Control of common ragweed by mowing and hoeing

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Control of common ragweed by mowing and hoeing
Gerhard Karrer
Institute of Botany, Department of Integrative Biology and Biodiversity Research,
University of Natural Resources and Life Sciences, Gregor Mendel Str. 33, A-1180 Wien,
e-mail: gerhard.karrer@boku.ac.at
DOI 10.5073/jka.2016.455.23
Introduction
Common ragweed (Ambrosia artemisiifolia L.) is an annual species that depends on regular seed
production for population persistence. By producing dormant soil seed banks (Basset and Cromp-
ton 1973, Toole and Brown 1946) this weed may overcome seasons with failure of seed production.
Consequently, the only sustainable way to control common ragweed is preventing seed production
(Bohren et al. 2008c, Karrer et al. 2011). Several actions and cutting experiments focus on the reduc-
tion of pollen produced by male inorescences of ragweed (Benoit 2003). Only few aim at estimat-
ing both male and female ower regeneration (Bohren et al. 2008a, Milakovic et al. 2014b). Karrer
et al. (2011) claim to focus more on control options that minimise seed production on regenerated
shoots.
This overview on eects of mowing and hoeing is mainly based on a literature review and some
provisional ndings of the HALT experiments
Papers considering mowing as a control measure can be grouped according to their designs by
explanatory factors:
A-simple designs:
cutting height,
cutting dates (timing),
cutting frequency,
+/- competition
B-mixed designs:
plant density and frequency,
timing and frequency,
height and frequency,
herbicide application and cutting,
competition and frequency
C-complex designs:
plant density and timing and frequency,
plant density and timing and frequency and competition
plant density and timing and frequency and competition and region
frequency and timing and herbicide application
Response variables were: simple resprouting, owering of resprouts, number of male racemes on
resprouts, biomass of shoots (uncut and/or resprouts), female owers of shoots (uncut and/or re-
sprouts), phenology of shoots/owers (uncut and/or on resprouts), seed number of resprouts and
seed viability of resprouts.
Experiments were either done under controlled conditions in the greenhouse (pots) or in the eld
diering in habitat type. Some experiments were performed in variable crops, others on roadsides
(road shoulders).

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Regeneration after cutting:
Regeneration after cut is well documented for common ragweed (Basset and Crompton 1975, Bar-
bour and Meade 1981, Bohren et al. 2005, 2008a, Karrer et al. 2011, Tokarska-Guzik et al. 2011, Meiss
et al. 2008). Generally the intensity of resprouting by lateral shoots is not limited throughout the
year. Even from axils where their lateral shoots have already nished growth (spontaneously or after
being cut), accessory buds can be developed prolonging the seasonal growth period (Karrer 2007,
Karrer et al. 2011). Gebben (1965) found that the development of lateral shoots tend to be more
intense at lower densities of A. artemisiifolia stands compared to crowded stands. Such can be in-
terpreted as self-thinning process (Londsdale 1990) or suppression of lateral branches by shading
neighbours of identical growth architecture (cohorts). Basset and Crompton (1975) report vegeta-
tive regrowth of plants by 80 % one week after they were cut at 5 cm (slightly above the cotyledons)
end of May. They observed also 100 % ragweed regrowth 10 days after grain harvest with cutting
height of 20 cm. Tokarska-Guzik et al. (2011) found 50% regeneration of ragweed individuals that
were cut once in early developmental stages (max. 12 cm high) just above the cotyledonary node,
both treatments that were cut above the rst foliar leaf pair as well as those cut above the second
foliar leave showed 100% resprouting. Meiss (2010) and Meiss et al. (2008) clipped solitary individu-
als at 5 cm height every month. This resulted in seven clipping dates and a signicant reduction of
total biomass by 40% – compared to the intact control. When added dense luzerne populations as
competitors the reduction total ragweed biomass was near to 100%.
No signicant eect on the allocation of reproduction (fecundity) were found after removing the
apical meristem only (MacDonald and Kotanen 2010).
It is known that A. artemisiifolia can germinate in Europe throughout the whole vegetation period
(end of March to October; Karrer et al. 2011, Kazinczi et al. 2008a). During the early season growth
in height is low (Gebben 1965, Klein 2011) producing several short internodes with a dozen of fo-
liar leaves (Karrer et al. 2011). But starting from mid of June rapid upright growth by elongating
the youngest internodes und all newly developed internodes is regular under full light conditions
(Klein 2011, Karrer et al. 2011). Seedling cohorts that start later in the year (May to August) gener-
ally produce less basal internodes, all of them elongated for rapid owering. Growth in height stops
at about mid of September (Kazinczi et al. 2008a, Klein 2011, Gebben 1965). Up to this date height
increment of early cut specimens can be compensated by elongated lateral shoots (branches of
rst order) (Simard and Benoit 2011, Karrer 2012, unpublished). A comparison between mown and
intact plants showed no signicant dierences with respect to the biomass produced all over the
season, anyway if they were cut early or late (Simard and Benoit 2011).
Patraccini et al. (2011) documented that the survival rate (resprouting after cut) was generally very
high: plants cut two or three times showed resprouting rates between 75 and 100%. Plants that
were cut at plant height 80 cm survived by 100%, the 50 cm cut height gave also 100% and the
20 cm plants about 70% survivers. The latter were cut more often (3-4 times) as they reached the
cutline earlier. Clipping even in the 4 times version resulted only in a death rate of 25 to 33%. In
all clipping experiments by Milakovic (summarised in Karrer et al. 2011, Milakovic et al. 2014a and
2014b) death rates of uncut and cut plants was very low (0-5%) throughout spring and summer.
Only starting from mid of September mortality increased successively until October.
Beres (2004) and Kazinczi et al. (2008b) also reported a strong allocation to shoots after early cut (in
May or June) nally compensating totally the biomass loss. A later cut (in July or August) resulted in
a signicant decrease of total biomass.
Considering its summer annual life cycle, A. artemisiifolia turned out to be very vital by compensat-
ing eciently biomass loss from cutting. However, cutting per se cannot control common ragweed.
Regeneration of male owers after cutting:
Aiming at the reduction of ragweed pollen load in the air (Buttenschøn et al. 2010, Bohren et al.
2005, Delabays et al. 2005, Karrer et al. 2011), blooming of male owers must be prevented. Of

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course cutting is an option as the male racemes are produced generally at the top of the main shoot
as well as on the lateral shoots (Bassett and Crompton, 1975). Several experiments focussed on this
response variable rather than on seed production.
The clipping experiment by Patracchini et al. (2011) resulted in a partial biomass reduction of the
surviving plants but did not prevent owering. In the high-stress treatment (4 times clipping at
20cm), more than 67% of the plants survived to the last clipping and, among these, more than
97% owered. Moreover, plants that reached 80cm height and experienced 2 cuts survived at rates
between 50 to 100%, and 100% of the survivors owered. Flower initiation on regenerative lateral
shoots happens quite quickly. Plants that were cut directly above the cotyledons failed to produce
buds of male inorescences after 2 weeks after the cutting date, but plants cut above the rst or sec-
ond foliar leaf pair showed already 60-80% and 80-90 %, respectively (Tokarska-Guzik et al. 2011).
Such quick recovery from being cut was also demonstrated by Beres (2004), Bohren et al. (2008a),
Delabays et al. (2008a), Simard and Benoit (2011), Karrer et al. (2011), Karrer and Milakovic (2011)
and, Bassett and Crompton (1975).
Beres (2004) and Kazinczi et al. (2008b) found a signicant reduction of male owers by 87 % when
ragweed was cut only once mid of July or even by 90 % for plants cut three times. Milakovic et al.
(2014a) found in a glasshouse experiment 8 times smaller inorescences numbers in early Septem-
ber in plants cut mid-August (at the beginning of male owering), compared to the uncut control.
Simard and Benoit (2011) found that mowed plants produced generally less pollen per unit ino-
rescence length and increasing plant density also reduces pollen production per inorescence unit.
In total, plants cut 2 times produced 6 times less pollen than intact plants. Mowing high density
plants show 3-5 times reduced lenghts of male inorescences compared to intact single plants (low
density). In general, the anthesis was delayed by mowing by 17 days, whereas higher densities had
no eect (Simard and Benoit, 2011). They summarized that the total pollen production was reduced
by 88.7 % when plants were mown twice (May and July). This fact, together with the experiments
by Klein (2011) illustrates well that the compensatory growth of lateral shoots tends to allocate bio-
mass to shoots primarily and less to pollen production i.e. when cut early in the year. When cut, later
in the year (late July to September), they tend to allocate biomass rather to lateral shoots that bear
female owers at their lower nodes (Bohren et al. 2008a, Klein 2011, Karrer et al. 2011). Allocation of
biomass to male inorescences seems to be typical for uncut individuals in the early phase of stem
elongation and initiation of inorescences. But it makes sense that the plants allocate resources
from pollen production towards the production of female owers (ripening seeds) in late summer
and autumn as the air is already overloaded with viable pollen at that time (Jäger 2000).
Production of female owers and seeds, seed viability:
Sustainable control measures against ragweed must focus on preventing seed production (Bohren
et al. 2005). Yet, only in very few experiments this response variable was measured when testing
dierent cutting treatments.
As there is a preference of ragweed to produce female owers in the middle and lower part of the
plant (Gebben 1965) cutting near the base never can really prevent seed production by 100 %.
Traditional cutting height used to manage the road shoulders rarely goes below 5 cm. On the other
hand we know that common ragweed tends to germinate directly along the roadside rather early
not facing tall competitors (Joly et al. 2011, Simard and Benoit 2010). In such habitats the early
development of the plant is rather free from competition but not optimal with reference to relative
growth ability. Those plants show short internodes at the base of their shoots and therefore several
buds remain below the cutting height that are able to develop regenerative shoots. Milakovic et al.
(2014a) found that early cuts (until mid of July) will not reduce total seed number, probably because
the resprouts overcompensate the biomass losses from cutting and produce many axillary shoots
with female owers. In this glasshouse experiment, total seed numbers per plant were reduced by
ca. 2-4 times compared to the control in cutting regimes with a late rst cut mid-August. Field exper-
iments by Milakovic et al. (2014b) showed as well that a cut in August is essential: 3-5 times smaller

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total numbers of seeds per plant were found in plants cut in August, compared to the uncut control.
Simard and Benoit (2011) reported that number and mean seed mass decreased 3-4 times with
increasing plant density and by mowing. Mown plant seeds were 0.65 times less viable, whereas
seeds from high density plants did not dier in this respect to single plants. Thus allocation to seed
biomass (weight and number of seeds) was only reduced by mowing not by higher densities.
If cut once a year the timing is rather important. Bohren et al. (2005) and Delabays et al. (2005) ar-
gued that one cut only in the rst half of September yielded no viable seeds on the few resprouts.
In more detailed experiments from 2005 to 2007, Bohren et al. (2008a) had to revise some advices
given that the year-to-year variation in the ripening dates of seeds showed the possibility that in
years with optimal climatic conditions ragweed already can produce viable seeds in late August.
Consequently, the rst cut should be set not later than August 20th. But this enabled the resprouts to
produce viable seeds between August and October.
All mowing treatments in Bohren et al. (2008a) resulted in a decrease of the total number of seeds
and their viability. When cut early (i.e. in June) ragweed regenerated seeds with only 50 % viability
compared to intact plants. Seed viability decreased to 30 % for shoots that developed from later
cutting dates.
Vincent and Ahmim (1978) and Vincent et al. (1992) showed that seed production was signicantly
reduced only at very low cutting heights of 2 cm which is not realistic in the eld.
Integrated treatments:
On crop elds production techniques contribute to the reduction of weeds like common ragweed:
crop rotation, mowing, mulching, hoeing, harrowing and tilling systems are applied. Hoeing is only
applied in specic crops mostly at early stages of development (Verschwele in the HALTAMBRO-
SIA-project, Buttenschøn et al. 2010). Karrer et al. (2011) promote hoeing for ragweed control in oil
pumpkin elds. Common ragweed is said to be easily controlled by rotary hoeing when less than
1/4” (MSU, weed science; http://www.msuweeds.com/worst-weeds/common-ragweed/).
Mechanical plus chemical treatments are generally used in crop elds; several treatments were test-
ed in the EUPHRESCO-project (Holst 2009). Hoeing once induced the highest values for ragweed
biomass produced, whereas hoeing two times did some harm. The eect of biomass loss by this
treatment was about the same as herbicide application followed by hoeing. But the most eective
combination was applying herbicide and afterwards hoeing. If herbicides are used as combined
treatments it is most eective to use herbicide in early developmental stages followed by mechani-
cal measures. The same was found in the U.S. (Donald 2000) for weeds in soybean where herbicides
were combined with mowing. Two times mowing after herbicide treatment worked well in reducing
weeds like common ragweed to a tolerable very low level.
Bohren et al. (2008b, 2008c) combined serial cuts and subsequent herbicide treatments of common
ragweed. The treatment with Florasulam 10 weeks after cut on 19th of June gave high ecacy by low
seed numbers and seed viability between 0.5 to 2.5 %. Other cutting/herbicide combinations gave
less valuable or insucient success.
Experiences by Kazinczi et al. (2008b), Delabays et al. (2005) and Bohren et al. (2005) indicate also
that hoeing alone (i.e., if not performed intensively enough) showed poor control ecacy. Never-
theless soil disturbance by hoeing can promote further emergence of ragweed seeds.
Competition by desirable plants (crops, lawn) acting against weeds and ragweed i.e. is documented
to work well (Kazinczi et al. 2008b, Holst 2009). Using competing plants against ragweed combined
with mowing showed high ecacy in reducing or totally deleting all ragweed individuals in dier-
ent trials. Meiss et al. (2008) and Meiss (2010) documented that ragweed grown together with high
densities of Lucerne and cut 7 times was outcompeted by 100 % after few cutting dates. The same
holds for the competition experiment with ragweed grown at dierent densities together with 3
dierent restoration seed mixtures by Milakovic et Karrer (2010) and (2011)) (see also Karrer et al.,
2011). Almost all ragweed plants died during the rst half of the experiments, obviously caused by

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the additive eect of damage due to cut and competition. In the glasshouse experiment conducted
by Milanova et al. (2010), Lolium perenne and Dactylis glomerata showed to be successful in outcom-
peting common ragweed when whole turfs were planted: number of emerged common ragweed
plants was decreased by 40% and 36%, respectively. The fresh biomass per pot was best reduced
by Lolium perenne planted as whole turf or sown (96% and 97%, respectively). In this experiment
Lucerne showed also an inhibitory eect on the growth of common ragweed, reducing its fresh
biomass per pot by 91%.
The growth type of the competing plants must be optimally adapted to the intensive cutting re-
gime. Therefore the seed mixtures used for the experiments consisted of 20 to 40 % Lolium perenne
which is well adapted to frequent cuts by intensive basal tillering. This grass develops a dense lawn
near the soil surface and regenerates within few days thus shading the resprouts of ragweed from
its basal nodes. The very few resprouts that recovered could not produce a reliable number of seeds.
Conclusions
Control options against common ragweed comprise of herbicide applications and several non-
chemical measures, both summarized by Buttenschøn et al. (2010). Hand pulling is generally the
cheapest and most ecient control option against small populations (less than 100 individuals).
Fumanal et al. (2007) made clear that pollen and seed production was closely related to plant vol-
ume and biomass, thus providing a means of estimating potential pollen and seed production in
given target areas. Such biological data could be integrated into population management strate-
gies or into airborne pollen modelling.
Cutting experiments designed to decrease the pollen production do not consider the problem of
seed production from regenerated shoots.
Basset and Crompton (1975) overdue their conclusion from the quick 100 % regeneration after one
cut when they claim “several cuts during August”. Based on the experience of Bohren et al. (2008a),
Karrer et al. (2011), Simard and Benoit (2011), Karrer and Milakovic (2011) and Pixner (2012), Kar-
rer and Pixner (2012) a three weeks interval between the cuts from July to September should be
enough to prohibit the development of ripened seeds above the cutting line. Even post-harvest
ripening of seeds on shoots left to the habitat could be avoided by 100 %.
Of course, the cutting height is problematic, because the regrowth from nodes below the lower-
most realistic cutting height of 5 cm (Simard and Benoit 2011, Karrer et al. 2011, Milakovic et al.
2014b) can produce seeds anyway. Thus, regrowth should be counteracted by desired strong com-
petitors like Lolium perenne (Karrer et al. 2011, Milakovic and Karrer 2009, Milakovic and Karrer 2010).
Preliminary Recommendations:
EPPO (2008) recommend fairly the same option for ragweed control like Bohren et al. (2008 c) and
Karrer et al. (2011). A late rst mowing just at the beginning or shortly after the start of male bloom-
ing is accepted by all scientists. Considering the detected post-harvest ripening of seeds on cut
branches (Pixner 2012, Karrer and Pixner 2012, Karrer et al. 2012) we would recommend subsequent
cuts every 3 weeks. Four (EPPO 2008) or more weeks (Bohren et al. 2008a) would enable serious
seed production from cut branches. This means at least four cuts from mid/end of July until end of
September.
Aiming at prohibiting the seed production a rst cut latest mid of August and one or two subse-
quent cuts would give optimal results (Bohren et al. 2008a, Karrer et al. 2011, Karrer 2012).

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