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Hydrilla verticillata — water-thyme

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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

Water-thyme is a widespread invasive aquatic plant, native to Asia. It was introduced to North America in the aquarium trade, and first escaped in Florida in 1959. It has a wide tolerance and can grow in almost any fresh or brackish body of water.

Habitat

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

Characteristics

Habitat
aquatic
New England state
  • Connecticut
  • Maine
  • Massachusetts
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
8–20 mm
Petal or sepal number
there are six petals, sepals, or tepals in the flower
Petal color
  • red
  • white
Specific leaf type
the leaf is not divided, rather the blade is made up of one segment
Floating leaf shape
NA
Underwater leaf blade width
1.2–4 mm
Fruit type (general)
the fruit is dry but does not split open when ripe
Underwater leaf length
8–20 mm
Show all characteristics

  • Flowers

    Carpels fused
    the carpels are fused to one another
    Flower lower lip length
    0 mm
    Flower number
    1
    Flower position
    • the flowers are below the surface of the water
    • the flowers are floating on the surface of the water
    Flower symmetry
    there are two or more ways to evenly divide the flower (the flower is radially symmetrical)
    Inflorescence type
    the inflorescence has only one flower on it
    Length of flower stalk
    0–0.5 mm
    Nectar spur
    the flower has no nectar spurs
    Number of carpels
    3–6
    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 color
    • red
    • white
    Petal fusion
    the perianth parts are separate
    Petal number
    3
    Petal or sepal number
    there are six petals, sepals, or tepals in the flower
    Pistil number
    1
    Sepal length
    1.5–3 mm
    Sepal number
    3
    Sepals fused only to sepals
    the sepals are separate from one another
    Spur length
    0 mm
    Stamen number
    3
    Stamens fused
    the stamens are not fused to one another
    Stamens fused to petals
    the stamens are not fused to the petals or tepals
    Style number
    1

  • Fruits or seeds

    Fruit length
    5–6 mm
    Fruit type (general)
    the fruit is dry but does not split open when ripe
    Fruit type (specific)
    the fruit is a capsule (splits along two or more seams, apical teeth or pores when dry, to release two or more seeds)

  • 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
    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
    Leaf arrangement
    whorled: there are three or more leaves per node along the stem
    Leaf blade length
    8–20 mm
    Leaf blade veins
    the lateral veins are parallel or slightly arched in the direction of the tip
    Leaf blade width
    1.2–4 mm
    Leaf position
    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 delicate and translucent
    Stipules
    the plant has stipules
    Trap-bladder length
    0 mm
    Underwater leaf air passage number
    At least 1439
    Underwater leaf air passage relative width
    At least 4336
    Underwater leaf air passage row number
    0
    Underwater leaf blade edges
    the underwater leaf blade edges are toothed
    Underwater leaf blade shape
    • the underwater leaf blade is elliptic (widest near the middle and tapering at both ends)
    • the underwater leaf blade is linear (very narrow with more or less parallel sides)
    Underwater leaf blade width
    1.2–4 mm
    Underwater leaf length
    8–20 mm
    Underwater leaf stalk
    no
    Underwater leaf stalk length
    0 mm
    Underwater leaf tip shape
    the tip of the underwater leaf is acute (sharply pointed)
    Veins in floating leaf
    0

  • Place

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

  • Stem, shoot, branch

    Flowering stem growth form
    the flowering stem trails along the substrate, or floats in the water

Wetland status

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

New England distribution and conservation status

Distribution

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

Conservation status

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

Massachusetts
not applicable (S-rank: SNA)

Native to North America?

No

Sometimes confused with

Egeria densa:
leaf blades 10-40 mm long, without pickles (vs. H. verticillata, with leaf blades mostly 8-15 mm long, with small prickles along the abaxial midvein).
Hippuris vulgaris:
leaves with entire margins and lacking prickles (vs. H. verticillata, with leaves with serrulate margins and prickles along the abaxial midvein).

Synonyms

  • Serpicula verticillata L. f.

Family

Hydrocharitaceae

Genus

Hydrilla

Help

From the dichotomous key of Flora Novae Angliae

1.  Hydrilla verticillata ( L. f. ) Royle E

water-thyme. Serpicula verticillata L. f. • CT, MA, ME. Lakes and slow-moving rivers, with a 
broad toleratance of pH, nutrient level, and salinity. This species can vegetatively perennate 
by stem fragmentation and by production of tuber-like turions (these structures can be produced underwater and beneath the substrate the plant is rooted in).