Hydrilla

Hydrilla verticillata is believed to have originated in the Old World tropics and subtropics, with its native range spanning parts of Asia, Africa, and Australia.

• Native range includes regions of South and Southeast Asia, parts of central Africa, and northern Australia
• First described scientifically from specimens collected in India
• Introduced to the United States in the 1950s or 1960s, likely through the aquarium trade
• Has since spread aggressively throughout the southern and eastern United States, becoming one of the most problematic invasive aquatic weeds in North America
• Now found on every continent except Antarctica

Hydrilla is a rooted submerged aquatic plant with a highly adaptable morphology that allows it to thrive in a wide range of freshwater habitats.

Stems:
• Slender, branching stems can reach lengths of 1 to 3 meters (occasionally up to 9 meters in deep water)
• Grow rapidly and form dense mats at or near the water surface
• Easily fragment — even small stem fragments can regenerate into entirely new plants

Leaves:
• Arranged in whorls of 3 to 8 (typically 4 to 5) around the stem
• Small, lanceolate to narrowly oblong, 6–20 mm long and 1–4 mm wide
• Leaf margins are distinctly serrated (toothed) — a key identification feature
• Midrib on the lower surface may bear small spines or teeth
• Bright green to dark green, sometimes with a reddish tinge

Roots & Rhizomes:
• Roots are adventitious, arising from nodes along the stem, anchoring the plant in sediment
• Produces underground tubers (turions) in the substrate — these are critical for overwintering and long-term survival
• Tubers are potato-like, 5–10 mm in diameter, and can remain viable in sediment for several years
• Also produces axillary turions (buds) on stems

Flowers:
• Dioecious — male and female flowers occur on separate plants (in most introduced populations, only female plants are present)
• Female flowers are tiny, solitary, with three translucent white sepals and three petals, borne on long, thread-like hypanthia that reach the water surface
• Male flowers are smaller, greenish, and either detach to float to the surface or remain attached
• Flowers are inconspicuous and rarely observed in many populations

Hydrilla occupies a wide range of freshwater habitats and profoundly alters the ecosystems it invades.

Habitat:
• Found in lakes, ponds, rivers, reservoirs, canals, ditches, and slow-moving streams
• Tolerates a broad range of water depths, from shallow margins to depths exceeding 6 meters
• Can grow in both oligotrophic and eutrophic waters
• Tolerates low light conditions — can photosynthesize at light levels too low for many competing species
• Tolerates slight salinity (up to ~7 ppt), allowing it to colonize brackish environments

Growth Conditions:
• Optimal temperature range: 20–30°C
• Can survive freezing conditions by dying back to tubers and regrowing in spring
• Tolerates a wide pH range (approximately 5.0–9.0)
• Extremely efficient at absorbing nutrients directly from both water column and sediment

Ecological Impact:
• Forms dense surface mats that block sunlight, suppressing native submerged vegetation
• Alters dissolved oxygen dynamics — can cause extreme diel fluctuations in oxygen levels
• Changes water chemistry, including pH and nutrient cycling
• Displaces native aquatic plant communities and the wildlife that depends on them
• Can impede water flow in irrigation canals and drainage systems
• Provides habitat for mosquito larvae in some settings

Reproduction & Dispersal:
• Primarily reproduces vegetatively through stem fragmentation, tubers, and axillary turions
• A single stem fragment with one node can regenerate into a new plant
• Tubers can survive desiccation, herbivory, and passage through waterfowl digestive tracts
• Dispersed by water currents, boats, boat trailers, fishing equipment, and waterfowl
• Sexual reproduction is rare in introduced populations (e.g., North America) where only one sex is typically present

Hydrilla verticillata itself is not considered directly toxic to humans or most mammals. However, it can harbor cyanobacteria (blue-green algae) in its dense mats, and some cyanobacterial species produce toxins (microcystins, cylindrospermopsin) that are harmful to humans, pets, and livestock.

• Dense hydrilla mats create stagnant microenvironments favorable to cyanobacterial blooms
• Contact with or ingestion of water containing cyanobacteria associated with hydrilla mats can cause skin irritation, gastrointestinal illness, and in severe cases, liver damage
• The plant is generally not toxic when consumed by waterfowl, which readily eat hydrilla foliage and tubers

Hydrilla is not intentionally planted in most jurisdictions and is classified as a noxious or prohibited weed in many regions. However, it is sometimes used in controlled aquarium settings and scientific research.

Aquarium Use (where legal):
• Can be used as an oxygenating plant in freshwater aquariums
• Grows rapidly and helps absorb excess nutrients, reducing algae
• Requires moderate to bright lighting
• Prefers temperatures of 18–28°C
• Easily propagated by stem cuttings — simply plant a stem fragment in substrate

Legal Status:
• Listed as a federal noxious weed in the United States under the Plant Protection Act
• Illegal to sell, distribute, or transport in many U.S. states
• Classified as an invasive species of concern in Australia, New Zealand, and parts of Europe
• Check local regulations before any cultivation

Control Methods:
• Mechanical harvesting — effective short-term but fragments can spread the infestation
• Herbicide application (e.g., fluridone, endothall, diquat) — most common large-scale management tool
• Biological control — grass carp (Ctenopharyngodon idella) feed on hydrilla; several insect biocontrol agents (e.g., the Asian hydrilla leaf-mining fly, Hydrellia pakistanae) have been introduced in the U.S.
• Drawdown — lowering water levels to expose and desiccate plants and tubers
• Prevention — cleaning boats and equipment to avoid fragment transfer between water bodies