The document discusses the biology and management of hydrilla (Hydrilla verticillata), an invasive aquatic plant. It describes hydrilla's rapid growth rate, ability to spread through vegetative fragments and specialized structures, and high genetic diversity, which allow it to quickly dominate water bodies. The document also outlines different control methods used, including herbicides and biocontrol with grass carp, and debates the feasibility of eradication efforts. Factors that influence hydrilla's establishment and spread, such as climate and water quality parameters, are also reviewed.

1. The Biology and Management of Hydrilla
Michael D. Netherland
US Army Engineer Research and Development Center
Gainesville, FL
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2. Does Hydrilla Pose a Greater
Threat to Missouri Waters
Than Other Invasive Plants ?
Hydrilla Eurasian Milfoil Curlyleaf Pondweed
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3. Overview
 Hydrilla
► Life history
► The “Perfect Weed”
► Invasion History
 Factors Impacting Establishment & Spread
► Abiotic contraints
 Control Methods
 Feasibility of Eradication
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4. Hydrilla verticillata L.f. Royle
 Hydrilla is a monocot - Hydrocharitaceae family
► Elodea, Egeria, and Vallisneria
► Early detection ?
 Native to tropical SE Asia
► Found on 6 Continents as far North as Poland
 Dioecious and Monoecious Biotypes
► Separate Introductions to the US
► Worldwide – Monoecious is tropical and Dioecious is
temperate
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5. Hydrilla - “The Almost Perfect Weed”
* Ability to cover thousands of contiguous acres
* propagates by fragments, turions, tubers, crowns
- tubers can remain quiescent for years (seed)
* Low light and CO2 compensation points
* Clonal plant - but high genetic diversity
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6. Why Plant Biology Matters ?
What is it about Hydrilla that
Allows it to Grow to Such
Abundance ?
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7. Does Hydrilla Grow an Inch per
Day ?
 Data often presented in terms of biomass
► E.g. 400 grams dry wt. /square meter
 Single 4” shoot grown for 35 days
1.2 M
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8. 3500
Hydrilla change in Week 1 Week 2 Week 3 Week 4 Week 5
inches over 35 d
3000
Laterals 2±2 13 ± 6 43 ± 11 109 ± 29 127 ± 38
191.7 New Stems 2±0 6±1 34 ± 13 71 ± 16 110 ± 63
Runners 0±0 1±1 3±2 9±6 35 ± 20
2500
Total Length of New Growth (in)
2000
-A single 9 inch shoot resulted
in over 3200 inches of growth in
1500 182.3
5 weeks
1000
500
58.8
2.9 11.5
0
7/23/09 7/30/09 8/6/09 8/13/09 8/20/09 8/27/09
Harvest Date
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9. Hydrilla
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10. Dioecious Hydrilla
 Southern Distribution in the US
► Clonal female population
 Tubers produced - day length < 12 hr
► Fall
production (short window in North)
► May switch to axillary turions in North (rapid process)
 Rapid Canopy formation
► Internodes measured in feet
 Fluridone Resistant Populations are in FL
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11. Monoecious Hydrilla
 Northern Distribution in the US
 Tubers produced in summer
 Dies Back in Winter (propagules)
 Rapid lateral expansion
► Competitive interactions not documented
 Multiple Eradication Programs Ongoing
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12. Greatest Threat to Missouri ?
 Monoecious Biotype May be Well Suited to
Missouri
► Tolerates much higher disturbance
► Reservoir fluctuations
► Found in Reservoirs throughout the S.E.
► Becoming well established in TVA reservoirs
• Expansion westward
► We know much less about monoecious hydrilla
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13. Significant Milfoil or Hydrilla Presence
Natural Lakes
Reservoirs
Ohio River – Hydrilla
Expanding where no SAV
Was previously noted
High Energy Reservoirs /
Turbid with water level fluctuations
Generally not conducive to SAV (Monoecious hydrilla ? )
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14. Progression of Hydrilla
Spread
Year Number States
of States
1960 – 1969 1 FL (1953)
1970 –1979 6 AL, CA, DE, GA, LA
1980 – 1989 13 CT, MD, MS, NC, SC, TX,
VA
1990 – 1999 17 AR, PA, TN, WA
2000 – 2010 29 ID, IN, KY, MA, ME, NJ,
OK, WI, WV, NY, KS, OH
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15. Water Bodies in Wake County, NC with Hydrilla
70
Number of Water Bodies
60
58
50
40
30
20
12
10
10
1 2
0
1980 1982 1984 1986 1989
Year Surveyed
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16. Each Water Body Requires an INTRODUCTION !
Lake Gaston, NC
Plants move from large to small
water bodies and vice versa
Small Landscape Pond in NC
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17. Hydrilla in Wisconsin Pond –
Response by DNR
Positive i.d. in fall of 07
Completely drained pond for
winter drawdown
Spring 08- refilled and
treated with fluridone
Why such a strong response ?
Where did the hydrilla come from ?
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18. Management Options Are
Limited
Grass Carp – SE Reservoirs
Selective Biocontrol
Environmental Extremes
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19. Hydrilla Management
 Grass Carp –
► “All or None control” in large systems
► generally non-selective or at least non-
predictable
► Still no efficient means for removal of fish
► Limited use in northern lakes due to low
preference for milfoil
• Cold water can reduce feeding
efficiency/predictability
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20. 13 Herbicides Labeled for Aquatic Use
(223 labeled for terrestrial use)
Copper (1900’s) 2,4-D (1950’s)
Endothall (1960) Diquat (1962)
Glyphosate (1977) Fluridone (1986)
*Amitrole, Dicamba, Dalapon Simazine, Fenac,
Dichlobenil, Silvex (most cancelled in 1987)
Triclopyr (2002)
Imazapyr (2003) Carfentrazone (2004)
Penoxsulam (2007) Imazamox (2008)
Flumioxazin (2010) Bispyribac (2011)
Blue = Plant Enzyme Specific Inhibitors
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21. Thousands of Harvesters - Winter Biomass Control , FL
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22. California – Eradication Policy
 Ongoing Eradication efforts for 30+years
► concern = agricultural irrigation
 Aggressive and Immediate Action
 Clear Lake (~ 40,000 acres)
► Eradication policy
► Treat 5 acres around individual plant finds
► Poor Economics on an acre basis / Good overall
Policy ?
 Nationwide Eradication projects > 6 years
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23. Hydrilla and Ecosystem Services
Increased water clarity
 Provides Structure and Food
► Hydrilla- low density = native plant
 Does Not Produce Toxins (e.g. golden algae)
 Valued by the Fishing and Hunting
Communities
►“ Fishing the Edge” “Ringneck duck buffet”
 Growth Rate , Canopy formation, & “Just leave us a little”
Ability to Occupy Vast Expanses
► Access, Flood Control, Fishery Mgmt.
► Native Plants, Water Quality
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24. Hydrilla Expansion
 Initial Establishment – difficult step
 2 to 10% may be the hardest step
 Once well established plant can
easily go from 30 to >70% coverage
- widespread cover and low biomass !
 Hydrilla creates habitat for more Hydrilla
• Water clarification and wave reduction in open fetches
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25. Unintended Consequences
of Hydrilla Growth
Stignematalan algae – produces a novel toxin
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26. Kissimmee Chain of Lakes
– Headwaters- C&SF Flood Control Project
- Extensive hydrilla management
In a cruel twist of irony –
Florida DEP recently declared Lake
Toho impaired for Excessive
Macrophyte Growth (HYDRILLA)
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27. What Limits Hydrilla Growth ?
 “It’s the light”, stupid
► Algal turbidity, suspended sediments
 Poor Sediment quality or nutrition
 Cyanobacteria blooms – toxins ?
 Herbivores – general and specialists
► Grass carp, snails, etc.
 Consistent Management ?
► Setting back the clock
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28. Unknowns for Hydrilla in MO
 What is the greatest environmental
constraint to hydrilla growth in MO waters ?
• Fluctuating Reservoirs
• Seasonal Extremes (temp, flow, etc.)
• Sediment Composition
• Water Clarity
 Can hydrilla colonize & proliferate in MO
Reservoirs ?
• We will likely find out
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