Once hydrilla has been introduced into a lake, complete elimination is very difficult. There is no quick fix for hydrilla but control options fall into three basic categories:

  • Mechanical controls:
    • Remove part or all of the plant either by hand or machine (harvesting)
      • This is costly and the plants grow right back, like mowing a lawn.
      • Dispose of all plant fragments on shore: Because new plants can sprout from fragments, all plant material cut or collected MUST be removed from the lake. Throwing hydrilla back in the lake can result in a maximum fine of $2000 per plant.
    • Winter lake lowering to expose the plant to drying and possibly freezing temperatures
      • This only impacts plants in less than 12 ft of water, and doesn’t kill roots
  • Biological methods:
    • Introduce fish (sterile grass carp) or insects that eat the plants
    • Reintroduce native plants- the City has done this on 20 sites on Lake Austin and Lady Bird Lake, but it takes a long time for the plants to outcompete invasives like hydrilla
  • Chemical control using aquatic herbicides to kill the plants. Lake Austin’s depth and constant water flow limit the effectiveness, and due to drinking water intakes and public perception, it has not been an acceptable solution in the past. The City does not recommend this, and private individuals must get approval from TPWD for this action.

September 2013

TPWD’s September 2013 vegetation survey of Lake Austin documented 203 acres of aquatic vegetation, mostly Eurasian watermilfoil (milfoil), which increased from 136 acres in June. The most notable change was the exotic plant hydrilla, which covered 330 acres in June but was not observed at all on the September survey.

Infesting the lake since 1999, this aggressive, invasive plant has posed significant public safety threats as its dense growth impacted flood flows, water intakes and recreation on the lake. It reached a historic high coverage of over 600 acres in February 2013, due primarily to the drought-induced warmer water that the plant prefers. The recent decline is a result of ongoing stocking of sterile grass carp that eat hydrilla, preferring it to other plants like milfoil.

As hydrilla declined, milfoil (a less problematic exotic) has increased and is providing critical ecosystem benefits like aquatic habitat, food and oxygen that helps keep the lake healthy without the extremely dense growth of hydrilla.

It is important to remember that hydrilla is under control but probably not eradicated; over time, the fish population will decline naturally and hydrilla may re-sprout from underground tubers. Changes in water flow and temperature can also impact growth rates, so the City and TPWD will continue to monitor Lake Austin vegetation and implement control efforts as needed.

Lake Austin Vegetation Survey September 2013

Summer 2013

TPWD’s June survey documented 338 acres of the exotic plant hydrilla on Lake Austin, a decrease from the lake’s historic high of 588 acres in February 2013. As hydrilla decreases, the less aggressive exotic milfoil increased to 136 acre from 60 acres in February, making 474 acres of total vegetative cover on the lake. Most of these plants are concentrated from upstream of Emma Long (City) Park to the headwaters, with hydrilla growing in more than 12 feet of water, and only small areas reaching the surface. This is a significant change since last summer, when hydrilla covered 580 acres, and dense mats impacted recreational use of the lake.

The decrease in hydrilla can be attributed to the stocking of grass carp in the lake, with the latest addition in May 2013 bringing the rate to 55.5 fish per hydrilla acre. The fish had kept hydrilla in check from 2005-2010, and the plant coverage stayed below 80 acres throughout that period. But since 2011, Lake Austin’s warmer water and lower flow (due to drought-stricken Lake Travis) allowed hydrilla to increase steadily, and required an increase in the fish stocking rate to gain control.

The extreme low water level on Lake Travis means there is insufficient water available for re-fill after a lake drawdown, so it is not anticipated that the City will request that LCRA conduct a lowering of Lake Austin in January 2013. If Lake Travis water levels rise significantly in the months ahead, this option can be re-evaluated as additional vegetation control. TPWD will continue to conduct quarterly Lake Austin vegetation surveys, using the results to determine appropriate control actions for hydrilla.

Lake Austin Vegetation June 2013

April 2013

TPWD’s February 2013 Lake Austin vegetation survey (see map below) documented over 580 acres of hydrilla (out of 647 acres total plant coverage), a slight increase since last fall’s survey of 554 acres of hydrilla. This increase over the winter prompted TWPD to approve the stocking of an additional 9,000 sterile grass carp, which the City will purchase and stock at Mary Quinlan Park in early April . This fish stocking is part of an ongoing program designed to limit impacts from hydrilla without removing all beneficial vegetation from the lake.

Estimating loss of fish due to mortality and migration downstream, TPWD has determined these additional fish will bring the stocking rate up to 55.5 fish per acre of hydrilla. This elevated stocking rate was recommended to address the drought conditions favorable to the plants (limited flow, warmer water) that are expected to continue throughout this summer. TPWD anticipates surveying the lake again in June or July to measure the impact from the higher stocking rate.

Lake Austin Vegetation Survey February 2013

October 2012

TPWD’s September 2012 survey showed a slight decrease in lake vegetation, with 554 acres of hydrilla, down from the April 2012 historic high of 585 acres. As a comparison, during the same period last year, hydrilla increased over 150 acres (see red arrows on graph below). While hydrilla coverage is still of great concern, and is a result of drought-induced warmer water and lower flow in the lake, this downward trend is encouraging, and is most likely due to the ongoing stocking of sterile grass carp.

Chart of Lake Austin Vegetation from Jan 2009 - Sep 2012.

The graph above shows plant growth and fish stocked over time on Lake Austin. 

The plants were kept in check from 2005 – 2010 primarily due to the large number of grass carp (fish that prefer hydrilla) stocked.   The City will stock 6,000 more grass carp in early November 2012, following this summer’s cooperative stocking of over 11,300 fish.  But it will take some time for these newly stocked fish to ‘catch up’ to the increased growth spurred by current environmental conditions. 

Over 47,700 fish have been stocked in the lake since the program began in 2003, but with natural mortality and other losses, TPWD estimates the 2012 population to be around 27,770 fish.  At current hydrilla coverage, this means there are about 50 fish per hydrilla acre, which is a level TPWD expects to provide control, as coverage has increased whenever the stocking level dropped below that in the past. 

While winter drawdowns can provide some control in areas less than 12 feet deep, hydrilla grows in depths well beyond the drawdown’s exposure, and affected plants grow back by the next summer.  LCRA has indicated that current water levels on Lakes Travis and Buchanan make a lake lowering unfeasible, but if water levels increase significantly this fall, that option will be re-considered. 


June 2012

TPWD approved 15,200 sterile grass carp for use in Lake Austin due to results of the April 2012 survey showing over 585 acres of  hydrilla in the lake.  This is 50% of the total number of fish stocked in the past nine years, and more fish than were stocked in all of 2011.   The City of Austin, LCRA and Friends of Lake Austin have agreed to share the cost of these fish.  However, current supplies of grass carp are very limited (2012 has been a banner year for hydrilla growth across the southern United States) so only 10,000 fish can be stocked at this time.  The City will release 3,000 of these fish on Wednesday June 20 at Mary Quinlan Park, and the remaining fish will be stocked in subsequent weeks.

May 2012

The April 2012 Texas Parks and Wildlife Department (TPWD) survey showed over 585 acres of hydrilla in Lake Austin, with an additional 86 acres of milfoil and other vegetation, for a total of 672 acres of vegetative cover, a historic high. This map shows the extent of the infestation. 

Lake Austin vegetation survey April 2012

The increase is most likely due to the drought-induced low water levels on Lake Travis, making the deep water releases into Lake Austin warmer than normal. Hydrilla has been increasing steadily over the past year, and has now reached levels that can limit shoreline access as well as other lake activities.  This trend is expected to continue, especially since LCRA is not releasing water for rice irrigation in 2012, so there is limited water flow through the lake.  

Chart of Lake Austin Vegetation

The graph above shows plant growth and fish stocking over time.

From 2003-2011, more than 30,000 fish were stocked in the lake (20,000 purchased by COA, 10,000 by TPWD), and for over five years, the fish were effective in keeping hydrilla below nuisance levels.  With the recent surge in hydrilla growth, TPWD approved an additional 15,200 grass carp, and the cost of these fish will be shared by the COA, LCRA and Friends of Lake Austin.  It is hoped that this large input of fish will provide a significant level of control, as the plants continue to grow and threaten uses of the lake.

Hydrilla sprouts new plants from fragments containing at least one circle of leaves, or leaf whorl. Once hydrilla becomes established, it is easily spread by waterfowl and boating activities.

Hydrilla forms dense mats of vegetation that can interfere with recreation and degrade fish and wildlife habitat. Hydrilla can grow extremely rapidly, up to one inch per day, until it reaches the water's surface and forms a thick mat that effectively shades any plants below it. Plants have been known to reach a length of 50 feet and produce a biomass of more than 130 tons per acre.

Does hydrilla provide good habitat for fish?  Initially, yes. Aquatic plants are an important part of Lake Austin's environment, providing habitat and food for organisms living there. Before it gets too dense, hydrilla can provide good habitat for fish. However, fish populations are negatively affected when hydrilla exceeds 30-40% coverage of the lake. Hydrilla will grow with less light and fewer nutrients than other aquatic plants, and can outcompete all other native and non-native plants. Eventually, decomposing plants can rob the water of oxygen needed for fish and a healthy aquatic community.

  • To minimize fragmentation and spreading of plants, avoid boating through dense hydrilla mats.
  • Remove hydrilla from your boat's propeller and trailer before and after boating.
  • Dispose of all plant fragments on shore: Because new plants can sprout from fragments, all plant material cut or collected MUST be removed from the lake. Throwing hydrilla back in the lake can result in a maximum fine of $2000 per plant.
  • Follow City of Austin hydrilla disposal guidelines:
    • Plants should be placed as far up on the shore as possible.
    • Plant material stockpiled within 75' of the water's edge should be surrounded on the downslope side by silt fencing.
    • Plant material pulled from the lake will contain small fish and other organisms, and will have an odor associated with it. The plants are mostly water, and piles will lose 90% of their bulk within 2-4 weeks. This material can be used to mulch flowerbeds or gardens.
  • Learn more about Friends of Lake Austin (FOLA) - a citizen group dedicated to preserving and enhancing Lake Austin for those who live, work and play on the lake. They have been working to help fight the hydrilla infestation on Lake Austin and are taking an active role in the approved management plan

In 2000, COA, TPWD, LCRA and Friends of Lake Austin finalized the Lake Austin Hydrilla management plan that integrates all control efforts appropriate for use in this situation.  Objectives are to return Lake Austin to a pre-hydrilla condition and to maintain a healthy lake ecosystem and fishery.  It integrates every appropriate control option, so that potential impacts from any one option are minimized while the possibility of success is increased.

Sterile grass carp have proven the most effective control, because they can move throughout the lake, eating plants in any water depth. Biennial winter drawdowns provide additional relief, but only in less than 12 ft of water.  From 2003 to 2011, over 30,000 grass carp were stocked, and hydrilla remained below 65 acres until late 2009, when drought-induced warmer water temperatures provided an advantage to the plants.

The graph above shows plant growth and fish stocking over time.

The graph above shows plant growth and fish stocking over time.

As of April 2012, aquatic vegetation (586 acres hydrilla, 86 acres Eurasian watermilfoil, and a mix of other plants) covered 41% of Lake Austin. Because of its dense and rapid growth, hydrilla has the potential to impact virtually every one of Lake Austin's uses:

  • Intakes for drinking water, power generation and irrigation can be clogged
  • Shoreline access and boating traffic can be restricted
  • Swimmers can get tangled in its thick growth
  • Water quality may degrade as dense vegetation dies and decomposes
  • Plant and animal diversity will decline as hydrilla takes over
  • Property values can decrease as recreation is limited by dense plant growth
  • Hydrilla may spread downstream to Town Lake and the lower Colorado River.