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Myriophyllum heterophyllum — variable-leaved water-milfoil

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Facts

Variable-leaved water-milfoil is a serious invader in New England, spreading rapidly by fragmentation, as well as by seed. Under optimum conditions, each stem may grow up to one inch per day. It is usually found first near boat launch sites, indicating that the leading means of spread may be boaters.

Habitat

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

New England distribution

Adapted from BONAP data

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North America distribution

Adapted from BONAP data

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Characteristics

Habitat
aquatic
New England state
  • Connecticut
  • Maine
  • Massachusetts
  • New Hampshire
  • Rhode Island
  • Vermont
Leaf position
the leaves are all submerged underwater
Leaf arrangement
whorled: there are three or more leaves per node along the stem
Leaf blade length
0.4–30 mm
Petal or sepal number
there are four petals, sepals, or tepals in the flower
Petal color
white
Specific leaf type
the leaf is once pinnately divided and the segments are very narrow, each side providing the appearance of a comb
Floating leaf shape
NA
Underwater leaf blade width
10–30 mm
Fruit type (general)
the fruit is dry and splits open when ripe
Underwater leaf length
15–40 mm
Show all characteristics

  • Flowers

    Anther length
    1–2.5 mm
    Carpels fused
    the carpels are fused to one another
    Flower lower lip length
    0 mm
    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
    30–370 mm
    Inflorescence type
    the inflorescence is a spike (a long unbranched stem with flowers along it that lack stalks)
    Length of flower stalk
    0 mm
    Nectar spur
    the flower has no nectar spurs
    Number of carpels
    4
    Ovary position
    the sepals and/or petals are attached above the ovary
    Palate on corolla
    no
    Petal and sepal arrangement
    the flower includes two cycles of petal- or sepal-like structures
    Petal appearance
    the petals are thin and delicate, and pigmented (colored other than green or brown)
    Petal color
    white
    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 length
    1.5–3 mm
    Petal number
    0–4
    Petal or sepal number
    there are four petals, sepals, or tepals in the flower
    Pistil number
    1
    Sepal appearance
    the sepals resemble leaves in color and texture
    Sepal number
    4
    Spur length
    0 mm
    Stamen length
    1.2–2 mm
    Stamen number
    • 0
    • 4
    Stamens fused to petals
    the stamens are not fused to the petals or tepals
    Style length
    0 mm
    Style number
    0

  • Fruits or seeds

    Fruit length
    1–1.5 mm
    Fruit type (general)
    the fruit is dry and splits open when ripe
    Fruit type (specific)
    the fruit is a schizocarp (when dry it splits into sections, each holding one or more seeds)
    Fruit width
    1–1.5 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
    the plant has turions
    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 1380 mm
    Bracts
    the flowers or their pedicels have bracts at their bases
    Floating leaf basal lobes
    NA
    Floating leaf blade width
    0 mm
    Floating leaf length
    0 mm
    Floating leaf shape
    NA
    Floating leaf tip
    NA
    Floral bract form
    • the bracts are much more lobed, or much less lobed, than the foliage leaves
    • the bracts are roughly as lobed as the foliage leaves
    Floral bract length
    4–18 mm
    Leaf arrangement
    whorled: there are three or more leaves per node along the stem
    Leaf blade length
    0.4–30 mm
    Leaf blade veins
    the lateral veins radiate from the base and continue to spread away from the centerline of the leaf, or branch off the central vein at intervals
    Leaf blade width
    1.5–5 mm
    Leaf position
    the leaves are all submerged underwater
    Leaf special features
    none of the mentioned special features are present
    Leaf-like branch segments
    5–12
    Leaf-like branch shape
    the leaf-like branches are round
    Specific leaf type
    the leaf is once pinnately divided and the segments are very narrow, each side providing the appearance of a comb
    Staminate bract edge (Myriophyllum)
    the edges of the staminate bracts have either forward-pointing teeth, or are pinnately lobed (with lobes projecting from both sides of the central axis of the leaf)
    Stipule appearance
    NA
    Stipule fused to leaf
    NA
    Stipules
    there are no stipules on the plant
    Stipules fused around stem
    NA
    Trap-bladder length
    0 mm
    Underwater leaf blade edges
    the underwater leaf blades are lobed
    Underwater leaf blade shape
    the underwater leaf blade is oblong (rectangular but with rounded ends)
    Underwater leaf blade width
    10–30 mm
    Underwater leaf length
    15–40 mm
    Underwater leaf stalk
    yes
    Underwater leaf stalk length
    0–2 mm
    Veins in floating leaf
    0

  • Place

    Habitat
    aquatic
    New England state
    • Connecticut
    • Maine
    • Massachusetts
    • New Hampshire
    • Rhode Island
    • 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)

In New England

Distribution

Connecticut
present, invasive, prohibited
Maine
present, invasive, prohibited
Massachusetts
present, invasive, prohibited
New Hampshire
present, invasive, prohibited
Rhode Island
present, invasive, prohibited
Vermont
present, invasive, prohibited

Conservation status

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

Massachusetts
not applicable (S-rank: SNA)

From Flora Novae Angliae dichotomous key

4.  Myriophyllum heterophyllum Michx. E

variable-leaved water-milfoil. CT, MA, ME, NH, RI, VT. Still or slow-moving, circumneutral to basic water of lakes and rivers.

4×Myriophyllum laxum Shuttlw. ex Chapman is a rare hybrid water-milfoil known from CT, ME. It tends to show some scattered (i.e., alternate) leaves at the base of the inflorescence (the others whorled), and the blades are often ± lobed (those of M. heterophyllum are whorled and serrulate). This hybrid could be confused with Myriophyllum pinnatum; however, the latter is smaller and has bracts that vary from pinnately lobed to prominently serrate.

Native to North America?

Yes and no (some introduced)

Sometimes confused with

Myriophyllum verticillatum:
carpellate bracts prominently serrate to pinnately lobed, fruits 2–2.5 mm long, and turions formed in the late summer and fall on the stems, these clavate in outline (vs. M. heterophyllum, with the carpellate bracts merely serrulate or sometimes pectinately lobed near the water line, fruits 1–1.5 mm long, and turions formed in the fall at the base of the stems or on the rhizomes, these not clavate in outline).

Family

Haloragaceae

Genus

Myriophyllum