philoxeroides (C. Martius) Griseb][ALRPH][CDFA Rating: A] Photographs
Map of Distribution
DESCRIPTION:Noxious herbaceous aquatic
to terrestrial perennial, with horizontal to ascending
stems 1 m long that root at the nodes. Aquatic form has hollow,
floating, emergent and submerged stems. Terrestrial
plants have solid stems. Typically, plants grow rooted in soil
in shallow water and form dense, interwoven floating mats that
extend over the surface of deeper water. Mats can become dense
enough to support the weight of a person. Floating mats can
break away and colonize new sites. Mats disrupt the natural
ecology of a site by reducing light penetration and crowding out
native species. Serious infestations can create anoxic, disease,
and mosquito breeding conditions. Introduced from South America.
The alligatorweed flea beetle (Agasicles hygrophila), stem
borer moth (Vogtia malloi), and alligatorweed thrips (Amynothrips
andersoni) have been released as biocontrol agents in the
Southeastern U.S. These insects can effectively control infestations
of alligatorweed. However, none are established in California
at publication time.
encountered because viable seed is rarely produced.
simple or branched and lacking hairs or with 2 opposing lengthwise
rows of hairs. Leaves opposite, more or less equal at a
node, sessile or with narrowly winged petioles (to 1 cm long)
that clasp the stem. Leaf blades mostly 4-11 cm long, 1-3 cm wide,
narrowly lanceolate (to obovate), with entire margins and a smooth
back to top of page
and UNDERGROUND STRUCTURES:Stolons root at the
nodes. Floating plants have shorter, finer roots than plants rooted
in soil. Stolon fragments with a node often develop into new plants.
Pleasantly fragrant. Spikes head-like,12-18 mm in
diameter on terminal or axillary stalks 4-9 cm long. Flowers
and bracts pearly white, glabrous. Flowers lack
petals. Sepals 5, separate 5-7 mm long. Stamens 5, opposite sepals,
and alternate with 5 longer sterile stamens (staminodia). Ovary
superior, with a single chamber containing 1 ovule.
and SEEDS:Utricles membranous, do not open to release the
single seed. Seeds smooth, disc-shaped to flattened wedge-shaped.
Mature fruits seldom encountered.
CHARACTERISTICS:Dead stems fall over and contribute to formation
of the mat. Mild frost kills leaves but not stems. Severe frosts
kill emergent stems but not submerged or buried parts.
or wet soils, ditches, marshes, edges of ponds and slow-moving
watercourses. Tolerates saline conditions (to 10% salt by volume).
Requires a warm summer growing season. Tolerates cold winters,
but cannot survive prolonged freezing temperatures.
Joaquin Valley (Tulare and Kings cos.) and Southwestern region
(Los Angeles Co.); to Southeastern U.S.; Central America. To 200
m (660 ft).
vegetatively from stolons. Each
node or fragment with a node is capable of producing a new plant.
Plants are highly competitive and have rapid growth rates. Plants
rarely grow in water deeper than 2 m. Seeds rarely develop,
and those that do are seldom viable.
back to top of page
FAVORING/ DISCOURAGING SURVIVAL:Plants grow best under high-nutrient (eutrophic)
conditions. Mechanical removal without careful removal of all
plant parts can facilitate spread. Stolon can regenerate from
burial to 30 cm (~12 in) deep.
SIMILAR SPECIES:Mat chaff-flower [A. caracasana
Kunth][ALRPE] is a prostrate terrestrial weed related to alligatorweed.
Unlike alligatorweed, mat chaff-flower has sessile flower
spikes, leaves mostly 1-3 cm long, and spine-tipped sepals.
It grows on drier soils of waste places, roadsides, vacant lots, and fields
in the Southwestern region, to 150 m (500 ft). Water smartweed [Polygonum
amphibium L.] is an aquatic perennial with rhizomes that is more
likely to be confused with alligatorweed. Water smartweed (Polygonum
amphibium L.) is distinguished by having alternate leaves with
fused, sheathing stipules (ocrea) and pink flowers.
back to top of page
A. philoxeroides generally will not establish in water deeper than 2 meters.
Proper pond construction can minimize shallow edges and prevent establishment.
Likewise, establishment of competitive grasses or other native species on
the banks of ponds and irigation ditches will reduce soil erosion and prevent
A. philoxeroides from gaining a terrestrial foothold.
Mechanical: Since this plant will regenerate from rootstocks and fragmented
stems, removal of the dense floating mats will only provide temporary control.
Care must be taken to prevent transport of detached stems downwater, where
re-establishment can rapidly occur. Tillage of terrestrial plants may severe
roots and shoots, which may increase the spread of the plant.
Biological: There have been three South American insect species released
between 1964 and 1971 to control A. philoxeroides, with varying degrees of
success. The alligatorweed flea beetle Agasicles hygrophilia may cause considerable
damage to aquatic mats of A. philoxeroides. It feeds on the leaves and bores
into stems, where it pupates before adulthood. Unfortunately, it will neither
feed upon nor reproduce in terrestrial plants. Considerable success has been
shown in the southeastern United Sates. However, repeated attempts at establishment
in California during 1967-1969 met with little success and no further colonizations
were attempted. The alligatorweed stem borer Vogtia malloi is a small moth
which lays eggs on the apical leaves. The larvae bore into the stem tips and
move down the stems. Infested stems rapidly wilt and droop. This damage can
be easily distinguished from the flea beetle's characteristic leaf stripping
of plants. The insects were initially released in Florida, Georgia, the Carolinas
and Alabama from 1971 to 1973, and have since been reported in Arkansas, Louisianna,
Mississippi, and Texas. A thrips species, Amynothrips andersoni, attacks and
deforms apical leaves of both aquatic and terrestrial plants. Damage, however
is relatively minor and scattered. Attempts to establish this species from
1967 to 1971 in Albany, California were unsuccessful. Most adults are wingless
and dispersal is somewhat limited.
Chemical: The following herbicide treatments have demonstrated considerable
success, although retreatment is necessary.
1) 2,4-D at 8 lb/A mixed with 8 oz of detergent applied in 50 gallons of water
per surface acre.
2) Glyphosate (Rodeo) at 6 pints per acre + X-77 non ionic surfactant at 3
pints per acre applied in 50 gallons of water per surface acre.
3) Dicamba (Banvel 720) at 1 gallon + Rodeo at 1 quart + X-77 at 1 pint applied
in 50 gallons of water over plants.
Read and follow all label directions before applying any herbicide to water.
Misuse may cause extensive damage to other nontarget plants, both native and
back to top of page