Stolon formation and fragmentation are two vegetative mechanisms by which hydrilla colonies expand.  These two mechanisms of spread were studied in ponds located in Lewisville, TX over a two-year period.  Stolons were determined to be the predominant mechanism for localized expansion in undisturbed areas.  While some fragments were produced, they accounted for only 0.1% of the establishment of rooted plants in new quadrats.  Peak production of fragments occurred in October and November, with fragment densities of 0.15 N m-2 d-1.  Expansion by stolons occurred between June and November of each year, with higher rates of spread (up to 4.0 cm d-1 radial growth) observed in the second season

Madsen, John D.

Smith, Dian H.

English

Aquatic Commons

J. Aquat. Plant Manage. 37: 1999. 25
J. Aquat. Plant Manage. 37: 25-29
Vegetative Spread of Dioecious Hydrilla 
Colonies in Experimental Ponds
JOHN D. MADSEN1 AND DIAN H. SMITH2
ABSTRACT
Stolon formation and fragmentation are two vegetative
mechanisms by which hydrilla colonies expand. These two
mechanisms of spread were studied in ponds located in
Lewisville, TX over a two-year period. Stolons were deter-
mined to be the predominant mechanism for localized
expansion in undisturbed areas. While some fragments were
produced, they accounted for only 0.1% of the establish-
ment of rooted plants in new quadrats. Peak production of
fragments occurred in October and November, with frag-
ment densities of 0.15 N m-2 d-1. Expansion by stolons
occurred between June and November of each year, with
higher rates of spread (up to 4.0 cm d-1 radial growth)
observed in the second season.
1U.S. Army Engineer Waterways Experiment Station, CEWES-ES-P, 3909
Halls Ferry Road, Vicksburg, MS 39180-6199.
2U.S. Army Engineer Waterways Experiment Station, Lewisville Aquatic
Ecosystem Research Facility, R.R. #3 Box 446, Lewisville, Texas 75056-9720.
Received for publication October 21, 1998 and in revised form March 10,
1999.
Key words: vegetative reproduction, fragment, stolon, dis-
persal, Hydrilla verticillata.
INTRODUCTION
Hydrilla (Hydrilla verticillata (L.f.) Royle) was first intro-
duced into North America during the 1960s (Pieterse 1981),
and since that time confirmed populations of dioecious hyd-
rilla have been identified in thirteen states (AL, AZ, CA, CT,
FL, GA, LA, MS, NC, OK, SC, TN, TX) (Les et al. 1997, Stew-
ard et al. 1984).
This highly invasive weed has two distinct biotypes in the
USA, with a number of dispersal mechanisms. Viable seeds
are produced by the monoecious biotype. Currently, the dio-
ecious biotype within the United States develops only pistil-
late flowers, and thus does not produce seeds (Langeland
and Smith 1984).
Like many submersed aquatic plants, both hydrilla bio-
types expand by the production of vegetative propagules
(Sculthrope 1967, Langeland and Sutton 1980). Root crowns
develop stolons that extend into the area surrounding the
parent plant and establish new plants. Tubers, also called
26 J. Aquat. Plant Manage. 37: 1999.
subterranean turions, are formed within the sediment and
remain viable for many years after above ground vegetation
has been destroyed (Van and Steward 1990). Fragments,
which are produced by the mechanical breakage of plant
stem apices, float away from the colony and settle to the sedi-
ment where they develop roots and produce new plants.
Additionally, axillary turions, which are produced on and
abscise from the stem, provide a mechanism for intermedi-
ate distance dispersal within a water body, and long distance
dispersal between waterbodies (Sutton 1996, Spencer and
Ksander 1991, Sutton and Portier 1985, Haller et al. 1976).
Axillary turion production is often considered to be quite
low on rooted dioecious plants, and may not provide the
most important mode for intermediate-distance dispersal
within a water body. Formation of turions on detached stems
may provide an excellent means of intermediate and long
distance dispersal.
The study objectives were to determine the prominence
of two modes of vegetative population expansion (stolon and
fragments) from a new, small colony of hydrilla; to deter-
mine the average daily rate of spread over two growing sea-
sons; and to analyze seasonal dispersal patterns of the two
modes of propagation.
MATERIALS AND METHODS
This study was conducted at the U.S. Army Corps of Engi-
neers Lewisville Aquatic Ecosystem Research Facility in
Lewisville, Texas, USA (Latitude 33°04’45” N, Longitude
96°57’33” W) in two 0.26 ha manmade ponds. An impervious
barrier divided each pond, providing four separate study
areas. Water depth within the ponds ranged from 70 cm to
148 cm (Madsen and Smith 1997). Lake Lewisville, a U.S.
Army Corps of Engineers reservoir, was the source of water
used for the experimental ponds. Water level was maintained
by screened standpipes in each experimental pond (Smart et
al. 1995). The water chemistry of the ponds was typically cir-
cumneutral (average pH of 7.1), with an average alkalinity of
100 mg CaCO3 L-1, average total phosphorus of 50 µg L-1,
average nitrate of 290 µg L-1, and an average ammonium of
50 µg L-1 (Smart et al. 1995). The pond sediment was 28%
sand, 33% silt, and 38% clay, with an average of 5% organic
matter, 9.2 mg g-1 of exchangeable nitrogen, and a bulk den-
sity of 1.2 g cm-3 (Smart et al. 1995).
Each of the four 18 m by 18 m sampling regions was delin-
eated into 324 1 m2 quadrats by a grid system using nylon
rope suspended from contiguous 3 m by 6 m PVC pipe
frames. The grid system was attached to floats and held sta-
tionary by metal posts driven into the sediment (Figure 1). A
total of 1,296 quadrats were used in this study.
An Omnidata EasyLoggerTM (Logan, Utah) underwater
thermistor, affixed to a float and held below the surface of
the water at a depth of 15 cm, measured water temperatures
at five minute intervals. The five minute intervals were aver-
aged to record an hourly average. Daily average water tem-
peratures were calculated from logged hourly averages.
Localized spread of hydrilla was investigated by planting
four root crowns in the sediment of each grid’s center four
quadrats (eight meters from each edge) in April 1995. The
root crowns planted were from plants grown in small pots in
the greenhouse from stem fragments planted in these pots in
November 1994. From April 1995 through November 1996,
presence or absence of rooted plants and fragments within
the quadrats of each grid was recorded at three week inter-
vals. No observations could be made from mid-November
1995 to mid-May 1996 due to high turbidity in the system.
During each observation period, the presence of rooted
plants or fragments within all quadrats (planted and outly-
ing) was used to calculate the percent frequency resulting
from the two types of vegetative propagules and overall
growth of the colonies. Quadrats which had fragments at one
time, and rooted plants the next were generally assumed to
have been colonized by fragments. Often, the stem fragment
could still be observed in these quadrats. Although we exam-
ined plants for turions, none were observed throughout this
experiment. Transition to presence or absence of the type of
vegetative propagule observed within a quadrat from one
sampling period to the next sampling period furnished data
for calculation of average daily expansion or mortality rates.
Colony expansion via stolons was analyzed via rooted plants
and their mortality. A stoloniferous expansion rate was charac-
terized by the transition from absence of rooted growth within
a quadrat during one sampling period to presence of rooted
growth during the next sampling period. Mortality was char-
acterized by transition from rooted growth present during
one sampling period to the absence of observable rooted
growth during the next sampling period. The units used to
express these changes are given as N m-2 d-1, or the number of
new ramets per quadrat (or square meter) per day.
Figure 1. Schematic of grid system arrangement within experimental study
ponds. Depth isopleths are in 25 cm intervals.
J. Aquat. Plant Manage. 37: 1999. 27
Distribution and colony expansion by fragments were ana-
lyzed via fragment settling, fragment persistence, establish-
ment of fragments into rooted growth, and fragment
mortality (Madsen and Smith 1997). Fragment settling was
defined as a stem fragment having drifted to the bottom, but
not yet rooted. Fragment persistence was defined as an
unrooted fragment having settled the previous time period,
and still occurring as an unrooted fragment. Fragment estab-
lishment was defined as a stem fragment forming adventi-
tious roots that extended into the sediment. Fragment
mortality was defined as a stem fragment occurring in the
previous time period, but the quadrat being unoccupied in
the current time period.
Data from all quadrats were combined across ponds and
depths for analysis.
RESULTS AND DISCUSSION
The major period of expansion for the colonies of Hydrilla
verticillata was in a six-month period from May 1996 through
November 1996 when the percent of quadrats with rooted
vegetation increased from 19.5% to 70% of the sampling
quadrats (Figure 2B). Stoloniferous growth accounted for
99.9% of the colony expansion while only 0.1% of the expan-
sion was attributable to fragmentation. Expansion followed a
period of quiescence during the winter months when water
temperatures ranged from zero to the mid-teens (Figure 2A).
During the major expansion period, the hydrilla colony
expanded radially at 4.0 ± 0.39 cm d-1. Stolons provided the
colony with a daily rate of spread ranging from 0.56 ± 0.16 N
m-2 d-1 to 1.48 ± 0.32 N m-2 d-1 during this major expansion
period (Figure 2C). Stolon mortality was less than 0.12 ± 0.04
N m-2 d-1 throughout the investigation (Figure 2D).
In contrast to the major expansion of hydrilla colonies
which occurred between May and November when water
temperatures were above 20 C, Eurasian watermilfoil (Myrio-
phyllum spicatum L.) grown under similar conditions demon-
strated an earlier major expansion period between
November and June when water temperatures were above 10
C (Madsen and Smith 1997). However, a similar radial popu-
lation expansion rate of 3.9 cm d-1 was observed for Eurasian
watermilfoil during its key expansion period (Madsen and
Smith 1997). A slightly higher rate of stolon mortality was
observed for Eurasian watermilfoil (≤ 0.20 N m-2 d-1, Madsen
and Smith 1997) as compared to the rate observed for hyd-
rilla. Observed mortality was minimal for both species, sug-
gesting that spread by stolons is a very efficient form of
vegetative propagation.
Phenology of hydrilla grown in this southern region of the
USA exhibits an active growth period during the summer
months, with a period of senescence during the winter (Mad-
sen and Owens 1998). The active growing period in Texas is
somewhat later than observed for dioecious hydrilla in Florida
(Bowes et al. 1979). In contrast, the active growth period for
Eurasian watermilfoil in Texas is during spring when tempera-
tures range from 10 to 25 C, with growth slowing during the
summer months as temperatures exceed 25 C (Madsen 1997).
The major episode of hydrilla fragment settling occurred
between October and November 1995 when fragments set-
tled into previously unvegetated quadrats at a rate of 0.02 N
m-2 d-1 to 0.15 N m-2 d-1 (Figure 3A). However, fragment persis-
tence was low with only one episode of persistence at the
time of fragment settling. During November 1995, the persis-
tence rate was 0.05 ± 0.04 N m-2 d-1 (Figure 3B). The mortality
rate following the pulse of fragmentation was 0.02 N m-2 d-1
from October 1995 through May 1996 (Figure 3D). Estab-
lishment of fragments into rooted plants was observed
between May and June 1996 when the rate of population
expansion by fragments was 0.01 ± 0.01 N m-2 d-1 (Figure 3C).
Both hydrilla and Eurasian watermilfoil exhibit a peak in
fragment settling during October and November. However,
Eurasian watermilfoil fragments had a settling rate (or pro-
duction rate) which was approximately 22 times greater, a
persistence rate approximately 9 times greater, and an estab-
lishment rate which was approximately 67 times greater than
hydrilla (Madsen and Smith 1997).
Data from this study of the spread of hydrilla and a similar
study of the spread of Eurasian watermilfoil (Madsen and
Smith 1997) delineate distinctive vegetative strategies uti-
lized by these nuisance weeds when grown in southern states.
Figure 2. Stolon dynamics within the four experimental study pond areas.
(A) Daily average water temperature during study period; (B) Percentage of
sample areas vegetated by rooted growth; (C) Daily expansion rate of colo-
nies by stolons; (D) Daily rate of stolon mortality.
28 J. Aquat. Plant Manage. 37: 1999.
Eurasian watermilfoil begins its growing season when water
temperatures are low and vigorously grows to the surface,
forming a mat which shades many species from required
light. The spread of an Eurasian watermilfoil colony slows as
water temperatures increase, in contrast to hydrilla which
begins its active growth period when higher water tempera-
tures prevail, growing rapidly to the surface and branching
profusely. Hydrilla has low light and carbon dioxide compen-
sation points, and is tolerant of high water temperatures and
high oxygen concentrations. It is therefore capable of main-
taining photosynthesis when many other submersed plants
are photosynthetically inactive, which provides a competitive
advantage for hydrilla (Bowes et al. 1977, Holaday and Bowes
1980, Langeland 1990). Once a surface mat is formed, hyd-
rilla also shades other submersed species from light.
In this study, there were no disturbances to cause break-
age of hydrilla stems (e.g., boat traffic, wave action, harvest-
ing, or other physical injuries to plants). This not only
reduced the production rate of hydrilla fragments in this
study, but may have also reduced the rate of hydrilla turion
formation. Our observations indicate that turions are often
formed on detached stems in Texas, as has been observed for
dioecious hydrilla in Florida (Miller et al. 1993).
Hydrilla depends on stoloniferous growth for localized
radial spread of a colony. In areas of relatively undisturbed
environments, vegetative expansion by stolons abet the estab-
lishment of new ramets by transporting nutrients from par-
ent plants; thereby reducing new ramet mortality (Grimes
1981). Production of subterranean tubers provides the spe-
cies with a vegetative strategy for localized regrowth or reveg-
etation after periods of high disturbance, such as drawdown
or herbivory. Finally, vegetative strategies for intermediate
distance dispersal (within and between connected waterbod-
ies) are provided by the development of axillary turions and
vegetative fragments which allow clones of the species to col-
onize areas further away from the rooted colony.
This study indicates that dioecious hydrilla is only spread-
ing during the time of active growth, from June through
November in Texas. The primary mechanism of spread in
this study was through stolon growth. Few fragments were
formed autochthonously. Therefore, other vectors of stem
breakage, either through human activity or from wave action
or water movement, are required to form large numbers of
fragments. Therefore, the spread of dioecious hydrilla within
a lake or river system may be reduced by reducing the level
of human activity in areas infested with hydrilla. Hydrilla
spread also exhibited a “lag phase” during the first year, in
which little spread occurred. Hydrilla did not form a dense
canopy on the surface during the first year, as well. Hydrilla
populations in lakes and reservoirs may exhibit a lag phase in
the initial year of introduction and establishment, during
which the plants do not form a dense canopy or spread
widely, and in the following year a large colony will then
reach the surface, forming a dense canopy.
ACKNOWLEDGMENTS
This research was conducted under the U.S. Army Corps
of Engineers Aquatic Plant Control Research Program, U.S.
Army Engineer Waterways Experiment Station. Permission
to publish this information was granted by the Chief of Engi-
neers. Technical assistance was provided by Chetta Owens
and Melissa Smith. Technical reviews were provided by
Michael Netherland and Susan Sprecher.
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Vegetative Spread of Dioecious Hydrilla Colonies in Experimental Ponds

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Vegetative Spread of Dioecious Hydrilla Colonies in Experimental Ponds

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