Description:
This aquatic plant forms branched leafy stems up to 3' long. The
slender stems are light to medium green, narrowly terete (filiform) to
slightly flattened, and glabrous. Alternate leaves are spaced about 1–7
cm. (½–2¾") apart along the stems, becoming more crowded toward their
tips. Mature leaves are 1.5–6 cm. in length (averaging about 2–2.5 cm.
or ¾–1") and 0.5–1.5 mm. across; they are medium green or olive green,
linear in shape, smooth along their margins, glabrous, and
sessile. Leaf venation is parallel; each leaf has a single central vein
and sometimes 2 lateral veins that are visible using a 10x hand-lens.
Both the stems and leaves are soft and flexible, rather than stiff.
At some leaf bases along the stems, there are pairs of translucent
glands that are slightly swollen and difficult to see. In addition,
membranous sheaths about 2.5–9 mm. in length wrap around the stems at
the bases of young leaves; they are usually light brown. These sheaths
are initially tubular in shape, but they soon split along one side and
later become completely detached from the stems without becoming
fibrous. Both terminal and axillary spikes of flowers are produced,
usually in considerable abundance; they are often held slightly above
the water surface.
These floral spikes are 3-10 mm. long and 3-4 mm.
across, consisting of 1-4 interrupted whorls of flowers. Each flower is
about 2 mm. across,
consisting of 4 green to greenish brown sepals (or sepaloid
connectives),
4 stamens, and 4 pistils. The sepals are clawed (contract abruptly) at
their bases. The peduncles are 12-52 mm. (½–2") long, narrowly
terete to slightly flattened, pale green, and glabrous; they are either
straight or curve upward toward the water surface. The blooming period
occurs during the summer or early fall, lasting 2-4 weeks for a colony
of plants. The flowers are cross-pollinated primarily by the wind and,
to a lesser extent, by water currents. Afterward, fertile flowers are
replaced by achenes that are initially green, but they later become
brown with maturity. Individual achenes (while still fresh) are 1.5–2.5
mm. in length, 1.0-2.0 mm. across, and somewhat flattened; they are
somewhat obovoid (where one edge is flat, while the other edge is
convex) with short stout beaks at their apices. The sides of fresh
achenes are convex, while the sides of dried-out achenes are flat to
slightly concave. These achenes lack significant keels. The root system
is fibrous. This plant reproduces vegetatively by the breakage of leafy
stems that can form roots in the mud. It also reseeds itself. Sometimes
colonies of plants are formed at favorable sites.
Cultivation:
The preference is full or partial sun, clear shallow water that is
relatively warm, and a muddy or silty bottom. The water should be
stagnant or
slow-moving. This pondweed can establish itself in more shallow water
than many other pondweeds. It sometimes spreads aggressively. To
discourage excessive growth of filamentous algae, which can smother
this plant, the water should not contain excessive amounts of nitrogen,
phosphorus, or other nutrients from fertilizer run-off.
Range
& Habitat: Slender Pondweed (
Potamogeton pusillus)
is relatively
common in most areas of Illinois, where it is native (see
Distribution
Map). This pondweed is widely distributed in North American,
and it
also occurs in Eurasia. Habitats include sheltered areas of lakes,
ponds, slow-moving creeks, drainage canals, and deep ditches. Slender
Pondweed can be found in both disturbed and higher quality wetlands
where there are bodies of standing or slow-moving water.
Faunal Associations: The larvae of some shore flies
(Ephydridae),
including
Hydrellia
ascita and
Hydrellia
bilobifera, feed on the
submerged leaves of this pondweed, as do the larvae of
Parapoynx
allionealis (Watermilfoil Leafcutter Moth). Larvae of the
aquatic
beetles
Neohaemonia
flagellata,
Neohaemonia
melsheimeri, and
Neohaemonia nigricornis
feed on the submerged stems and roots of
various pondweeds (
Potamogeton
spp.); see the
Insect Table for a more complete list of insect feeders. Because of its soft fine foliage
and relative abundance, Slender Pondweed is one of the more important
sources of food for vertebrate animals among the various pondweeds.
Both the seeds and foliage are eaten by many ducks, geese, swans, and other birds
(see
Bird Table).
Submerged pondweeds
are also eaten by carp and other kinds of fish (see Forbes,
1888), and they are eaten by such turtles as
Chelydra serpentina
(Snapping Turtle),
Chrysemys
picta (Painted Turtle),
Emydoidea
blandingii (Blanding's Turtle),
Graptemys ouachitensis
(Ouachita Map
Turtle),
Graptemys
pseudogeographica (False Map Turtle),
Sternotherus
odoratus (Musk Turtle), and
Trachemys scripta
(Slider); see Lagler
(1943) and Ernest et al. (1994). Muskrats also feed on these plants
(Hamerstrom & Blake, 1939; Martin et al., 1951/1961).
Because Slender Pondweed often forms dense mats of leafy stems, it
provides good cover for minnows, aquatic insects, and other aquatic
organisms.
Photographic
Location: The photographed plants were taken from a
slow-moving stream at Meadowbrook Park in Urbana, Illinois.
Comments:
In Illinois, there are two common species of pondweed with grass-like
leaves that occupy similar habitats:
Potamogeton foliosus
(Leafy
Pondweed) and
Potamogeton
pusillus (Slender Pondweed). Unless the
flowering spikes or achenes are present, it is very difficult to
distinguish these two species. At the achene-bearing stage, the floral
spikes of Leafy Pondweed are more compact than those of Slender
Pondweed. In addition, the peduncles of Leafy Pondweed are more short
(less than 50 mm. in length) and more stout than those of Slender
Pondweed. The achenes of these two species are also different in
appearance: the achenes of Leafy Pondweed have prominent crested keels,
while the achenes of Slender Pondweed lack significant keels. Because
there is some variability in the characteristics of Slender Pondweed
across its range, 2 different subspecies have been described: the
typical subspecies (as described here) and
ssp. tenuissimus.
The latter
subspecies differs by having stipules with inrolled margins that are
not joined together (therefore, they are not truly tubular), its floral
spikes are more compact and continuuous at the achene-bearing stage,
and the sides of its dried achenes are convex, rather than flat or
concave. Sometimes
Potamogeton
pusillus tenuissimus is classified as a
distinct species,
Potamogeton
berchtoldii (Berchtold's Pondweed). In
Illinois, this latter pondweed has been found in the NE section of the
state, although it has not been collected since the 19th century. Even
within the typical subspecies of Slender Pondweed,
Potamogeton pusillus pusillus,
there is significant variation in how widely separated the whorls of
flowers are on the spikes. The floral spikes of this subspecies in
Illinois tend to be less interrupted. Another common name of this
subspecies is Small Pondweed.