AQUATIC INVASIVE PLANTS FACT SHEETS

Prepared by:

Nez Perce Tribe
Department of Fisheries Resource Management
Watershed Division
Mr. Richard L. Christian, Project Leader
P.O. Box 909 / 500 Main Street
Joseph, OR
(541) 432-2506

April 13, 2010

Aquatic Invasive Plant List
Common Name
Flowering Rush

Scientific Name

USDA Code

Page

Butomus umbellatus

BUUM

1

Hydrilla
Eurasian
Watermilfoil

Hydrilla verticillata

HYVE3

2

Myriophyllum spicatum L.

MSP2

3

Common Reed

Phragmites australis ssp.
australis

PAHU7

4

Lagarosiphon major

LAMA15

5

Didymosphenia geminate

NA

6

Nymphoides peltata

NYPE

7

African Elodea or
African Waterweed
Rock Snot or
Didymo
Yellow Floating
Heart
Water Hyacinth
Giant Salvinia
Yellow Flag Iris
European Water
Chestnut
European Frogbit
Carolina Fanwort
Feathered
Mosquitofern
Parrot Feather
Watermilfoil
Twoleaf
Watermilfoil
White Watercress
Narrow Leaf
Cattail

Eichhornia crassipes (Mart.)
Solms
Salvinia molesta
Iris pseudacorus

EICR

8

SAMO5
IRPS

9
10

Trapa natans L

TRNA

11

Hydrocharis morsus-ranae
Cabomba caroliniana

HYMO6
CACA

12
13

Azolla pinnata R. Br.

AZPI

14

Myriophyllum aquaticum

MYAQ2

15

Myriophyllum heterophyllum
Michx.
Rorippa nasturtium-aquaticum

MYHE2

16

RONA2

17

Typha angustifolia

TYAN

18

FLOWERING RUSH
(Butomus umbellatus)
Family: Butomaceae

Flowering rush, also known as grassy
rush and water gladiolus, is not a true
rush species.
Currently there are no known sites of
flowering rush located in Oregon. It has
been found on the Snake and other
regions of the Columbia River system are
considered highly susceptible to invasion.
Most western infestations are a sterile
triploid form. However, there is still
considerable risk of the diploid form
infestations, as the plant is still being sold
online and by pond plant retailers.

Description This weed is distinctive and easy to identify when flowering, difficult when
not in flower. It is a tall plant growing to a height of 1.5 meters in marshes and in aquatic
habitats. The plant crown is topped by an umbel of showy-white or pink flowers that
emerge in late summer to early fall. Leaf stems are triangular in cross-section. Flowering
rush reproduces both asexually by rhizomes and bulblets and sexually through seed
production. Moving water is the predominant dispersal mechanism for localized as well as
long-distant weed movement, human activities and wildlife also contribute.

Impacts: This plant has the potential to invade
and disrupt native marshlands. In the lower
Columbia, it may compete strongly with reed
canary grass. Evidence of economic damage is
now emerging in the western United States.
Unlined irrigation canals are becoming clogged
as well as drainage ditches. Maintenance of these
structures is expensive. Public and private boat
access is now being impeded at some locations in
Idaho (Rice 2008). It is unclear at this time of the
effects on fishing access and success. Shallow
lakes could become seriously infested and
rendered unfishable. Reservoirs with variable water levels provide great habitat for this
species.
Images courtesy of Leslie J. Mehrhoff, University of Connecticut, Bugwood.org and FoxyIslandWalks.com.

1

HYDRILLA
(Hydrilla verticillata)

weeds.hotmeal.net

Hydrilla was imported from Asia in the 1950’s for use in aquariums and was first
introduced to the wild in Florida. Now established in several states including Washington
and California. Clogs irrigation and drainage canals, interferes with recreation, displaces
native vegetation and damages sportfish populations. Transported on boats and trailers.

Description: Perennial aquatic plant. Grows rooted to the bottom with long stems that
reach water’s surface. Can be monoecious or dioecious. Leaves are 1/16 to 1/8 inch wide,
1/4 to 3/4 inch long and occur in whorls of five. Small, axillary leaf scales are found next to
the stem and inserted at the base of the leaf, a character that distinguishes hydrilla from
other family members. The nut-like turions (tubers) are a key identifying feature.

Impacts: Hydrilla is the most
serious threat to aquatic
ecosystems in temperate climate
zones. Dense stands of hydrilla
provide poor habitat for fish and
other wildlife altering water
quality by raising pH, decreasing
oxygen, and increasing
temperature. Stagnant water
created by mats provides good
breeding grounds for
mosquitoes. Hydrilla interferes
with recreational activities such
as swimming, boating, fishing,
and water skiing and will clog irrigation ditches and intake pipes.
2

EURASIAN WATERMILFOIL
(Myriophyllum spicatum L.)

www.ecy.wa.gov/.../plants/weeds/images/fish2.jpg

Initial introduction is not certain, but it has been suggested that it was accidently
introduced to Chesapeake Bay in the 1800s from Eurasia or northern Africa, probably
with ship ballast. Common throughout western Oregon and the Columbia Basin. It is not
known to be in Wallowa County, but it is abundant in Umatilla and Morrow Counties.

Description: Perennial, aquatic plant. Purplish red branching stems grow up to five feet
long. Feather-like leaves in whorls of four at each node. Male flowers purplish and appear
at the end of the flowering stem. Lower, whitish
flowers are female. Determination between these
natives and Eurasian water milfoil is difficult to do
without flowers, but typically leaves of EWM have
more than 14 leaflet pairs (28 individual leaflets) and
the leaves are very lax, collapsing around the stem
when taken out of the water.

Impacts: Eurasian watermilfoil has significant
economic impacts to waterways, irrigation ditches,
and drainage canals where it can inhibit flow and
increase maintenance costs. It is also a pest of rivers,
lakes and ponds were it reduces water quality,
impeded recreation and boat access and have adverse
impact to fish habitat. Elevated nutrient levels created
by erosion, fertilizers or urban effluent create a rapid
growth response that can render a waterway clogged
within a short period of time. Expensive control
projects frequently target this plant across North
America.
weeds.hotmeal.net/weeds/Eurasian_Watermilfoil.jpg

3

COMMON REED
(Phragmites australis ssp. australis)

Also known as giant reed, phragmites, giant reedgrass, Roseau cane and yellow cane,
common reed has been widely introduced and is naturalized in the United States. Large
populations of Phragmites can be observed at Klamath Lake, Summer Lake, Garrison
Lake, John Day River, in North Portland adjacent to Smith and Bybee Lakes, and along
the Columbia River, but no determination has yet been made regarding native or
introduced lineages. Of these populations the Ft. Stevens and Smith-Bybee are currently
known to be introduced.

Description: A large, perennial, clonal grass species with
creeping rhizomes and stolons, and terminal, plume-like flowering
stalks. Known This plant has woody hollow stems that can grow
1-4 meters tall with stem diameters of 0.5-1.5 cm. Leaves are 1540 cm long with an open leaf sheath. Phragmites grows in a wide
range of sites that hold shallow water, including roadside ditches,
marshes, swamps, brackish estuaries, and alkaline wetlands.
Reproduction is primarily vegetative, through an extensive
network of rhizomes, which can grow horizontally up to 1.8 m per
year depending on the climate. Differences between the native and
invasive phragmites can be subtle and may partially depend on
ecological conditions. ODA recently posted this link to help folks
to determine the differences between native and non-native
(http://www.oregon.gov/ODA/PLANT/WEEDS/phragmitesidentification.shtml).

Impacts: Phragmites dominated areas exclude large wading birds, increases land
elevation, reduces habitat for important fish species, disrupts trophic transfers, blocks fish
passage, reduces refuge by steepening creek banks. In addition, Phragmites can have
adverse impacts on waterfront property values and recreation such as hunting and fishing.
Disturbances or stresses such as pollution, dredging, and increased sedimentation favor
invasion, and spread of non-native Phragmites.
James H. Miller, USDA Forest Service

4

AFRICAN ELODEA OR AFRICAN WATERWEED
(Lagarosiphon major)
It is believed that this plant was in New Zealand for
some time before it was recognized as a plant
distinct from Elodea canadensis in the 1950s. By the
time it was recognized, Lagarosiphon major was
already a major weed there.

Description: As a submersed, long-stemmed plant
having many small narrow leaves, Lagarosiphon
major might be confused with three other plants in
the U.S. As chance would have it, two of these three
other plants are themselves also not native to the
U.S.; however they are here, whereas Lagarosiphon
is not (early 2008). This plant is rooted in the
hydro-soil by numerous thread-like unbranched
roots. The stems are submersed, brittle (3mm or 1/8
in) growing to 20 feet long, branching every 10-to12 nodes. The 3-veined with visible midvein leaves
are
submersed, greatly recurved and alternate
spirally along the stem (16 mm (1 in.) long by 2
mm (1/16 to 1/8 in.) wide). The flowers are tiny
transparent to white or pinkish. African
waterweed will grow in lakes, rivers, streams
and ponds, oligotrophic to eutrophic. It prefers
cooler waters and is winter hardy (optimum
temperature, 20-23o C (68-74oF); maximum
temperature, 25o C (77o F)). The determinant of
its maximum depth (about 7 m) is pressure (in
non-light-limited environments).

Impacts: Happily, Lagarosiphon major does not
yet occur in the wild in the United States, as 2008,
so far as is known. However, experts have reason
to believe that should this plant be introduced to
the U.S., the resulting problems could be as
consequential as those caused by another plant in
the Hydrocharitaceae family, hydrilla (Hydrilla
verticillata).

5

ROCK SNOT OR DIDYMO
(Didymosphenia geminata)
The native distribution of D. geminata is the cool
temperate regions of the Northern Hemisphere,
including the rivers of northern forests and alpine
regions of Europe, Asia and parts of North
America. Until its recent discovery in New
Zealand, where it was introduced, it was never
previously found in the Southern Hemisphere.
The distribution of didymo in the last two decades
appears to be gradually expanding outside its
native range. The exact pathway of introduction
is unknown, but it spreads easily through
contaminated boats and fishing gear.

Description: Didymosphenia geminata is a
diatom, which is a type of single-celled
algae unique for their silica (SiO2) cell
walls. The life history of diatoms includes
both vegetative and sexual reproduction,
though the sexual stage is not yet
documented in this species. Although it is
symmetric only along the apical axis,
typical of gomphonemoid diatoms, it is a
cymbelloid, which are typically symmetric
along both primary axes. Cells contain a
raphe, which allows them to move on
surfaces, and an apical porefield, through
which a mucopolysaccharide stalk is secreted.
The stalk can attach to rocks, plants, or other submerged
surfaces. When the diatom cell divides, through vegetative
reproduction, the stalk divides too, eventually forming a
mass of branching stalks. The nuisance build-up is not the
cell itself, but their massive production of extracellular
stalks. Extracellular polymeric substances (EPS) that form
the stalks are made primarily of polysaccharides and
protein, forming complex, multi-layered structures that are
resistant to degradation. The microscopic algae can be
spread in a single drop of water.

Impacts: Didymosphenia geminata, if it overgrows, it can form large mats on the bottom
of lakes, rivers and streams. It is not considered a significant human health risk, but it can
affect stream habitats, disrupt ecological processes, causing a decline in native plant and
animal life, and make recreational activities unpleasant. It is considered a nuisance
organism or invasive species.
6

YELLOW FLOATING HEART
(Nymphoides peltata)
Other common names: Asaza, entire marshwort, floating heart, and fringed water lily.

Yellow Floating Heart was
introduced to the U.S. as an
ornamental water plant from
the Mediterranean area.
There are several known sites
in Oregon in the recent past.
One site was found in
Washington County (2004),
Lane County (2005), Douglas
County (2007), Lane County
(2007), and Jackson County
(2008). It appears that this
plant is spreading in the past
few years.

Description: Aquatic perennial. Grows rooted to
the bottom in water depths of 2-13 feet. Floating
leaves heart-shaped to circular, 1 to 4 in long,
purplish underneath with slightly wavy margins.
Leaves attached to long stalks that arise from
creeping underwater rhizomes. Flowers bright
yellow, 5-petaled, and 1 to 1 1/4 inch in diameter.
Reproduces by seed and by rooting at nodes and will
regenerated from plant fragments.

Impacts: Yellow floating heart grows
in dense patches, excluding light for
native species and creating stagnant
areas with low oxygen levels underneath
the floating mats. These mats make it
difficult to fish, water ski, swim or
paddle. It displaces native plants and
animals and decrease biological
diversity. This troublesome aquatic
changes how natural ecosystems work.
Grows on slow-moving rivers, lakes,
reservoirs, and ponds.
7

WATER HYACINTH
(Eichhornia crassipes (Mart.) Solms)
Water hyacinth is native to South
America, and was introduced to the
United States in the 1880s. It’s beautiful,
large purple and violet flowers have made
it a popular ornamental, and the plant is
now naturalized in most of the southern
United States. Currently it is only known
to occur in two Oregon counties, but
doesn’t appear to be overwintering in
either. Waters with thermal springs
upstream, or cooling ponds for power
plants (the case in a reoccurring
infestation in WA) would be prime spots
for infestations to persist.

Description: Water hyacinth has been called the worst aquatic plant in the world! Its
growth rate is among the highest of any plant known: hyacinth populations can double in
as little as 12 days. The plant is a free-floating, robust plant that can grow up to three feet
off the water's surface with a mass of fine purplish black and
feathery roots hangs in the water underneath the plant. The
shiny green leaves are round to oval, four to eight inches in
diameter, with gently incurved sides. The leaf veins are
dense and numerous, so leaves stand erect. Water hyacinth
stalks are bulbous and spongy, and help keep the plant
buoyant. The flowers have six petals, purplish blue or
lavender with yellow. There are several flowers grow at the
top of a single stalk. The plant reproduces by seeds and
vegetatively through daughter plants that form on rhizomes
and produce dense plant beds. In one study, two plants
produced 1,200 daughter plants in four months. By this
mechanism, water hyacinth can form impenetrable mats of
floating vegetation. Individual plants break off the mat and can be dispersed by wind and
water currents. A single plant can produce as many as 5,000 seeds and waterfowl eat and
transport seeds to new locations. Seedlings are common on mud banks exposed by low
water levels. Another tropical floating-leaved nursery plant, called water lettuce, may be
mistaken for water hyacinth However; water lettuce doesn't have showy flowers and has
large ribbed leaves. Water lettuce is much less cold-tolerant than water hyacinth and is not
thought able to survive in cold climates

Impacts: Forms dense colonies that block sunlight, clog water intakes, and crowd out
native species. Incredibly dense mats of free-floating vegetation block boat traffic and
prevent swimming and fishing, and keep sunlight from reaching the water column and
submerged plants.

8

GIANT SALVINIA
(Salvinia molesta)
Other Common Names: giant salvinia, Kariba weed, African pyle, aquarium watermoss, koi
kandy

Salvinia molesta, one of the world’s most
noxious aquatic weeds, is notorious for
dominating slow moving or quiet
freshwaters. Giant salvinia is native to
South America.

Description: It is a small free-floating plant that grows in clusters and develops into
dense, floating mats or colonies in quiet water, undisturbed by wave
action. The floating leaves of giant salvinia are oblong (0.5 to 1.5 inches
long) with a distinct midrib along which the leaf may fold forming a
compressed chain-like appearance. Salvinias have stiff leaf hairs on the
upper surface of the leaves. In giant salvinia the leaf hairs have a single
stalk that divides into four branches that reconnect at the tip, giving the
hair a cage-like or egg-beater appearance. Underwater the leaves are
modified into small root-like structures. The entire plant is only about 1
to 2 inch in depth. Salvinias are ferns and have no flower. Giant salvinia
has sporangia but are thought to reproduce only by fragmentation. Giant salvinia can
double in size in 4 to 10 days under good conditions. Giant salvinia is an aggressive invader
species. Under optimal conditions (light, temperature and nutrient) in the laboratory, plant
populations have been found to double in size every 2-4 days. Salvinias have no known
direct food value to wildlife and is considered an exotic and highly undesirable species.

Impacts: Its rapid growth, vegetative reproduction and tolerance to environmental stress
make it an aggressive, competitive species known to impact aquatic environments, water
use and local economies. These colonies will also eliminate native submerged plants by
blocking sunlight penetration. Resulting dense surface cover prevents light and
atmospheric oxygen from entering the water. Meanwhile, decomposing material drops to
the bottom, greatly consuming dissolved oxygen needed by fish and other aquatic life.
9

YELLOW FLAG IRIS
(Iris pseudacorus)
Other common names: Water Flag, Yellow Flag and Yellow Iris
Yellow flag is native to most European countries with the
exception of Iceland. It is also known to occur in North Africa
and the Mediterranean regions. It has significant freeze
tolerance and can be found in Scandinavia to 68 degrees
North. Yellow flag iris is a popular ornamental in North
America that is planted in natural and artificial wet areas
within urban and rural landscapes. It is particularly popular
as a large and colorful flowering element in ponds and has
been planted in wastewater ponds where it is used to remove
heavy metals. The species has naturalized extensively and is
currently distributed across the United States. Once
established, it is an aggressive invader in most wet habitats. In
Oregon, significant populations can be found in the Willamette
valley, in coastal lakes and streams, in the Columbia and
Snake Rivers and in Central Oregon on irrigation canals. This plant is known to occur in
two locations in Wallowa County and other small populations throughout northeast OR.

Description: Yellow flag is a very showy species growing
3-4 feet in height with the most vigorous growth attained in
the wettest environments. The leaves are long, flattened and
sword-like, typical of most iris. Large plant clumps are
formed from the lateral growth of rhizomes sometimes
attaining 20 feet in width. It has erect plant stalks with
multiple flowers produced on each. Fruit capsules are large,
3-angled and up to 4 inches in length. Disk-like seeds are
shed from the capsules throughout the fall and winter.
Floating mats of seed can be observed in backwaters and marshes aiding dispersal.
Reproduction can occur asexually through rhizome fragmentation or by seed production.

Impacts: An infestation of yellow flag iris presents a dual impact on both human interests
and native environments. This plant displaces native plants including
sedges and rushes. This can reduce the carrying-capacity of wetlands
for waterfowl and disrupt other ecological relationships. Irrigation
canals and flood control ditches can be severely restricted by the
physical nature of the plant clumps. Removal can be costly requiring
large excavation equipment or herbicides. Control of heavily infested
waterways can be cost prohibitive due to the huge volume of plant
material needing to be removed. Any rhizome fragments that remain quickly reestablish a
population. Many over-wintering waterfowl species are dependent on sedge and rush seeds
as a high-energy food source. Replacement of this food source with yellow flag iris would
reduce the carrying capacity of these marshes to sustain waterfowl populations. A small
volume of yellow flag still exists in the nursery trade. A variegated variety is popular with
aquatic gardeners and can be found in several catalogs and web sites.
10

EUROPEAN WATER CHESTNUT
(Trapa natans L)
Other common names: Singhara Ling, bullnut
Little information is available as to how
this plant got established in the United
States. It is most likely that T. natrans was
brought from Europe as an ornamental.
Currently, there are no known
populations established in the state of
Oregon.

Leslie J. Mehrhoff, University of Connecticut

Description: Annual aquatic plant. Upper
floating leaves diamond-shaped with toothed
Leslie J. Mehrhoff,
edges, arranged on inflated, spongy stalks and
University of Connecticut
occur in clusters up to 20 inches across.
Submersed leaves long and narrow or often replaced with green feather-like structures.
Flowers small, solitary and white to light purple. Fruit a large swollen nut with two to
four sharp spines.

Impacts: This plant is a fierce competitor in shallow
bodies of water possessing soft, muddy rooting
substrates. In the Northeast it creates nearly
impenetrable mats across wide areas of water creating
barriers to boaters, swimmers and fisherman alike.
Spiked nuts
drift to shore
where their
sharp spines may hurt bare feet. Mono-cultures
severely limit light penetration and water mixing,
creating conditions of reduced oxygen levels which
may increase the potential for fish kills. It is of little
value to waterfowl.

Alfred Cofrancesco, U.S. Army Corps of Engineers

11

EUROPEAN FROGBIT
(Hydrocharis morsus-ranae L.)
This plant occurs around the
Great Lakes and is only
found in New York State
within the United States. It
is found in Ontario and
Quebec in Canada. Frogbit
is currently on the
Washington State’s noxious
weed list.

Description: Plants perennial, of fresh waters.
Rhizomes absent; stolons present. Stems floating on or
suspended in water, rooted or not, unbranched, short.
Leaves basal, emergent or floating, petiolate; blade
cordate to reniform or orbiculate, base reniform or
cordate, apex obtuse to almost truncate; midvein
without rows of lacunae along sides, uniform in color
throughout, abaxial surface without prickles, smooth on
emergent leaves or with aerenchymous tissue on
floating leaves; intravaginal squamules entire. Inflorescences 1-flowered or cymose, sessile
or short-pedunculate; spathe not winged. Flowers unisexual, staminate and pistillate on
different plants [on same plants], emersed, pedicellate; petals white to pinkish. Staminate
flowers: filaments distinct or basally connate, distinct portion longer than connate; anthers
oval; pollen in monads. Pistillate flowers: ovary 1-locular; styles 6, 2-fid less than ½ length.
Fruits spheric, smooth to ridged, dehiscing
irregularly. Seeds ellipsoid, minutely
tuberculate or muricate.

Impacts: The impacts associated with this
weed are very similar to other aquatic
invasive species. It forms dense mats that
eliminate native species and creates
conditions of reduced oxygen levels which
may increase the potential for fish kills.

12

CAROLINA FANWORT
(Cabomba caroliniana)

Fanwort is a native of the southeastern U.S, but is considered to be a non-native invasive
plant in the northeastern U.S. and in the northwest. The plant is known to be present in
Columbia, Clatsop, Lane and Douglas Counties in Oregon. Two species of Cabomba occur
in Florida.

Description: Fanwort is a rooted submersed plant. It may have
submersed and floating leaves of different shapes. Fanwort
generally grows in three to ten feet of water, and is found in ponds,
lakes and quiet streams. Fanwort stems
are long and much-branched. Fanwort has
fan-like underwater leaves, which are
about two inches across. The submersed
leaves are frequently divided, and are
arranged oppositely or in whorls along the stem. The floating
leaves are small, diamond-shaped,
and are infrequent. Fanwort
flowers are white to pink to
purplish and are about 1/2 inch
across. The flowers are on stalks
which arise from the tips of the
stems.

Impacts: Although the potential impacts are not clear at
this time, it is believed that they would be similar to other
aquatic invasive plant species. By forming dense mats, this
plant out-competes native species and reduces the overall
ecological diversity.
13

FEATHERED MOSQUITOFERN
(Azolla pinnata R. Br.)
Other common names: Feathered Mosquitofern, Mosquito Fern or Water Velvet

Azolla pinnata can spread rapidly, and has
the ability to survive on moist soil in and
around rivers , ditches, and ponds . Azolla is
usually found in stagnant or slow-moving
water of ponds, lakes, marshes, swamps, and
streams. Plants turn reddish when under
stress, such as from poor nutrition, salinity,
or high temperatures. A. pinnata has been
cultivated for many centuries in rice paddies
of northern Vietnam and southeastern China,
where it acts as a fertilizer after it
decomposes.

Description: Plants small, 1.5 - 2.5 cm long, with a
straight main axis with pinnately arranged side
branches, progressively longer towards the base ,
thus roughly triangular in shape. The basal
branches themselves become pinnate and eventually
fragmenting as the main axis decomposes to form
new plants. Roots with fine lateral rootlets, having a
feathery appearance in the water. Leaves minute, 1
-2 mm long, overlapping in 2 ranks , upper lobe
green, brown green or reddish, lower lobe
translucent brown; minute, short, plae, +/cylindrical unicellular hairs often present on the
upper lobes. When fertile, round sporocarps 1 - 1.5
mm wide can be seen on the under-side at the bases
of the side
branches. The
leaves often have a maroon-red tinge and the water
can appear to be covered by red velvet from the
distance. The upper surfaces of the leaves are totally
water-repellant, and if completely submerged the
plants quickly refloat with the right side up.

Impacts: It forms dense surface mats, which degrade water quality by reducing oxygen
levels, and can interfere with boating, fishing and recreational activities.

14

PARROT FEATHER WATERMILFOIL
(Myriophyllum aquaticum)
Other common names: Brazilian Water-Milfoil, Brazilian Watermilfoil, Parrot Feather, Parrot Feather
Watermilfoil, Parrot´s-Feather, Parrot's Feather, Parrot's-Feather, Parrot-Feather, Parrotfeather, Parrotfeather Water
Milfoil, Parrotweed, Thread-Of-Life, Water Milfoil, Water-Feather

M. aquaticum is a bright or glaucus green
perennial freshwater herb. Parrots feather
exhibits two different leaf forms depending on
whether it is growing as a submerged plant or as
an emergent. It is found in freshwater lakes,
ponds, streams, and canals and appears to be
adapted to high nutrient environments, and does
well in good light and a slightly alkaline
environment. Almost all plants are female, and
male plants are unknown outside of South
America. It has been introduced for use in indoor
and outdoor aquaria and is also a popular
aquatic garden plant. It tends to colonize slowly moving or still water rather than in areas
with higher flow rates. While it grows best when rooted in shallow water, it has been
known to occur as a floating plant in the deep water of nutrient-enriched lakes. The
emergent stems can survive on wet banks of rivers and lake shores, so it is well adapted to
moderate water level fluctuations.

Description: Herbs perennial, rarely annual, aquatic or
terrestrial, monoecious or dioecious. Stem creeping,
ascending or erect, often rooting at lower nodes. Leaves
estipulate, 2-4-whorled, decussate, or alternate, in terrestrial
species always simple, in aquatic submersed species always
pectinately lobed. Inflorescence an
indeterminate or determinate spike
of 1-5-flowered dichasia borne in
axils of leaflike primary bracts. Flowers minute, actinomorphic ,
epigynous , protandrous, anemophilous or entomophilous, bisexual
or unisexual. Stamens 1 or 2 Ã as many as sepals; filaments short,
slender; anthers basifixed , mostly oblong or linear , 4-loculed,
dehiscing by slits, antisepalous anthers sometimes somewhat longer than antipetalous ones;
styles as many (rarely half as many) as sepals, alternisepalous, free , mostly short, grading
into globose or subulate stigmas, spreading in fruit; stigmatic papillae hairlike; ovules as
many as styles, rarely 1, apical, pendulous, anatropous . Fruit nutlike or a drupe, variously
ornamented with wings, ribs, and tubercles, indehiscent and 1-seeded or breaking up into
(2-4) 1-seeded mericarps; Seeds with thin testa; embryo cylindric, surrounded by thick,
white albumen.

Impacts: It has escaped cultivation and spread via plant fragments and intentional
plantings. It forms dense surface mats, which degrade water quality by reducing oxygen
levels, and can interfere with boating, fishing and recreational activities.
15

TWOLEAF WATERMILFOIL
(Myriophyllum heterophyllum Michx.)
Other common names: Variable milfoil
It was most likely a “stowaway” fragment attached to a
boat or trailer that came to the northeastern US region.
Milfoil can live out of water for many hours if it
remains moist, like when it’s wound around a wet
carpeted bunk on a boat trailer or in a live well. Milfoil
is usually first found near boat launch sites when it
infests a new waterbody, a sure sign that transient
boaters are the leading means of spread. Another
theory is that milfoil was introduced to a New
Hampshire waterbody through the dumping of a home
aquarium. The plant is new to the Pacific Northwest,
but commonly used as ornamental plant in fish
aquariums.

Description: Milfoil is a submerged aquatic plant
with fine densely packed, featherlike leaves whorled
around a main stem. It can grow up to 15 feet and may
exhibit a three to six inch green spikelike flower above
the waterline in late June or in July. A crosssection of
the stem will reveal
“pieshaped” air chambers.
Boat propellers chop
These fragments
of the wind and lake
these fragments. If
take hold creating a
until every suitable
alternative form of
This vegetation type
form and can persist

milfoil plants into small fragments.
float on the surface and are at the mercy
currents. In a short time, roots form on
washed ashore, these plants eventually
new colony of milfoil. The cycle goes on
area is filled in with these weeds. An
the plants develops during low water.
is more succulent than the submersed
for moderate periods of low water.

Impacts: This species is not native to our state and is very difficult to control once it
becomes fully established. Milfoil reproduces through fragmentation whereby plant
fragments break off from the parent plant through wind or boat action, grow roots, and
settle in a new location. Seeds are also a means of spread within an infested waterbody.
Milfoil spreads rapidly and displaces beneficial native plant life, often forming
monoculture of growth around the shallows of a waterbody. It makes swimming difficult
and can devalue waterfront property. Where this species grows in its native environment,
insects and fish may feed on this plant at such a rate as to control its growth. In New
Hampshire, variable milfoil has no abundance of natural predators to keep its population
in check. Under optimum temperature, light and nutrient conditions, milfoil may grow up
to an inch per day.
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WHITE WATERCRESS
(Rorippa nasturtium-aquaticum syn. Nasturtium nasturtium-aquaticum, Nasturtium officinale,
Sisymbrium nasturtium-aquaticum)
White watercress is native to Europe. The
plant grows in lakes, ponds, rivers, streams,
springs and moist ground.

Description: Alternate, nearly toothless,
pinnately lobed with 3 to 9 leaflets. The
terminal leaflet is larger than the side ones.
Leaflets oval shaped.

Flowers: White, small, 0.2 inches wide, 4
petalled flowers borne on long stems,
appearing April through September.

Fruit: Slender pods are about 0.4 to 0.6
inches long. The seeds within the pod are
small and are contained in two rows.

Look-alikes: Onerow yellowcress
(Rorippa microphylla), a non-native,
is very difficult to distinguish from
white watercress in the field. They
both occur in moist soils and behave
similarly. Onerow yellowcress seed
appear in one row in the pod and a
slightly larger and more prolific than
white watercress seeds.

Impacts: White watercress rapidly forms
dense stands that exclude native plants. White
watercress is uncommon but spreading rapidly.
It is highly threatening to natural plant
communities. All detected occurrences should
be eradicated.
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NARROWLEAF CATTAIL
(Typha angustifolia)
Other common names: Cattail, Narrow-leaved Cattail, and in the UK, Bulrush, Lesser Bulrush, Small Reed Mace
Narrow-leaved cattails are believed to have been
introduced to the Atlantic seaboard from the dry
ballast of European ships. This plant has since
spread westward and occurs throughout much of
the United States, and is found in southeast OR.
The hybrid cattail is concentrated in the northeast,
but may occur wherever both the native and the
narrow-leaved species are present. It is found in
wetlands, ditches, stream and lake shores and wet
depressions; tolerates high levels of silt, nutrients
and salt. Typha angustifolia hybridizes with the
native common cat-tail (Typha latifolia) to produce
Typha X glauca. The hybrid cat-tail is larger than
parents, sterile; reproduces vegetatively through
rhizomes; tolerates a greater range of conditions
than parents.
Photo: Ohio Department of Natural Resources, Division of Natural Areas and Preserves,
http://www.dnr.state.oh.us/dnap/invasive/11cattail.htm

Description: An erect, rhizomatous, semiaquatic or aquatic, perennial herb. The leaves
are erect, linear, flat, basal, very narrow and flattened (¼"-½" wide and 3'-6' tall). Twelvesixteen leaves arise from each vegetative shoot. The stems are stout and erect (3-6 feet).
Rhizomes are stout, produced at the leafbase and up to 27 inches long; typically ¾"-1½" in
diameter. The flower structure is a dense, fuzzy,
cylindrical spike on the end of stem, with a distinct gap
of 1"-3" of naked stem between the upper, male portion
(staminate) and the lower, female (pistillate) portion.
Both male and female sections are roughly the same
length. Male flowers are lighter brown; female flowers
often green during bloom turning dark brown during
seed maturation. Individual blossoms are minute and
closely packed on spike, blooming May-June. The fruits
are cigar-shaped and 2"-6" long, with soft, downy
seeds. Seeds of Thinleaf cattail are a tiny nutlet, about 1
mm long with downy hairs underneath Seeds remain
viable in the seed bank for up to 100 years.

Impacts: Narrow-leaved and hybrid cattail will outcompete native plants in wetland systems. These plants
establish dense monocultures that enable them to shade
out native vegetation. They are also thought to be allelopathic, producing chemicals which
discourage growth of other plant species. Cattails reproduce both vegetatively by rhizomes
and sexually through massive amounts of seed.
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