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 blocked.