Wild taro (Colocasia esculenta (L.) Schott), is an exotic, emergent

perennial that has established in many shallow-water

wetlands throughout the southern United States. Although

wild taro is a cultivated crop in many tropical and subtropical

areas of the world, its invasion in riverine and lacustrine wetlands

in the U.S. has resulted in the loss of habitat for native

plant species. Once established, wild taro forms dense, monotypic

stands that reduce the diversity of native vegetation, as

has occurred in Louisiana, Florida, and Texas (Akridge and

Fonteyn 1981, Simberloff et al. 1997). Akridge and Fonteyn

(1981) reported that although wild taro is considered naturalized

in south-central Texas, its present dominance along

the San Marcos River has altered the native vegetational

structure and dynamics of this river system.  The

objective of this study was to evaluate the efficacy of four

aquatic herbicides for control of wild taro

Getsinger, Kurt D.

Nelson, Linda A.

English

Aquatic Commons

70 J. Aquat. Plant Manage. 38: 2000.
Herbicide Evaluation for Control of Wild Taro
LINDA S. NELSON1 AND KURT D. GETSINGER1
INTRODUCTION
Wild taro (Colocasia esculenta (L.) Schott), is an exotic, emer-
gent perennial that has established in many shallow-water
wetlands throughout the southern United States. Although
wild taro is a cultivated crop in many tropical and subtropical
areas of the world, its invasion in riverine and lacustrine wet-
lands in the U.S. has resulted in the loss of habitat for native
plant species. Once established, wild taro forms dense, mono-
typic stands that reduce the diversity of native vegetation, as
has occurred in Louisiana, Florida, and Texas (Akridge and
Fonteyn 1981, Simberloff et al. 1997). Akridge and Fonteyn
(1981) reported that although wild taro is considered natu-
ralized in south-central Texas, its present dominance along
the San Marcos River has altered the native vegetational
structure and dynamics of this river system. One of the con-
cerns for using herbicides to remove wild taro, and thereby
restore the native emergent plant community at this site, is
the potential impact of chemical treatment to neighboring
stands of Texas wild rice (Zizania texana Hitchc.), a federally
listed endangered plant that grows exclusively in the San
Marcos River.
Restoration and preservation of native plant communities
in conjunction with chemical management practices are
achievable goals. Recent research on the use of herbicides to
selectively control nuisance, exotic species has demonstrated
that aquatic herbicides can be safely used to remove a target
species with minimal harm to non-target communities
(Getsinger et al. 1997, Netherland et al. 1997, Petty et al.
1998). Little information is available on the use of currently
labeled aquatic herbicides for control of nuisance popula-
tions of wild taro. Moreover, the non-target effects of herbi-
cide application on Texas wild rice are also unknown. The
objective of this study was to evaluate the efficacy of four
aquatic herbicides for control of wild taro.
1U.S. Army Engineer Research and Development Center, Waterways
Experiment Station, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199.
Received for publication December 21, 1999 and in revised form May 16,
2000.
MATERIALS AND METHODS
The study was conducted in large, outdoor tanks located
at the U.S. Army Engineering Research and Development
Center, Waterways Experiment Station, Vicksburg, MS. Wild
taro plants were collected from Florida and transplanted in-
to 7.6-L plastic pots (one plant per pot) filled with a potting
soil-sand mixture amended with Osmocote® plant fertilizer.
Potted plants were placed into large outdoor tanks (75 cm
tall by 122 cm diameter) and water was added to the tanks to
a depth sufficient to cover the soil surface with 3 to 5 cm of
water. Plants were allowed to grow for 1 month prior to
chemical treatment. Water was added to the tanks as needed
to maintain saturated soil conditions.
The aquatic herbicides evaluated in this study included:
diquat (Reward®), 2,4-D (2,4-D 4 Amine IVM®), triclopyr
(Renovate®), and glyphosate (Rodeo®). Each herbicide was
applied as a 1- and a 2% solution based on a total spray vol-
ume of 935 L ha-1 (100 gal acre-1), using a CO2-pressurized
backpack sprayer equipped with a hand-held, single-nozzle
spray boom. All herbicide rates were within the maximum la-
bel rate of application for use on emergent vegetation in
aquatic sites using hand-held equipment. A non-ionic surfac-
tant approved for aquatic use (X-77®), was added to all spray
mixtures at a rate of 0.25% volume:volume. At the time of
treatment, potted plants of equal size (50 to 60 cm height with
3 to 4 mature leaves per plant) were selected and removed
from tank cultures for herbicide treatment. After herbicide
application and when plant surfaces were dry, the treated
plants were replaced in the large tanks for post-treatment
monitoring. Treatments were arranged in a randomized
block design (a tank consisted of a block) with 4 replicates.
Visual ratings of herbicide injury were recorded at 1, 2, 3,
and 4 weeks after treatment (WAT) and were assessed on a 1
to 9 scale where 1 = death or necrosis of all plant tissues; 2 =
severe damage, 75% stand kill; 3 = severe injury on 50% of
plant tissues; 4 = distinct symptoms, more severe than 5; 5 =
visible symptoms with possible reduction in biomass; 6 =
clear symptoms, may not affect overall biomass; 7 = mild
symptoms, acceptable appearance; 8 = very mild symptoms,
suggestion of effect; 9 = no visible injury. Plant stunting, dis-
J. Aquat. Plant Manage. 38:70-72
J. Aquat. Plant Manage. 38: 2000. 71
coloration, wilting, plant deformity, and necrosis were con-
sidered when making the visual injury rating. Changes in
injury ratings over time may also reflect plant regrowth. Re-
maining live shoot (leaves, stems, and stolons) and root
(roots and corms) biomass were harvested 6 WAT, dried to a
constant weight at 70C, and dry weights recorded. Data were
subjected to analysis of variance and treatment means were
separated using the Waller-Duncan K-ratio T test procedure.
RESULTS AND DISCUSSION
Results showed that both application rates of triclopyr, 2,4-
D, and glyphosate provided complete control of wild taro
above- and below-ground biomass 6 WAT (Table 1). Al-
though diquat significantly reduced shoot and root biomass
by as much as 87 and 79%, respectively when compared with
untreated plants, it did not provide complete control of the
plants. There were no differences between the two diquat
rates of application.
Symptoms of herbicide injury were noted one week after
application with all chemical treatments (Table 1). Petiole
bending and twisting (epinasty) and leaf necrosis were noted
on plants treated with 2,4-D and triclopyr and are characteris-
tic symptoms for growth regulator-type herbicides. Herbicide
injury was least severe with glyphosate 1 WAT but by the next
evaluation period, injury ratings decreased indicating in-
creased chemical activity. Treatment with diquat provided im-
mediate necrosis of aboveground plant tissues by 1 WAT
which is a typical response from a contact herbicide however,
new, healthy aboveground plant tissues regrew from surviving
corms by 3 WAT. In terms of visual symptoms of herbicide inju-
ry, plants treated with the higher rate of diquat (2% solution)
recovered more slowly than those treated with the low rate.
In conclusion, the results of this evaluation showed that a
1% solution of either 2,4-D, triclopyr, or glyphosate was suffi-
cient to eliminate shoot and root biomass of wild taro under
these experimental conditions. Treatments with diquat were
unsuccessful for long-term control due to regrowth from sur-
viving underground plant tissues. Since a federal permit for
conducting experimentation with an endangered species us-
ing herbicides has not been granted at this time, the direct ef-
fect of these herbicide treatments on Texas wild rice has yet to
be investigated. It should be noted, that two of the active in-
gredients evaluated in this study, 2,4-D and triclopyr, are regis-
tered for post emergence broadleaf weed control in
agricultural rice (Oryza sativa) production (Meister Publishing
1998). Moreover, 2,4-D has little to no activity against estab-
lished grasses (Weed Science Society of America 1994), but
can be effective on some aquatic, broad-leaved monocot spe-
cies such as waterhyacinth (Eichhornia crassipes (Mart.) Solms)
(Gopal 1987). Triclopyr is also used to selectively control
weeds in many grass crops. Although Texas wild rice and culti-
vated rice varieties are classified within different genera, both
are members of the grass family (Gramineae). It is likely that
2,4-D and triclopyr could be utilized for wild taro manage-
ment with minimal impact to adjacent stands of Texas wild
rice. Glyphosate, on the other hand, is a broad-spectrum her-
bicide that may pose a greater hazard to non-target vegetation.
It should be noted however, that herbicide applications for
controlling emergent vegetation (e.g., wild taro) are usually
sprayed onto the foliage above the water line with little chemi-
cal lost to the water itself (Joyce and Ramey 1986). Therefore
proper herbicide application should pose little hazard to sub-
mersed vegetation. Leaves, stems and stolons of Texas wild rice
grow completely submerged in 0.3 to 2.0 m of flowing water,
with only the inflorescences emerging above the water surface
(Terrell et al. 1978). Timing herbicide application for control
of wild taro prior to flowering of Texas wild rice, would mini-
mize the risk of non-target chemical injury to emergent plant
tissues. Further evaluation is required to confirm the toler-
ance of Texas wild rice to herbicide applications. The use of
selective herbicides for preserving threatened plant species
through elimination of aggressive, exotic competitors that
share the same habitat should be a management priority.
TABLE 1. HERBICIDE INJURY AND SHOOT AND ROOT BIOMASS OF WILD TARO FOLLOWING TREATMENT WITH FOUR AQUATIC HERBICIDES.1
Treatment2
Rate3
kg ae ha-1
Dry weight biomass, g (+ S.E.)
3 WAT 4 WAT
Shoot Root Herbicide injury
6 WAT 6 WAT 1 WAT 2 WAT
Untreated Control 0 38.2 (5.1) a 70.4 (9.2) a 9.0 a 8.5 a 9.0 a 9.0 a
1% Diquat 4.2 (ai) 5.1 (2.7) b 14.8 (6.7) b 2.0 c 1.5 c 8.3 a 9.0 a
2% Diquat 8.4 (ai) 9.8 (2.2) b 23.5 (5.5) b 1.3 d 1.5 c  6.6 b 8.8 b
1% 2,4-D 4.3 0.0 c 0.0 c 2.3 c 1.5 c 1.0 c 1.0 c
2% 2,4-D 8.5 0.0 c 0.0 c 2.0 c 1.0 d 1.0 c 1.0 c
1% Triclopyr 3.4 0.0 c 0.0 c 2.0 c 1.0 d 1.0 c 1.0 c
2% Triclopyr 6.7 0.0 c 0.0 c 2.0 c 1.0 d 1.0 c 1.0 c
1% Glyphosate 4.5 0.0 c 0.0 c 5.5 b 2.0 b 2.0 c 1.0 c
2% Glyphosate 9.0 0.0 c 0.0 c 5.0 b 2.0 b 1.5 c 1.0 c
1Data shown are means of four replicates. Comparable means in a column followed by the same letter are not significantly different at α = 0.05 according to
the Waller-Duncan K-ratio T test. Herbicide injury based on a 1 to 9 scale: 1 = death of all plant tissues and 9 = no visible injury. WAT = Weeks after treatment.
2Rate of application as percent solution based on a total spray volume of 935 L ha-1.
3Rate in terms of kg ha-1 as the acid equivalent (ae) or active ingredient (ai). For diquat, the active ingredient is reported as diquat dibromide salt.
72 J. Aquat. Plant Manage. 38: 2000.
LITERATURE CITED
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community using the herbicide triclopyr. Regul. Rivers: Res. Manage. 13:
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Gopal, B. 1987. Aquatic Plant Studies I: Waterhyacinth. Elsevier Science
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