Agronomy 2020, 10, 129; doi:10.3390/agronomy10010129 www.mdpi.com/journal/agronomy
Flaming, Glyphosate, Hot Foam and Nonanoic Acid
for Weed Control: A Comparison
Luisa Martelloni *, Christian Frasconi, Mino Sportelli, Marco Fontanelli, Michele Raffaelli
and Andrea Peruzzi
Department of Agriculture, Food and Environment, University of Pisa, Via del Borghetto 80, 56124 Pisa, Italy;
email@example.com (C.F.); firstname.lastname@example.org (M.S.); email@example.com (M.F.);
firstname.lastname@example.org (M.R.); email@example.com (A.P.)
* Correspondence: firstname.lastname@example.org (L.M); Tel.: +39-050-2218966
Received: 18 December 2019; Accepted: 11 January 2020; Published: 15 January 2020
Abstract: Synthetic herbicides are commonly used in weed management, however, 70 years of use
has led to weed resistance and environmental concerns. These problems have led scientists to
consider alternative methods of weed management in order to reduce the inputs and impacts of
synthetic herbicides. The aim of this experiment was to test the level of weed control using four
weeding methods: glyphosate applied at an ultra-low volume, the organic herbicide nonanoic acid,
flaming, and hot foam. The results showed that weed control was effective only when flaming and
hot foam were applied (99% and 100% weed control, respectively). Nonanoic acid at a dose of 11 kg
a.i. ha−1 diluted in 400 L of water did not control developed plants of Cyperus esculentus (L.),
Convolvulus arvensis (L.) and Poa annua (L.). Glyphosate at a dose of 1080 g a.i. ha−1 (pure product)
only controlled P. annua (L.), but had no effect on C. esculentus (L.) and C. arvensis (L.). After the
aboveground tissues of weeds had died, regrowth began earlier after flaming compared to hot foam.
There was no regrowth of P. annua (L.) only after using hot foam and glyphosate. Hot foam was
generally better at damaging the meristems of the weeds. In one of the two experiment sites,
significantly more time was needed after the hot foam to recover 10% and 50% of the ground
compared to flaming. The time needed to recover 90% of the ground was on average 26–27 days for
flaming and hot foam, which is the time that is assumed to be required before repeating the
application. A total of 29 days after the treatments, weeds were smaller after flaming, glyphosate
and hot foam compared to nonanoic acid and the control, where they had more time to grow.
Keywords: alternative methods; herbicide; organic; pelargonic acid; thermal tools; ultra-low
Synthetic herbicides are commonly used in weed management, however, after 70 years of use,
this has led to weed resistance [3,4] and environmental concerns . These problems have stimulated
scientists into investigating alternatives and integrated systems of weed management to reduce the
inputs and impacts of synthetic herbicides .
One method of reducing the environmental impact of chemically synthesized herbicides is to
use ultra-low volume (ULV) spraying, i.e., rates of less than 5 L ha−1 . Rotary atomizers distribute
large quantities of small droplets efficiently, enabling a less active ingredient to be applied compared
to conventional water-based methods using emulsifiable concentrate formulations [7,8]. Glyphosate
applied in low water volumes means that lower doses of herbicides can be used on more sensitive
weed species, but it also improves the herbicide activity on weed species that are difficult to control
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Alternative methods to the use of chemical-synthesis herbicides are alternative organic
herbicides and thermal weed control [5,10,11]. Nonanoic acid (also called pelargonic acid) is a contact,
non-selective, non-translocating, post-emergence non-synthetic herbicide [10,12]. It is a fatty acid,
which kills plants by destroying the cell membranes, leading to rapid desiccation of plant tissues, and
providing non-residual weed control [10,12]. Thermal weed control involves heat being transferred
to plant material (leaves, stems, flowers, propagules, etc.) to destroy cell structures, and leads to the
denaturation of proteins [11,13,14]. Flaming is the primary heat source for weed control in agriculture
and on hard surfaces in urban areas [11,15–18]. Hot foam is an evolution of the hot water weed control
method, modified by the addition of biodegradable foaming agents, and was first patented in 1995
[14,19]. Hot foam weed control is a non-toxic technique and is applicable to numerous weed species
When a method of weed control is applied, in addition to its effectiveness, it is important to also
evaluate the weed regrowth after the above-ground tissues of weeds have died . In fact, most
thermal methods affect the above-ground portion of the plants, however, some weeds (i.e., perennial
weeds) may regrow from their below-ground components and thus a repeated application of the
thermal control is required [5,21,22].
Hot foam has been used to control weeds in cotton fields , but the application of this high-
energy demand weed control method (due to the high thermal capacity of water ) is more realistic
in urban area contexts (e.g., on pavements) . The growth of weeds on road pavements is different
from that in a field, because the characteristics of the pavements affect the weed growth (i.e., fewer
appear on frequently used roads with small joints than in infrequently used pavements with medium
or wide joints) [24,25]. In Sweden hot foam was used to control weeds along railways . Flaming
can be used successfully for controlling weeds in both agricultural and urban area context
The aim of this experiment was to test the weed control effect of different weeding methods:
glyphosate applied at an ultra-low volume, organic herbicide nonanoic acid, flaming, and hot foam.
Weed regrowth after the death of the vegetative weed tissues and weed dry biomass 29 days after
treatment application were also evaluated.
2. Material and Methods
2.1. Experimental Set up, Design and Treatment
A two-site experiment was conducted at the experimental farm of the University of Pisa (Pisa,
Italy) (43°40′33.1′′ N 10°18′41.2′′ E). The study was replicated twice at each site. The two sites (sites I
and II) differed in terms of weed population composition typology.
At site I, the major weeds was Cyperus esculentus (L.), Convolvulus arvensis (L.) and Poa annua (L.),
each accounting for 25% of the weed population. Other weeds randomly present in the field were
Anagallis arvensis (L.), Avena fatua (L.), Cirsium arvense (L.), Conyza canadensis (L.), Eleusine indica (L.),
Inula viscosa (L.), Lolium rigidum (Gaud.), Picris echioides (L.), Plantago major (L.), Silene vulgaris
(Moench), Sonchus oleraceus (L.), Stellaria media (L.), Tordylium apulum (L.), Trifolium repens (L.), and
Veronica persica (L.), with an overall total of 25% of the weed population. The majority of C. esculentus
(L.) were at the 6-tiller visible growth stage, P. annua (L.) was at the inflorescence emergence
(inflorescence fully emerged) growth stage, and C. arvensis (L.) was at the 8–9 true-leaf growth stage
. At site II, the major weed was C. esculentus (L.), which accounted for 90% of the weed population.
Other weeds randomly present in the field were A. arvensis (L.), C. canadensis (L.), C. arvensis (L.),
Erodium cicutarium (L.), Euphorbia prostrata (Aiton), P. echioides (L.), P. major (L.), S. oleraceus (L.), T.
repens (L.), and V. persica (L.), with an overall total of 10% of the weed population. C. esculentus (L.)
was at the 6-tiller visible growth stage .
Weed species and percentages of single species in the total weed population were identified
based on visual estimates. The sites were uncultivated (i.e., meadows under orchards) and the weeds
had been managed periodically with mowing before the experiments were carried out. The soil was
loam in both sites.
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Glyphosate (360.00 g L−1 of active ingredient) was applied pure (i.e., without diluting in water).
The product used was GLIFENE HP (Diachem S.p.A., Caravaggio, Italy), which contained glyphosate
as isopropylamine (IPA) salt and surfactants. The product was applied with an ultra-low volume
sprayer (MANKAR-P 30–50 Flex, Mantis ULV®, Geesthacht, Germany) (Figure 1b) at a dose of 3 L
ha−1 (i.e., 1080 g a.i. ha−1). The machine was equipped with a segment rotation atomizer, which
produces small droplets with a uniform size of about 150 μm . Flaming was applied manually
with a prototype of a back-pack flaming machine developed at the University of Pisa  (Figure 1a).
The dose was 150 kg ha−1 of liquefied petroleum gas (LPG) based on previous experiments where this
dose was found to be effective in controlling developed weeds [16,17]. The burner was 0.3 m wide
and operated at 6 cm above the ground. Hot foam was applied using a Foamstream® MW Series
(Weedingtech Ltd., London, UK) . The solution used (Foamstream V4) was a 100% blend of plant
oils and sugar (e.g., alkyl polyglucoside surfactants) . The emission class is equivalent to a Euro 5
. The machine flow rate was 0.2 L s−1 (96% water and 4% Foamstream V4) and the dose applied
was 8.33 kg m−2. The manufacturer advised that Foamstream V4 percentage in the total flow rate
could be varied between 0.5% and 5% depending on the client’s application. This dose was based on
a previous experiment where it provided the highest weed control effect and the slowest weed
regrowth . The hot foam distribution tool was 0.3 m wide and operated at 5 mm above the ground
(Figure 1c). Pure nonanoic acid (Beloukha, Novamont, Novara, Italy) was applied using a sprayer
(Acuspray, Techneat engineering ltd, Ely, Cambridgeshire, UK) (Figure 1d) at a dose of 16 L ha−1 (i.e.,
11 kg a.i. ha−1) diluted in 400 L of water.
Figure 1. Machines used in the experiments for weed control: (a) prototype back-pack flaming
machine; (b) ultra low volume sprayer (MANKAR-P 30–50 Flex, Mantis ULV®, Geesthacht, Germany)
used for glyphosate application; (c) hot foam distribution tool (Weedingtech Ltd., London, UK); (d)
sprayer (Acuspray, Techneat engineering ltd, Ely, Cambridgeshire, UK) used for nonanoic acid
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Treatments were applied on 14 May 2019 (repetition I) and on 02 July 2019 (repetition II) in both
sites. Cumulative rainfalls were 93, 4, 94 mm in May, June and July, respectively, and the average
temperatures were 15, 23, 25 °C in May, June, and July, respectively.
The experimental design was a randomized block design with four blocks. The five treatments
(control, flaming, glyphosate, hot foam, and nonanoic acid) were applied in each block for a total of
20 plots per site. Plots were 2 m long and 0.3 m wide. Plots were 0.3 m wide based on the width of
the hot foam application tool and flaming burner. A space of 2.5 m between the plots has been left in
order to avoid drift effect due to the use of the herbicides.
2.2. Data Collection
Measurements of ground covered by the total population of weeds were used to estimate weed
control (i.e., from treatments application to death of weeds above-ground tissues) and weed regrowth
(i.e., from death of weeds above-ground tissues to 27 days after the treatment application). These
measurements were estimated from digital images using IMAGING Crop Response Analyser .
The digital images, one for each plot, were taken from an area of 0.075 m2 (30 cm × 25 cm) at the center
of each plot (with the same geographical coordinates). Photographs of the weed cover for evaluating
the weed control were taken 1 day before, and 1 and 2 days after treatments. Weed cover photographs
for the evaluation of the weed regrowth were taken 3, 7, 10, 17 and 27 days after treatments. The
distance between the weeds and the camera was constant (i.e., 30 cm from the ground), and high
contrast was prevented by using an umbrella. The brightness of the digital images was equalized
before analysis. The digital image analysis was as described in Rasmussen et al. , which,
summarizing, counted the percentage of green pixels on the whole pixels of the photograph. The
green weed biomass was collected 29 days after treatment at the center of each plot (i.e., 0.075 m2
area) by cutting the weeds at ground level. Cut plants were dried at 105 °C to a constant weight. The
dry weight was then converted into g m−2.
2.3. Statistical Analysis
Data normality was assessed using the Shapiro–Wilk test. Other tests consisted of the Student’s
t-test to verify that the mean error was not significantly different from zero, the Breusch-Pagan test
for homoscedasticity, and the Durbin–Watson test for autocorrelation.
The weed control in each site was modeled in a linear mixed model using the R software 
extension package ‘lmerTest’ (tests in linear mixed effects models) . A logit transformation of
weed cover data was performed. The treatment, evaluation date and repetition of the experiment
were fixed factors. Correlated random intercepts and slopes were fitted between blocks and fixed
factors. Weed regrowth was modeled in a full model as above, however, a comparison between the
full models and the reduced models (without the logit transformation and no random factors)
resulted in p-values equal to 1 and higher Akaike information criterion (AIC) and Bayesian
information criterion (BIC) (AIC = 705.02 and BIC = 875.57 of the full model vs AIC = −303.55 and BIC
= −230.45 of the reduced model, at site I; and AIC = 533.03 and BIC = 703.58 of the full model vs AIC
= −458.79 and BIC = −385.70 of the reduced model, at site II), therefore the reduced models were used.
The weed dry biomass was modeled in a mixed model where the treatment, site, and repetition were
the fixed factors. Correlated random intercepts and slopes were fitted between blocks and fixed
factors. An analysis of variance was performed for each model. The extension package ‘ggplot2’
(elegant graphics for data analysis)  was used to plot all the graphs.
The comparisons between pairs of estimated values were computed by estimating the 95%
confidence interval of the difference between the values (Equation (1)):
(difference)=()± 1.96+ , (1)
where (x1) is the mean of the first value, (x2) is the mean of the second value, (SEx1) is the standard
error of (x1), and (SEx2) is the standard error of (x2) . If the resulting 95% confidence interval (CI) of
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the difference between values did not cross the value 0, the null hypothesis that the compared values
were not different was rejected.
3.1. Weed Control
The p-values resulting from the analysis of variance are reported in Table 1. At site I, weed
control was influenced by the treatment, evaluation date, and interaction between the two. At site II
the interaction between the treatment, evaluation date, and repetition of the experiment were also
significant. Tables 2 and 3 report the weed control least squares means and standard errors of weed
cover percentage logit transformed one day before, and one and two days after treatments at sites I
and II, respectively. The inverse transformed values and 95% confidence intervals are plotted in
Figures 2 and 3, for site I and II, respectively.
Table 1. Weed control type III analysis of variance with Satterthwaite’s method resulting from the
linear mixed model where the treatments (control, flaming, glyphosate, hot foam, and nonanoic acid),
evaluation date (one day before, and one and two days after the treatments) and repetition of the
experiment (I and II) were used as fixed factors at sites I and II, respectively. Significant p-values are
shown in italics.
Treatment: date: repetition
Table 2. Weed control least squares means and standard errors (SE) of weed cover percentage logit
transformed as affected by the different treatments, repetition of the experiment, and evaluation date
(one day before, and one and two days after treatments) at site I.
Logit [Weed Cover (%)] (±SE)
1 DBT, 1 DAT, and 2 DAT: one day before, and one and two days after the treatments, respectively.
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Figure 2. Weed control bar graph of back-transformed means (Table 2) and the 95% confidence
interval as affected by the treatments (control, flaming, glyphosate, hot foam and nonanoic acid),
repetition (I and II) and evaluation date at site I; 1 DBT, 1 DAT and 2 DAT: one day before, and one
and two days after the treatment, respectively.
Table 3. Weed control least squares means and SE of weed cover percentage logit transformed as
affected by the different treatments, repetition of the experiment, and evaluation date (one day before,
and one and two days after treatments) at site II.
logit [Weed Cover (%)] (±SE)
1 DBT, 1 DAT, and 2 DAT: one day before, and one and two days after the treatments, respectively.
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Figure 3. Weed control bar graph of back-transformed means (Table 3) and the 95% confidence
interval as affected by the treatments (control, flaming, glyphosate, hot foam and nonanoic acid),
repetition (I and II) and evaluation date at site II; 1 DBT, 1 DAT and 2 DAT: one day before, and one
and two days after the treatment, respectively.
In both sites and repetitions of the experiment, only the flaming and hot foam treatments were
able to control weeds and showed a significant reduction in weed cover one and two days after
treatments compared with one day before their application. On the other hand, after the nonanoic
acid and glyphosate applications, there was no weed cover reduction (Figures 2 and 3, Tables 2 and
In the two repetitions at site I, one and two days after the application of flaming, weed cover
was statistically similar and was on average 0.7%. Also after hot foam application, there were no
statistical differences the weed cover in the two repetitions, both one and two days after the
treatments, which on average was 0% (Figure 2, Table 2). At site II, weed cover one day after flaming
was similar in the two experiment replications (on average 0.3%), and significantly lower compared
with two days after its application in both replications, which was on average 1.3%. This thus
suggests that there was an early start of the regrowth already two days after the treatment
application. Weed cover after hot foam was similar between one and two days after the treatment
application in both replications, which was on average 0.4% (Figure 3, Table 3).
Weed cover estimated one and two days after the application of treatments was statistically
lower in plots where hot foam was applied compared with the flamed plots in both repetitions of site
I. At site II, in both repetitions, weed cover one day after the treatments was similar between flaming
and hot foam, whereas two days after treatments, weed cover after hot foam was significantly lower
than after flaming (Figures 2 and 3, Tables 2 and 3).
3.2. Weed Regrowth
At site I, the weed composition observed 27 days after the application of the treatments showed
a shift in the plots (both repetitions) where glyphosate and hot foam were applied compared to that
observed before the start of the experiment. In these plots P. annua (L.) was no longer present,
whereas it was still observed in the plots where the other treatments had been applied. At site II, an
increase in C. arvensis (L.) was observed, which represented 30% of the final weed population (27
days after treatments) in all the treated plots and the control. In both sites and repetitions, regrowth
was observed starting from the meristems, and the weed coverage was not due to a new weed
infestation from seeds.
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The p-values from the analysis of variance are reported in Table 4, and the coefficients of the
regressions in Table 5. The regression lines with all the points and 95% confidence interval bands of
percentage weed cover as affected by the treatments (control, flaming, glyphosate, hot foam and
nonanoic acid), the repetitions (I and II) and the evaluation dates (3, 7, 10, 13, 17 and 27 days after the
treatments) for both sites are shown in Figure 4. At site I, weed cover regrowth was affected by the
treatments, evaluation date, and their interaction, whereas at site II, the interaction between
treatments, evaluation date and the repetition was also significant (Table 4).
Table 4. Weed regrowth analysis of variance resulting from the linear model, where weed coverage
was affected by the treatments (control, flaming, glyphosate, hot foam, and nonanoic acid), the
evaluation date (3, 7, 10, 13, 17 and 27 days after the treatments) and the repetition of the experiment
(I and II) were used as factors. Significant p-values are shown in italics.
Treatment: date: repetition
Table 5. Multiple linear regression coefficients used to estimate the percentage weed cover per day of
interest for each treatment, repetition, and site using the linear equations. Linear regressions lines are
plotted in Figure 5.
Regression Coefficient (±SE)
Glyphosate: evaluation date
Flaming: evaluation date
Hot foam: evaluation date
Nonanoic acid: evaluation date
Glyphosate: repetition II
Flaming: repetition II
Hot foam: repetition II
Nonanoic acid: repetition II
Evaluation date: repetition II
Glyphosate: evaluation date: repetition II
Flaming: evaluation date: repetition II
Hot foam: evaluation date: repetition II
Nonanoic acid: evaluation date: repetition II
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Figure 4. Regression lines with all the points and 95% confidence interval bands of percentage weed
cover as affected by the treatments (control, flaming, glyphosate, hot foam and nonanoic acid), the
repetition (I,II) and the evaluation date (time) in the two sites. (Site I) residual standard error = 0.123;
multiple R-squared = 0.859; adjusted R-squared = 0.846. (Site II) residual standard error = 0.089;
multiple R-squared = 0.916; adjusted R-squared = 0.909.
In both sites and repetitions, except for glyphosate at Site I, the weeds grew again. Glyphosate
at site I followed a different trend, showing a slight weed cover decrease of 15% (±7%) 27 days after
the application of the treatment in repetition I, whereas in repetition II, the weed cover did not
increase and remained statistically similar for 27 days (Figure 4).
At site I, the application of treatments showed three response trends. Glyphosate followed the
trend described above. Hot foam and flaming started with a weed cover of 0% and 1%, respectively,
and reached a similar average of 90–94% (± 5%) after 27 days (i.e., regrowth). Nonanoic acid and the
control started with an average weed cover of 73–81% (± 4%) and reached 98–100% (± 5%) after 27
days. Flaming after 27 days was also similar to the control and nonanoic acid, whereas the hot foam
was significantly lower (Figure 4).
At site II, after hot foam and flaming, weed cover regrew from 0% (hot foam) and 7–11% ± 3%
(flaming). After the other two treatments, the weeds did not die and continued to grow, starting with
a weed cover percentage of 61–79% (±3%). The hot foam in repetition I showed the lowest significant
weed coverage after 27 days (71% ± 4%). In repetition I, weed cover after flaming treatment reached
88% (±4%) after 27 days. This was similar to flaming in repetition II (95% ± 4%), hot foam in repetition
II (90% ± 4%), glyphosate (89% and 90% ± 4%, respectively for Replication I and II) and the control in
repetition II (95% ± 4%). However, it was different from nonanoic acid (98% and 99% ± 4% for
repetitions I and II) and the control in repetition I (99% ± 4%), respectively. The other treatments
showed similar results to each other (Figure 4).
The time (days) estimated to reach 10%, 50% and 90% weed cover regrowth, is reported in Table
6. The time to reach 10% and 50% weed cover regrowth was only biologically significant for the
flaming and hot foam because, in the control, glyphosate and nonanoic acid plot weeds did not die
and weed cover was already above 10% and 50%, respectively (Figure 4).
In both sites and repetitions, weeds re-covered 10% of the ground in a similar average time of
four days after flaming, and in a similar average time of six days after hot foam. At site I, there were
no statistical differences between flaming and hot foam in re-covering 10% of the ground after
treatment. The times estimated after flaming at site II were, instead, statistically lower than that
estimated after hot foam treatments, which reached 10% weed cover about three days later (Table 6,
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Table 6. Estimated time (days) to reach 10%, 50% and 90% weed cover regrowth (ET10, ET50, and ET90,
respectively) as affected by the different treatments (control, flaming, glyphosate, hot foam, and
nonanoic acid), the repetition (I and II) and the evaluation date in the two sites. The linear regression
lines are plotted in Figure 4.
Estimated Time (Days) (±SE) for Weed Cover Percentage Regrowth
NA: not available (i.e., the estimation had no biological meaning).
A total of 50% weed cover regrowth was reached after flaming in a similar average time of 16
days in the two repetitions and in both sites. Also after the hot foam, there were no statistical
differences between repetitions and sites, and 50% weed cover was reached after an average time of
17 days. At site I, again, there were no statistical differences between flaming and hot foam in re-
covering 50% of the ground after treatment, whereas at site II, 50% weed cover after hot foam was
reached three days later than the time needed after flaming (Table 6, Figure 4).
A total of 90% weed cover regrowth (or natural growth where weeds were not dead) was
reached after all the treatments, except for glyphosate at site I. After an average of 18 days from the
start of the experiment, the control reached 90% of the ground covered by weeds in the two
repetitions and in both sites. The nonanoic acid plots reached it after an average time (average
between repetitions and sites which were similar) of 19 days. The control and nonanoic acid times
were statistically similar. A total amount of 90% weed cover was reached in an average time (average
between similar values of replicates and sites) of 26 days after flaming. After the hot foam, repetition
I of site II showed a significantly higher time (average of 34 days) to reach 90% weed cover regrowth
compared to repetition II of site II and repetitions I and II of site I, which showed a similar average
time of 27 days. At site II, the resulting high standard errors due to the high variability in the plots
after the glyphosate application averaged 27 days, which was similar to the times estimated for all
the other treatments to reach 90% weed cover. At site I, the time to reach 90% weed cover after flaming
was significantly higher compared with the control (+8 days) and nonanoic acid (+9 days), and similar
to that of hot foam. At site II, the time after flaming was similar to that of the control and hot foam in
repetition II, higher than nonanoic acid (+4 days) and lower (−8 days) than hot foam in repetition I.
At site I, after the hot foam, the significant time delay to reach 90% weed cover compared with the
control and nonanoic acid was 9 days and 10–11 days, in repetition I and II, respectively. At site II,
the significant delay after hot foam in repetition I compared to the control and nonanoic acid was 14
days and 13 days, respectively. On the other hand, after the hot foam in repetition II, the time was
similar to the control and higher (+6 days) than the nonanoic acid (Table 6, Figure 4).
3.3. Weed Dry Biomass
Weed dry biomass collected 29 days after treatment application was influenced by the treatment
and the interaction between treatment and site (p-values < 0.001, respectively). The other factors and
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interactions were not significant. Least squares means and 95% confidence intervals for each
treatment, repetition and site are plotted in Figure 5.
Figure 5. Weed dry biomass bar graph and 95% confidence interval as affected by the treatments
(control, flaming, glyphosate, hot foam and nonanoic acid), repetition and evaluation date at sites I
and II, respectively.
At site I, the weed dry biomass in both repetitions of the control and nonanoic acid plots were
similar and significantly higher compared with both the repetitions of glyphosate, flaming and hot
foam, whereas the dry biomass was similar (Figure 5).
At site II, the weed dry biomass in both repetitions of the control was similar to that estimated
in both repetitions of nonanoic acid and higher compared with both repetitions of flaming,
glyphosate and hot foam. Both repetitions of hot foam were significantly lower than both repetitions
of nonanoic acid, whereas both repetitions of flaming and glyphosate were significantly lower only
than repetition I of nonanoic acid. Both repetitions of glyphosate and repetition II of flaming were
similar, whereas repetition I of flaming was lower than repetition II of nonanoic acid. Weed dry
biomass in both repetitions of glyphosate was similar to those of flaming and hot foam in repetition
I, whereas glyphosate in repetition I was significantly higher than hot foam in repetition II. Weed dry
biomass in both repetitions of flaming was similar to those of hot foam.
Weed control was effective only when flaming and hot foam were applied. Hot foam was the
most effective method, leading to 100% weed control one and two days after the treatment in both
sites and replications. At site I, flaming was statistically a little less effective than hot foam, but in any
case, provided 99% of weed control. At site II, also flaming led to 100% weed control, but this effect
lasted only one day (Figures 2 and 3).
Although the effectiveness of a herbicide should increase if the droplet size is reduced (i.e., an
increase in droplet number obtained with ultra-low volume applications increases the likelihood of
impacting the weed leaf surface) , a dose of 1080 g a.i. glyphosate per ha−1 was probably not high
enough to control C. esculentus (L.) and C. arvensis (L.), whereas this dose was effective against P.
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annua (L.). This effect of glyphosate on P. annua (L.) was visible in the photographs taken 13 days
after the treatments, which showed the delayed death of this weed species. In Figure 5, the decrease
in the total weed cover in repetition I of glyphosate was due to the death of P. annua (L.). This decrease
was not significant in repetition II (where the weed cover was similar to that three days after
treatment). This was probably because the simultaneous growth of C. esculentus (L.) and C. arvensis
(L.) minimized the reduction in the total weed population coverage due to the death of P. annua (L.).
Because only P. annua (L.) died, and the total weed population coverage was never lower that an
average of 50%, the weed control due to the use of glyphosate cannot be considered effective in this
Nonanoic acid was not effective in controlling weeds probably because the species in these
experiments were too developed for the herbicide to have an effect. Previous research reported that
nonanoic acid needs to be applied to very young or small plants for acceptable weed control , and
repeated applications are suggested . Rowley et al.  obtained a moderate reduction in weed
coverage, density, and dry biomass compared to the untreated control, but the dose of nonanoic acid
used (39 L a.i. ha−1) was above that indicated on the product label. Other authors  found a
reduction in Microstegium vimineum (Trin.) coverage compared to the untreated control when
pelargonic acid was applied at 11.8 kg a.i. ha−1, 5% volume.
The regrowth of weeds after the death of the aboveground vegetative tissues is an important
indicator to validate the effectiveness of a weed control technique. In fact, it determines how many
times a weeding method needs to be applied during a weed management program. Given that a
technique should kill the weeds after being applied, the time weeds take to regrow and cover the
ground again is an indicator of how many times the technique needs to be repeated in the annual
management of weeds. This management depends on whether the weeds grow in urban areas or
agricultural fields, with crops that may vary in sensitivity to competition from weeds.
Weed regrowth started earlier after flaming than after hot foam, in fact, at site II, just two days
after the flaming application, the weed cover was higher than one day after. Three days after flaming,
the weed cover estimated at site II was already 7–11% (±3%), whereas in the hot foam plots, the weed
cover was still 0%. At site I, 27 days after treatments, the weed cover after hot foam was still
significantly lower than the control and nonanoic acid. However, in the flaming plots, the weed cover
was similar to hot foam, but also to the control and nonanoic acid, thus suggesting greater damage
of the hot foam to the weeds’ meristems. This was more evident in the repetition I at site II, where
the weed cover after 27 days from the hot foam application was still significantly lower than the other
treatments (Figure 4). At site II, the delay of time needed after hot foam to recover 10% and 50% of
the ground compared to flaming was also significant, and this delay was still significant in repetition
I to re-cover 90% (Table 6). P. annua (L.) did not regrow after hot foam, suggesting that the meristems
of this species were severely damaged, which flaming did not achieve.
The time needed to recover 90% of the ground was on average 26–27 days for flaming and hot
foam. The time of 34 days was estimated only for hot foam in repetition I of site II (Table 6). A time
of 26–27 days was estimated to be the time after which a new weed control application was needed
for a real infested field during high weed season (i.e., May, June, July in Italy). For glyphosate and
nonanoic acid, the time needed to reach 90% weed coverage of the ground was less relevant because
in these plots there was no weed control. The dose of 1080 g glyphosate per ha−1 had no effect on the
growth of C. esculentus (L.) and C. arvensis (L.), which continued their natural growth, whereas P.
annua (L.) died and was not able to regrow. In the nonanoic acid and control plots, the growth
observed naturally occurred in 27 days (i.e., weeds did not die).
Twenty-nine days after the treatment application, the weed dry biomass was similar when
flaming and hot foam were applied. This suggests that the weed cover in repetition I at site II was
lower, but was made up of larger weeds. Also the lowest weed coverage after glyphosate at site I was
made up of the largest weeds, in fact, the weed dry biomass was similar to flaming and hot foam. At
site I, in the control and nonanoic acid plots, weed dry biomass was always higher than flaming,
glyphosate and hot foam, suggesting that during their growth these weeds, in addition to expanding
laterally, had time to grow in size. At site II, the differences in weed dry biomass were less marked
Agronomy 2020, 10, 129 13 of 15
than at site I, suggesting a more homogeneous weed growth, but in any case the control had more
time to grow in size.
Weed control was effective only when flaming and hot foam were applied, providing
respectively 99% and 100% of weed control two days after the treatments. Nonanoic acid at a dose of
11 kg a.i. ha−1 diluted in 5 L of water was not effective at controlling the developed plants of C.
esculentus (L.), C. arvensis (L.) and P. annua (L.). Glyphosate at a dose of 1080 g a.i. ha−1 without water
dilution only controlled P. annua (L.), but had no effect on C, esculentus (L.) and C. arvensis (L.).
Flaming and hot foam controlled these three major species of the weed population effectively
together with the other weeds that were observed in the field experiments.
Weed regrowth started sooner after flaming that after hot foam. P. annua (L.) did not regrow
only after the hot foam and glyphosate application, and there was generally more damage to the
weeds’ meristems after hot foam. At site II, a significant time delay was needed after hot foam to
recover 10% and 50% of the ground compared to flaming. The time needed to recover 90% of the
ground was on average 26–27 days for flaming and hot foam. This time of 26–27 days was estimated
to be the time after which a new weed control application was needed after flaming and hot foam for
a real infested field during the high weed growth season (e.g., May, June, July in Italy). After 29 days
from treatment application, weeds were smaller in size when flaming, glyphosate and hot foam were
applied compared with nonanoic acid and the control. From a practical standpoint, hot foam and
flaming applications could be repeated once a month in spring and beginning of summer, and less
frequently when the weeds growth is slower. Flaming can also be used to control weeds after the
emergence/transplant of heat-tolerant crops, whereas hot foam is recommended applied in bands of
soil before high-income crop transplant and/or for controlling weeds under vineyard rows, in order
to reduce heat production costs compared to the application of the whole ground surface.
Author Contributions: conceptualization, L.M., C.F., M.S., M.F., M.R. and A.P.; methodology, L.M., C.F., M.S.;
validation, L.M., C.F., M.S., M.F., M.R. and A.P.; investigation, L.M., C.F., M.S., M.F., M.R. and A.P.; resources,
L.M., C.F., M.S., M.F., M.R. and A.P.; data collection, L.M., C.F., and M.S.; data analysis, L.M.; writing—original
draft preparation, L.M.; writing—review and editing, L.M.; visualization, L.M., C.F., M.S., M.F., M.R. and A.P.;
supervision, L.M., C.F., M.S., M.F., M.R. and A.P.; project administration, L.M., C.F., M.F., and A.P. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: This study was self-financed by the Department of Agriculture, Food and Environment of
the University of Pisa (Pisa, Italy). The authors would like to thank Cosmin User, Franck Balducchi and Edward
Cutler from Weedingtech Ltd. who provided the hot foam machine and technical support; Lorenzo Greci and
Romano Zurrida from the Department of Agriculture, Food and Environment of University of Pisa for their
Conflicts of Interest: The authors declare no conflict of interest.
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