Hydrilla!  You mention this aquatic plant and some reservoir constituents are all smiles (primarily anglers) while others label it the world’s worst aquatic weed (swimmers, water skiers, homeowners, industrial and municipal users, etc.).  Both sides of the issue have merit in certain situations.  Before we explore both beneficial and detrimental scenarios associated with hydrilla (Part 2 of this topic in a future Lakecaster issue), a little biological/life history information is in order.

Hydrilla, known to scientists as Hydrilla verticillata, is an exotic plant.  Originally native to Asia, it was first discovered in the United States in 1960 (Florida), in Texas in 1969 (Houston area), and in Toledo Bend and Sam Rayburn reservoirs in the mid-1970s.  Hydrilla has been called the perfect aquatic plant, due to its growth characteristics, adaptable nature, and wide tolerance of conditions.  The current distribution of the plant is reflective of these features (it is found in over 15 states as far north as Pennsylvania).

During typical conditions in the late fall and winter, hydrilla growth will slow considerably and at least some of the plant biomass is lost due to mortality.  Most of the hydrilla loses its rigidity, causing it lay down on itself.   At this time, the plants are more susceptible to fragmentation by wave action.  It is common to see fragmented hydrilla floating or wind-rowed along exposed banks during these seasons.  Hydrilla growth can occur at temperatures as low as 51F, but rates accelerate as temperatures reach 65F and peak in the middle of the summer at up to one inch per day.  Growth is negatively impacted at temperatures exceeding 95F.  As hydrilla matures and reaches the surface, most of its growth is directed towards the surface.  In fact, after the plant reaches the surface over one half of its biomass is found in the upper 2 feet of the water column, resulting in the thick surface mats characteristic of hydrilla.  Below the mat, the water column is relatively open, producing shade and desirable habitat for a host of aquatic organisms. 

In addition to outgrowing other species of submergent vegetation, hydrilla has other characteristics that provide a competitive advantage.  Sunlight is essential to aquatic plant growth.  Relative to other species of aquatic plants, hydrilla requires little sunlight to grow.  This allows the plant to grow deeper than other species (up to 15 feet or more in Sam Rayburn and Toledo Bend) and gives it a growth advantage early in the morning and late in the evening.  In addition, the matting characteristic of the plant allows it to outcompete other plants by physically blocking sunlight.

Reproductive characteristics reflect the adaptability of the plant.  Although both male and female hydrilla plants exist in the United States, only female plants are found in Texas.  Reproduction of the female plants is by reproductive structures (turions and tubers) and by fragmentation.  Turions are small, green, spiny buds that form at the base of the leaves and fall from the plant when mature.  Tubers, white structures that form on the roots, can survive up to 4 years in the soil until conditions are favorable for growth.  Plant fragmentation is probably the most adaptable means of reproduction and explains the rapid rate of expansion once hydrilla is introduced into a reservoir.  A single clump of leaves can lead to establishment of the plant.  It is widely believed that many reservoir introductions were accidental due to presence on boats and trailers.

Unlike many other species of aquatic vegetation, hydrilla survives under a wide range of water quality conditions.  It needs less sunlight so it grows in more turbid water than other species.  Hydrilla is present in both clear, infertile reservoirs and those that contain high levels of nutrients.  It can withstand a relatively high amount of salinity and tolerates a wide range of pH values.

Although hydrilla is a beneficial plant in certain situations, hydrilla is considered a nuisance plant, due to its adaptive and invasive nature.  As a result, the plant is listed under the federal Prohibited Aquatic, Wetland, and Invasive Plants list and the Texas Harmful or Potentially Harmful Exotic Plants list found in the Texas Parks and Wildlife Department (TPWD) Proclamations.  The Texas list is maintained by the TPWD Commission.  As part of this list, it is an offense to release into public waters, transport, propagate, or possess hydrilla in the state of Texas.  In a future issue, beneficial and detrimental scenarios regarding the presence of hydrilla will be discussed.  Contact us with questions or concerns about area fisheries by phone (409-384-9572) or email ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ).  Good luck and good fishing!

Todd Driscoll is a district fisheries management biologist for Texas Parks and Wildlife Department where he has worked for 10 years.  He received a B.S. in Fisheries Biology from Kansas State University and a M.S. in Fisheries Management from Mississippi State University.  His primary responsibilities include fisheries management of Sam Rayburn and Toledo Bend reservoirs in Southeast Texas.  Todd is an avid bass angler and participates in approximately 25-30 local and regional bass tournaments per year.  He also represents Lowrance Electronics as a Technical Pro Staffer, working tournament support and service at BASS, FLW, and owner’s tournaments across the country.