Hydrilla (Hydrilla verticillata), also commonly called water thyme, is a submersed perennial herb. The plant is rooted in the bed of the waterbody and has long stems (up to 25 feet in length) that branch at the surface where growth becomes horizontal and forms dense mats. Small, pointed, often serrated leaves are arranged around the stem in whorls of 4 to 8. Southern populations are predominantly dioecious female (plants having only female flowers) that overwinter as perennials. Populations north of South Carolina are essentially monoecious (having both male and female flowers on the same plant) that set some fertile seed, and depend on tubers for overwintering.
The dioecious form of Hydrilla is believed to originate from the Indian subcontinent, specifically the island of Sri Lanka, although random DNA analysis also indicates India's southern mainland as a possible source location. The monoecious form is believed to have arrived on our shores from Korea.
Introduction & Spread
The dioecious strain of H. verticillata was imported as an aquarium plant in the early 1950s. Discarded (or intentionally planted ) colonies were found in canals in Miami and Tampa shortly after. The monoecious strain was introduced separately decades later in the Potomac Basin.
Both dioecious and monoecious Hydrilla propagate primarily by stem fragments, although turions (buds) and subterranean tubers also play an important role. The main means of introduction of Hydrilla is as castaway fragments on recreational boats and trailers and in their live wells. New colonies can often be found near boat ramps as such stem pieces become rooted in the substrate (even very, very small fragments can become the start of new populations). Boat traffic through established populations can shatter and spread Hydrilla throughout the waterbody, similar to the spread of Eurasian watermilfoil.
Hydrilla is often a contaminant on popular watergarden plants and may be unwittingly transported and established in private ponds in this manner. As with most invasive aquatic plant species, Hydrilla is a very opportunistic organism and can often be found taking over waters that have had populations of Eurasian watermilfoil chemically removed without a management plan for reestablishing native vegetation.
Hydrilla can invade deep, dark waters where most native plants cannot grow. The plant’s aggressive growth (hydrilla’s 20 - 30 foot stems can add up to an inch per day) can spread into shallow water areas and form thick mats that block sunlight to native plants below, effectively displacing the native vegetation of a waterbody. Major colonies of hydrilla can alter the physical and chemical characteristics of lakes:
- It is one of the world's worst aquatic invasive plants
- It blocks sunlight and displaces native plants below with its thick, dense surface mats
- Stratification of the water column and decreased dissolved oxygen levels can lead to fish kills
- The weight and size of sportfish can be reduced when open water and natural vegetation are lost
- Waterfowl feeding areas and fish spawning sites are eliminating by dense surface mats
- Thick mats of vegation can obstruct boating, swimming and fishing
- The value of shorefront property can be significantly reduced, hurting both homeowners and the communities that rely on taxation of shoreline property
- In severe infestations, intakes at water treatment, power generation, and industrial facilities can be blocke
Hydrilla has pointed, bright green leaves about 5/8 inches long. The leaves grow in whorls of 3 - 10 along the stem, 5 being most common. The margins of the leaves are serrated (toothed). The underside of the leaf has a reddish central spine and one or more small spines that give it a rough feeling when rubbed between your fingers. Thin stalks from the stem end in a single, small, floating white flower at the water's surface. A key identifying feature is the presence of small (up to half inch long), dull-white to yellowish, potato-like tubers which grow 2 to 12 inches below the surface of the sediment at the ends of underground stems. These tubers form at the end of the growing season and serve to store food to allow Hydrilla to overwinter.
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