthernIllinois University Edwardsville. We thank Dr. L. A. P. Kohnfor reading an earlier draft of this manuscript.

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Comparison of Invasive Shrub Honeysuckle Eradication Tactics for Amateurs: Stump Treatment versus Regrowth Spraying of Lonicera maackii