Hydrilla

Scientific name

Hydrilla Rich.

Common names

hydrilla

Family

Hydrocharitaceae

Similar genera

Egeria, Elatine, Elodea, Hemianthus, Lagarosiphon, Micranthemum, Nechamandra

Native distribution

Europe, Africa to Australia

Species cultivated

Hydrilla verticillata (L.f.) Royle

U.S. Federal Noxious Weed: Hydrilla verticillata

Identification: Hydrilla can be distinguished from Egeria, Elodea, and Lagarosiphon by its whorledwhorled:
(n) bearing whorls; a type of leaf arrangement (phyllotaxis) in which leaves are in whorls
leaves, which bear small spines on the midveins on the underside of each leaf. (See below for further details.)

See Hydrilla verticillata disseminule fact sheet.

Adventive distribution

almost cosmopolitancosmopolitan:
(adj) essentially worldwide in distribution
, apparently absent from South America

Weed status

Hydrilla is a serious environmental weed and a weed in irrigation systems in numerous countries. Hydrilla is an aquatic weed on the U.S. federal noxious weed list.

Habit

submersedsubmersed:
see submerged
stem plantstem plant:
(n) (a term used in the aquarium and pond plant trade) having an elongate stem (as opposed to a compact stem)
, rooted but often breaking fee and floating in mats

Brief description

Annual or perennialperennial:
(adj) (of a plant) having a life cycle of more than two years
, attached. Monoeciousmonoecious:
(adj) having separate male and female flowers on the same individual
or sometimes dioeciousdioecious:
(adj) having separate male and female flowers on different individuals of the same species
. Stems elongate, branched regularly, ascending or stoloniferous. Leaves caulescentcaulescent:
(adj) having a more or less well developed above-ground stem
, arranged in whorls of 3-10, but whorles other than at nodes mostly 5 per whorlwhorl:
(n) three or more similar organs arranged in a circle at the same point around an axis
on well-developed stems. Leaves relatively evenly distributed along stem, crowded at apexapex:
(n) the point farthest from the point of attachment; the tip (often pointed)
, sessilesessile:
(adj) attached directly, without a stalk
; leaf bladeblade:
(n) (syn. lamina) the flat, expanded part of a leaf, frond, or petal (excluding, e.g., the petiole)
linear to lanceolatelanceolate:
(adj) lance-shaped; widest point below the middle, tapering to the apex
, midveinmidvein:
(n) the primary, usually central vein of a leaf or leaflet
distinct; marginmargin:
(n) edge; rim
finely serrateserrate:
(adj) (of a leaf margin) bearing sharp teeth pointing forward or to the apex
, but coarser than Elodea serrations, usually visible without magnification. Turions develop either on stolons below substrate or are axillaryaxillary:
(adj) in, of, or produced from an axil
on stems. Inflorescenceinflorescence:
(n) the arrangement of flowers on the floral axis
axillaryaxillary:
(adj) in, of, or produced from an axil
; spathespathe:
(n) a large bract or bracts subtending and often enclosing an inflorescence
of two united bracts. Flowers unisexualunisexual:
(adj) (of a flower) with either stamens (male) or pistils (female) but not both; consisting of only male or female flowers
; male flowers sessilesessile:
(adj) attached directly, without a stalk
or subsessile, subtending spathespathe:
(n) a large bract or bracts subtending and often enclosing an inflorescence
opens violently, liberating flowers which then float on water surface; female flowers sessilesessile:
(adj) attached directly, without a stalk
but appearing long-pedicellate (hypanthiumhypanthium:
(n) the cup-shaped structure formed from the fusion of the basal parts of the calyx, corolla, and stamens, on the rim of which these parts arise
), opening at water surface. Dispersal by seed, turionturion:
(n) a hibernating bud produced by certain aquatic plants from which growth emerges in the following spring
or stem fragments.

Natural habitat

still waters of lakes, ponds, swamps, and rivers

Additional comments

Hydrilla verticillata is the only species in this genus. Hydrilla is a very fast growing plant which quickly outcompetes other plant species. Extensive growth of this plant quickly chokes waterways, limiting their use by people. Variation in leaf number, stem elongation, and leaf shape can result in misidentification with other genera similar in appearance. Small spine-like processes along the abaxialabaxial:
(adj) the side facing away from the axis, as in the underside of a leaf
surface of the leaf midveinmidvein:
(n) the primary, usually central vein of a leaf or leaflet
is an important diagnostic character in plants that are not flowering, but is not always present.