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Potamogeton illinoensis — Illinois pondweed

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New England distribution

Adapted from BONAP data

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North America distribution

Adapted from BONAP data

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Facts

Within New England, Illinois pondweed is restricted to Connecticut, Massachusetts and Vermont. It can be difficult to distinguish from grassy pondweed (Potamogeton gramineus).

Habitat

Lacustrine (in lakes or ponds), riverine (in rivers or streams)

Characteristics

Habitat
aquatic
New England state
  • Connecticut
  • Massachusetts
  • Vermont
Leaf position
  • some of the leaves are floating at the surface of the water
  • the leaves are all submerged underwater
Leaf arrangement
  • alternate: there is one leaf per node along the stem
  • whorled: there are three or more leaves per node along the stem
Leaf blade length
50–200 mm
Petal or sepal number
there are four petals, sepals, or tepals in the flower
Petal color
  • brown
  • green
Specific leaf type
the leaf is not divided, rather the blade is made up of one segment
Floating leaf shape
  • the leaf blade is elliptic (widest near the middle and tapering at both ends)
  • the leaf blade is oblong (rectangular but with rounded ends)
Underwater leaf blade width
2–45 mm
Fruit type (general)
  • the fruit is dry but does not split open when ripe
  • the fruit is fleshy
Underwater leaf length
50–200 mm
Show all characteristics

  • Clonal plantlets

    Turion length
    0 mm

  • Flowers

    Anther length
    0.6–2 mm
    Carpels fused
    the carpel is solitary or (if 2 or more) the carpels are not fused to one another
    Flower lower lip length
    0 mm
    Flower number
    16–60
    Flower position
    the flowers are above the surface of the water
    Flower symmetry
    there are two or more ways to evenly divide the flower (the flower is radially symmetrical)
    Inflorescence length
    25–60 mm
    Inflorescence type
    the inflorescence is a spike (a long unbranched stem with flowers along it that lack stalks)
    Inflorescence width
    8–10 mm
    Length of flower stalk
    0–0.5 mm
    Length of peduncle
    40–300 mm
    Nectar spur
    the flower has no nectar spurs
    Number of carpels
    4
    Ovary position
    the sepals and/or petals are attached below the ovary
    Palate on corolla
    no
    Petal and sepal arrangement
    the flower includes only one cycle of petals or sepals
    Petal appearance
    the petals are green and/or leafy in texture
    Petal color
    • brown
    • green
    Petal fringed edges
    the petals are not fringed
    Petal fusion
    the perianth parts are separate
    Petal hairs on inner/upper surface
    there are no hairs on the inner/upper petal surface
    Petal number
    4
    Petal or sepal number
    there are four petals, sepals, or tepals in the flower
    Pistil number
    4
    Sepal appearance
    NA
    Sepal length
    0 mm
    Sepal number
    0
    Sepals fused only to sepals
    NA
    Spur length
    0 mm
    Stamen number
    4
    Stamen position relative to petals
    NA
    Stamens fused to petals
    the stamens are fused near the bases of the petals or tepals
    Style number
    4

  • Fruits or seeds

    Fruit beak length
    0.5–0.8 mm
    Fruit length
    2.5–3.6 mm
    Fruit type (general)
    • the fruit is dry but does not split open when ripe
    • the fruit is fleshy
    Fruit type (specific)
    • the fruit is a drupe (fleshy, with a firm inner ovary wall that encloses a single seed)
    • the fruit is an achene (dry, usually 1-seeded, does not separate or split open at maturity)
    Fruit width
    2.1–3 mm

  • Glands or sap

    Oil glands on nodes
    none of the nodes have oil glands
    Sap
    the sap is clear and watery

  • Growth form

    Lifespan
    the plant lives more than two years
    Root septa
    the roots do not have transverse septa
    Roots floating in water
    there are no clusters of roots floating in the water
    Turions
    there are no turions on the plant
    Underground organs
    the plant has a rhizome (a horizontal underground stem with roots growing from it)

  • Leaves

    Bract position (Sparganium)
    NA
    Bract relative length
    At least 3854 mm
    Bracts
    neither the flowers nor their pedicels have bracts
    Floating leaf basal lobes
    no
    Floating leaf blade width
    20–65 mm
    Floating leaf length
    40–190 mm
    Floating leaf shape
    • the leaf blade is elliptic (widest near the middle and tapering at both ends)
    • the leaf blade is oblong (rectangular but with rounded ends)
    Floating leaf tip
    • the tip of the floating leaf blade is caudate (has a very long tail-like projection ending in a point)
    • the tip of the floating leaf blade is rounded, with no point
    Floral bract form
    NA
    Floral bract length
    0 mm
    Leaf arrangement
    • alternate: there is one leaf per node along the stem
    • whorled: there are three or more leaves per node along the stem
    Leaf blade length
    50–200 mm
    Leaf blade veins
    the lateral veins are parallel or slightly arched in the direction of the tip
    Leaf blade width
    2–45 mm
    Leaf position
    • some of the leaves are floating at the surface of the water
    • the leaves are all submerged underwater
    Leaf special features
    none of the mentioned special features are present
    Leaf-like branch segments
    0
    Leaf-like branch shape
    NA
    Specific leaf type
    the leaf is not divided, rather the blade is made up of one segment
    Staminate bract edge (Myriophyllum)
    NA
    Stipule appearance
    the stipules are firm or fibrous, and colored white, green or brown
    Stipule fused to leaf
    the stipules are not attached to the leaf blade at all
    Stipules
    the plant has stipules
    Stipules fused around stem
    the stipules do not forma closed tube around the stem
    Trap-bladder length
    0 mm
    Underwater leaf air passage number
    At least 1440
    Underwater leaf air passage row number
    2–5
    Underwater leaf blade edges
    • the underwater leaf blade edges are toothed
    • the underwater leaf has smooth edges, without teeth
    Underwater leaf blade shape
    • the underwater leaf blade is elliptic (widest near the middle and tapering at both ends)
    • the underwater leaf blade is lanceolate (lance-shaped; widest below the middle and tapering at both ends)
    • the underwater leaf blade is linear (very narrow with more or less parallel sides)
    Underwater leaf blade veins
    7–19
    Underwater leaf blade width
    2–45 mm
    Underwater leaf length
    50–200 mm
    Underwater leaf stalk
    • no
    • yes
    Underwater leaf stalk length
    0–40 mm
    Underwater leaf tip shape
    the tip of the underwater leaf is caudate (has a very long tail-like projection ending in a point)
    Veins in floating leaf
    13–29

  • Place

    Habitat
    aquatic
    New England state
    • Connecticut
    • Massachusetts
    • Vermont
    Specific habitat
    • in lakes or ponds
    • in rivers or streams

  • Stem, shoot, branch

    Flowering stem growth form
    the flowering stem is upright

Wetland status

Occurs only in wetlands. (Wetland indicator code: OBL)

New England distribution and conservation status

Distribution

Connecticut
present
Maine
absent
Massachusetts
present
New Hampshire
absent
Rhode Island
absent
Vermont
present

Conservation status

Exact status definitions can vary from state to state. For details, please check with your state.

Massachusetts
fairly widespread (S-rank: S4)

Native to North America?

Yes

Sometimes confused with

Potamogeton nodosus:
submersed leaves with petioles mostly 5–13 cm long, the larger leaves with blades 10–35 mm wide and acute at the apex, and mature drupes red to red-brown (vs. P. illinoenensis, with submersed leaves sessile or with petioles up to 4 cm long, the larger leaves with blades 20–45 mm wide and mucronate at apex, and drupes gray-green to olive-green).

Synonyms

  • Potamogeton angustifolius Bercht. & K. Presl
  • Potamogeton heterophyllus Schreb.

Family

Potamogetonaceae

Genus

Potamogeton

Help

From the dichotomous key of Flora Novae Angliae

13.  Potamogeton illinoensis Morong N

Illinois pondweed. Potamogeton angustifolius Bercht. & K. Presl; P. heterophyllus Schreb. 
• CT, MA, VT; also reported from NH by Haynes and Hellquist 2000, but specimens are 
unknown. Shallow, still or slow-moving, circumneutral to basic water of lakes and rivers. Potamogeton illinoensis can be difficult to separate from P. gramineus. The former 
has submersed leaves with an acute and apiculate apex and with 7–15 veins and the 
drupes are 2.5–3.6 mm long. The latter has submersed leaves with a short-acuminate to acuminate apex and with 3–9 veins and the drupes are 1.9–2.3 mm long.

2×13. Potamogeton amplifolius × Potamogeton illinoensis → Potamogeton ×luxurians Z. Kaplan is a rare pondweed hybrid known from CT, MA, VT. It has been known by the name )P. ×scoliophyllus Hagstr., but the type material of that name represents a collection of P. illinoensis. This hybrid generally resembles P. amplifolius, but the submersed leaves are weakly arched, have mostly 13–15 veins (rather than 19–49), and are relatively elongate and narrow for that species. The floating leaf blades are broad-cuneate at the base (rather than rounded to cordate), and the stems are unspotted (rather than often spotted with red-brown).

11×13. Potamogeton gramineus × Potamogeton illinoensis → Potamogeton ×‌spathuliformis (J.W. Robbins) Morong is a rare pondweed hybrid known from CT, MA, VT. It has submersed leaves that are subsessile to petiolate, the petioles as long as 19 mm. The submersed leaves have 5–11 veins (usually 7) and have stipules 11–35 mm long.

13×15. Potamogeton illinoensis × Potamogeton nodosus → Potamogeton ×‌faxonii Morong is a rare and difficult to detect pondweed hybrid known from VT; also reported from MA by Angelo and Boufford (2000), but the specimens (at MASS!) either are misidentified (a specimen is actually P. ×‌spathuliformis) or have qualified (i.e., tentative) determinations. This hybrid is recognized by its leaves that are sessile or borne on petioles up to 35 (–55) mm long, the principal ones mostly 16–20 mm wide, and are scarcely or not mucronate at the apex.

13×23. Potamogeton illinoensis × Potamogeton richardsonii → This very rare pondweed hybrid is known from CT. It has sessile, broad-lanceolate to narrow-ovate leaf blades that vary from not clasping to weakly clasping, have mostly 18–21 veins, the principal ones mostly 48–77 mm long. The lower stipules disintegrate into fibers.