Alligator weed, Alternanthera philoxeroides, is a non-woody perennial
aquatic/shoreline plant found in Florida but non-native to the U.S.
Leaves and stems vary greatly in size and shape. Fleshy, succulent stems can grow horizontally and float on the surface of the
water, forming rafts, or form matted clumps which grow onto banks. The
horizontal stems (called stolons) may reach a length of 10 m. The leaves are
opposite in pairs or whorls, with a distinctive midrib, and range in size from
5-10 cm (Environment Waikato undated, Virginia Cooperative Extension undated).
Fibrous roots arising at the stem nodes may hang free in water or penetrate
into the sediment/soil. Flowers, which appear from December to April, are thin
and clover-like in shape. The white flowers grow on stalks and are
approximately 1.25-7.6 cm in length and 13 mm in diameter (Virginia Cooperative
Alligatorweed occurs in a range of habitats ranging from dry terrestrial to
aquatic. To facilitate buoyancy, plants growing in aquatic habitats tend to
have stems that are hollow and larger than those of plants growing on land
(Julien et al. 1992).
Potentially Misidentified Species
A number of non-native Alternanthera species occur in the United States,
including sessile joyweed Alternanthera sessilis which is similar in appearance
to A. philoxeroides and co-occurs with it in the IRL region of Florida. Mis-identification of these two species has had negative ecological
ramifications elsewhere. A. philoxeroides was accidentally spread in Australia by members of
the Sri Lankan immigrant community because it was mistaken for its congener A.
sessilis which is the Indian herb mukunawanna (Hosking, et al 1996).
Several species of aquatic primrose (genus Ludwigia) and spiderwort
(genus Tradescantia) that occur in Florida also have appearances and/or
growth habits somewhat similar to those of A. philoxeroides.
HABITAT AND DISTRIBUTION
The native range of Alternanthera philoxeroides is reportedly the Parana
River region of South America (Maddox 1968, Vogt et al. 1979). It can now be
found elsewhere in South America, and also on the continents of North
America, Australia, and Asia, and on a number of adjacent islands.
In the United States, A. philoxeroides occurs throughout the southeast
from Virginia south to Florida and west to Texas. It has been reported as far
north as Illinois, and now also occurs in California (Virginia Cooperative
Extension undated, USDA).
Distribution records indicate Alternanthera philoxeroides occurs in the
northern IRL counties of Volusia and Brevard, and also at the far southern end
of the IRL region in Palm Beach County (Wunderlin and Hansen 2004).
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Alternanthera philoxeroides Alligator weed is a summer perennial that typically grows to around 1-2 m and whose horizontal stolons can reach 10 m
Alternanthera philoxeroides was substantially more abundant in the
southeastern U.S. four decades ago than it is now. In 1963, over 65,000 ha of
land in 8 southern states were overrun with invasive alligatorweed.
Within two decades, however, the infested land in those states was brought down
to only about 1% of that total, primarily through successful biological control (Coulson 1977). During this same period,
however, the amount of infested land in Texas and Louisiana increased
(Cofrancesco 1988). Biocontrol of A. philoxeroides is further discussed below.
Reproduction in Alternanthera philoxeroides is predominantly through
vegetative means; individuals rarely produce seeds, and when they do the seeds
are typically non-viable. Vegetative growth occurs at the apical stem buds and
axillary stem and root buds and the plant is spread through fragmentation
(Julien et al. 1992, Virginia Cooperative Extension undated).
Alligatorweed thrives in subtropical to cool climates; optimum growth
temperature of alligatorweed has been experimentally determined to occur
between approximately 15 and 20°C. The plant is susceptible to winter die-back
of exposed portions at higher latitudes (Julien et al. 1992, 1995).
Ensbey (2004) reports that Alternanthera philoxeroides can withstand
salinities of approximately 10% full strength seawater in static aquatic
environments and up to 30% full strength seawater in flowing brackish water.
The ability of this plant to grow in aquatic as well as terrestrial habitats allows it to persist in regions with pronounced wet and dry seasons, including Florida.
Closely associated with Alternanthera philoxeroides is the alligatorweed flea beetle, Agasicles
hygrophila, an insect not native to the U.S. but intentionally introduced
here in the mid-1960s as potential biological control agent of A.
philoxeroides (Buckingham 2002). A. hygrophila has the distinction
of being the first biocontrol insect released in the U.S. in order to combat an
invasive plant. Overall, management impacts on alligatorweed have been
excellent as indicated by the dramatic decrease in the amount of infested
aquatic habitat since the the insect was first released. However, it is not considered
to be effective against plants occurring in terrestrial habitats (UF/IFAS CAIP
Additional alligatorweed-associated insects purposely introduced to the U.S. as
potential biocontrol agents are the alligatorweed thrips, Amynothrips
andersonii, the alligatorweed stem borer moth, Arcola (=Vogtia) malloi and another species of moth, Arcola malloi. Several
additional insects are being studied to determine their suitability as A.
philoxeroides control agents, primarily in Australia (Buckingham 2002).
Alternanthera philoxeroides is native to South America. It was first
reported in the U.S. in 1897 near Mobile, AL. The introduction of this aquatic
/terrestrial plant is interesting in that it is believed to have been the
result of accidental release from ship ballast (Carley and Brown 2006).
Ballast release has also been implicated as the vector for introduction to
Alligatorweed now occurs as an invasive exotic in subtropical to temperate
regions of the Americas, Asia, Australia, New Zealand and a number of Pacific
island nations. In the U.S. A. philoxeroides occurs throughout the
southeast as far north as Illinois, and west to Texas, and coastal California.
The ability of A. philoxeroides to persist in terrestrial, semi-aquatic,
and aquatic environments, the ability to rapidly take root along waterway
banks, and the ability to propagate via vegetative fragmentation and waterborne
dispersal of vegetative propagules all contribute to its success as an invasive
Potential to Compete With Natives
Dense mats of aquatic alligator weed displace native aquatic and shoreline
vegetation and also alter aquatic systems by decreasing water flow, increasing
sedimentation, shading submersed plants, reducing oxygen levels, and filling in
and choking off formerly open water column habitat (Quimby and Kay 1976, Holm
et al. 1997, Carley and Brown 2006).
Possible Economic Consequences of Invasion
Alternanthera philoxeroides has adverse economic impacts as an invasive
species in several ways. It is capable of choking waterways and thereby
impacting boating and sportfishing activities. Aquatic alligatorweed mats
provide breeding habitat for mosquitoes. The terrestrial form of the plant can
also invade agricultural and pasture lands, and drainage and irrigation may be
impacted as well (Coulson 1977, Julien and Bourne 1988, Julien and Broadbent
Mechanical removal of A. philoxeroides mats is costly, and often results
in the dispersal of large numbers of vegetative fragments that can exacerbate
the infestation (GBEP/HARC 2006). Although biocontrol by means of the aligatorweed flea beetle (Agasicles
hygrophila) and other control agents has greatly attenuated the threat of
this plant, the cost associated with carefully studying, planning and managing
the release of biocontrol agents is substantial.
Alligatorweed is a federal noxious weed and a prohibited or noxious plant in
Arizona, California, Florida, and South Carolina (USDA/NRCS).
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